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Splicing rafters in the ridge area: we splice the rafters along the length and installation methods in the ridge area. Types of gable roof truss system: for small and large houses Why do you need a run under the ridge

The truss system is the basis of your future roof, so its construction must be taken very seriously. Before you start work, you need to sketch out a rough plan of the system for yourself in order to understand what the overall structure will look like and what functions its individual elements perform.

In order to calculate the parameters and technical characteristics of the truss system for large objects, it is best to resort to the services of professionals. If your roof is intended for a private building of a relatively small size (house area up to 100 m 2), then you can install it using the materials below.

The first step is to determine the angle of inclination of the slopes. Usually, the average calculations are based on the amount of materials, which has a very good effect on the material component of the issue, it is generally accepted that the smaller the angle of inclination, the more profitable and cheaper the construction will cost. In fact, it is necessary to choose the angle of inclination from two main indicators - wind loads and the weight of precipitation (in particular in winter), as you can see, the issue of pricing in technical parameters is not taken into account. The universal angle of inclination for our climate is 45-50 degrees, with such parameters, the strength indicators are maximally balanced before loads, both wind and those that can be caused by precipitation pressure. Sometimes it happens that about 180 kg of snow falls on one square meter of the roof. In addition, the financial component will also be at an average level, which is much better than saving by reducing the angle of inclination, but subsequently overpaying two prices for the elimination of defects that will be caused by the above factors.

Tree selection

For the rafter part, two parameters are important - strength and lightness of construction, so ordinary pine is suitable for installation. It is often used for these types of structures, as it has these two qualities, plus it has a favorable price compared to noble woods. It is necessary to use a board of the first grade, 150-200x50x6000 mm in size, we also need a bar with a section of 200x200 mm.

An important technical point is the moisture content of the wood. A freshly sawn tree has a 50% moisture coefficient, it is impossible to mount such a tree, because if it dries out in a state of tension, it can lead, it will bend and crack in places where the knots are located. It is necessary to purchase material with a 15-20 percent moisture content.

When buying, check that all boards are even and without rot, the strength and durability of the structure depends on this.

When the tree is delivered to your construction site, it must be treated with antiseptic preparations and laid in the most ventilated area. The laying of the tree must be done in a certain way: first we lay three or four transverse slats, on them, along, we lay the boards, so that there is a distance of 0.5-1 cm between each board, then again a row of transverse battens and a row of boards.

Thanks to this, we will create an air space between each piece of lumber, they will be ventilated in the right conditions, which will allow us to avoid rotting and moisture accumulation.

We put the ridge beam

The ridge beam is the central top beam, which is designed to evenly transfer the total weight of the roof to the gables, distributing the pressure area along the entire side perimeter. Installing a beam is a very complicated process. First of all, let's decide on its length. As a rule, from the sides of the roof, according to the plan, there are small peaks (from 0.5 to 1.5 m), the ridge beam must exactly lie along this length with all the protrusions outside the gables. On concrete foundations, in places of contact with the timber, we lay pieces of roofing material, so that the tree does not touch the gable directly - only through waterproofing. We bend the roofing material around the timber, and drill on the sides and insert two segments of the 12th reinforcement, 0.4 m each. We do not drill the beam itself to avoid cracks.

Elongated beam

Very rarely enough for the "horse" standard 6 meters. In most cases, this length has to be increased. The build-up takes place at the installation site, otherwise the spliced beam will be very difficult to lift up and install. The place of joining the timber must be chosen in such a way that it is as close as possible to some partition or other point on which a temporary vertical support can be placed. For a vertical support, we measure and cut off a board, on the sides of which we nail two small boards, so we get something like a wooden fork, between the teeth of which there will be a joint of a ridge beam. On the upper side of the ridge, we stretch the thread, which will serve as a level for us before we fasten the beam together. They must be fastened with two one and a half meter sections of the board, the connecting sections are located exclusively on the sides, in which case the load will be applied to the tree in the right direction, reducing the risk of breaking at the junction. The boards are fastened with nails, because if you try to organize bolted connections, the beam can give a number of cracks when drilling.

Mauerlat

This element serves to join the rafters with the longitudinal bases of the bearing wall, for point distribution of the load of the entire structure. It is necessary to lay it using roofing material (as in the case of a ridge). Choose the most even boards, they should be as close as possible to the wall surface. The Mauerlat is fixed using anchor bolts 0.2 m long. The points where the anchors will be placed must be calculated in advance, their location should be in the gaps between the future rafter boards, so that the anchor caps do not interfere with us when further fastening the following elements.

If the standard length of the board is not enough - feel free to get the boards and fasten them in the same way as the joint between the Mauerlat boards will be organized - it does not matter, the main thing is that they fit snugly against the concrete.

Don't forget to put the Mauerlat in short lengths behind the gables, where you have the roof peaks.

Device and installation of rafters

The first step is to determine the number of rafters, for this we take the total length of the roof and divide by approximately 1.2-1.4 m, after we get an integer, we divide the length of the roof by it. An integer is the number of rafters on one side, dividing the length by this number will give us a more accurate step between them, for example, if the length of the roof is 9 meters:

9 m / 1.3 m = 6.92(round up) \u003d 7 - the number of rafters;
9 m / 7 = 1.28 m- a step between the rafters.

We multiply the number of rafters by two and again by two, thanks to these calculations we will get the total number of boards that will need to be used to make the structure.

The next step is to cut the boards at the angle of the roof. To do this, on one side of the board, the perpendicular between the cut and the longitudinal part must be shifted down by the required number of degrees. With the help of a protractor and a pencil, everyone can perform this procedure. Next, we cut the board along the intended line, we will get a template according to which we will trim all the other boards.

First, we mount the extreme rafters, which are located inside the zone between the gables. Installation of rafters is carried out on two levels, the first at the ridge, the second near the Mauerlat. The marking of the step between the rafters should be done both at the top and at the bottom. This line is the middle of the rafters, the design of one rafter consists of two boards, the distance between them is 50 mm.

We cut off 9 boards 30 cm long and fasten them on the ridge beam clearly according to the step markings. Fastening is carried out using self-tapping screws and corners, the board should lie on top and perpendicular to the ridge. These segments will serve as a link for attaching two opposite rafters.

Similarly, we fasten 9 segments on each side to the Mauerlat, only the length of the board should be 20 cm, and it should be vertical, this knot will be used to attach the lower sides of the rafters.

Now you can proceed to the main procedures. On each upper segment (30 cm) it is necessary to draw an average vertical line, it will play the role of a guide, where two boards cut at an angle are joined. Installation of rafters begins with the fact that the first board is aligned in the center from above and grabbed onto a nail to a 30-centimeter segment. Then, on the other side, a second board is nailed. It is necessary to ensure that the boards are at the same horizontal level, for this it is necessary to undermine the board that is planted below and raise it to the level of the second board, fixing it on a nail to the connecting jumper. It is highly not recommended to make cuts in the ridge beam. From below, to level the level between the boards, the opposite procedure is performed, the board, which turns out to be slightly higher, is drowned in the Mauerlat, for this it is necessary to hollow out a small groove with a chisel.

After the boards are leveled, it is necessary to tighten the bottom of the rafter with two nails and make two bolted connections, one at the top, the other at the bottom, in places where the boards are baited on the nails. The bolted connection must be through three boards.

After that, we get an almost finished rafter, which needs to be strengthened in order to give it rigidity. We conditionally divide the length of the rafter into four parts, you can sketch out the markings with a pencil. At the junction of the first and second quarters, we fasten a 60-centimeter segment between the boards to tighten the rafter. We use nails as fasteners. We perform a similar procedure at the junction of the third and fourth quarters.

After the four rafters are mounted, we will form two extreme triangles, at the bases and at the top it is necessary to stretch the threads along the entire roof, which we will use as guides to adjust the level of all diagonally located elements.

After the side rafters, the central part is mounted, now you can knock out the support, which is located at the junction of the ridge beam, we no longer need it, at this stage the structure already has a sufficient margin of safety. Next, all the other rafters are placed, one section on each side in a checkerboard pattern, to evenly distribute the loads. At the top, at the joints of opposite rafters, it is necessary to further strengthen the connections, we use connecting plates and self-tapping screws for this.

When all the rafter sections are in place, it is necessary to cut off with a hand saw all the corners that go beyond the level of the rafters, in particular, these are the corners of the connecting boards on the timber and on the Mauerlat.

Installation of bows

A bow is a connecting board that is located approximately at the level of the middle line of the rafter triangle. It serves to reduce the load on the sides of the roof, thanks to the bows, the likelihood of roof deflection under the weight of precipitation and the likelihood of vibrations under wind loads are greatly reduced.

In our case, the height of the ridge beam is a little more than 4 meters, which means that the location of the bows can be done strictly in the center, so all loads are distributed evenly, plus the height of the attic ceiling will be relatively normal and there will be no obstacles for movement of a person with average height in it.

As in the case of rafters, the first bows are attached to the sides, after which two threads are pulled, they will help us maintain the level. After that, the central bow and all the others are attached. On the extreme rafter triangles, bows are not needed, this will spoil the appearance of the roof, and besides, there are very light loads, so from a technical point of view, this step is not required.

One side of the bow is brought into the middle of the rafter and mounted on a nail, the second side, after observing the horizontal level, is also baited on the nail, then we perform two bolted connections. It is very important to stick to the level at this stage, since the bow is not only a spacer, but also the basis of the ceiling of the attic or attic room.

In fact, this technology is very simple, no matter how complicated it may seem at first glance. Armed with a sheet of paper and a pencil, draw the roof in stages, as indicated in the article, then the whole puzzle will turn into one accessible and elementary picture.

Using a standard set of construction tools, two people can build a similar roof in 5-6 working days.

Evgeny Ilyenko, rmnt.ru

The ridge beam is the top beam to which the roof rafters are attached. Installing a ridge beam is considered a special skill in the work of builders: they must make a special calculation of the size of the room, attachment point, attic.

The ridge wooden beam and the rafters attached to it are designed to perform the following tasks in the construction of housing:

Create a stable structure of the truss system.
Evenly distribute the pressure force and area along the side perimeters.
Distribute the weight of the roof correctly on the gables.
Maintaining roof geometry that is longer than 4.5 m. This allows you to install rafters without using a template. If the dimensions of the roof are large, then a rafter beam (upper part) is placed on the ridge wooden beam, and the lower one is attached to the Mauerlat.

An important condition for the installation of a ridge beam is the calculation of the correct section of such a support, which will make it possible to make a stable structure.

Let's figure out how to calculate and fix the timber. The cross section of the run is calculated very simply: all load data from the horizontal projection of the roof are added up. The dimensions of the ridge beam depend on 2 main parameters:

Bar runs.
Building dimensions.

The calculation of the parameters of the timber provides that for large buildings a powerful, heavy and rather weighty run is needed. But it should be borne in mind that such dimensions of the ridge beam will require the use of a crane. The average length of a regular beam is approximately 6 m, so to make a larger run, you will need to look for a tree or a so-called glued beam.

The fixed ends of the ridge, pre-treated with an antiseptic, rest against the wall into which they are embedded. Additional processing is carried out with roofing felt and roofing felt, which perfectly protects the wood from decay. A solid wood beam is installed differently:

The end is bent at an angle of 60°.
The ends are left open so that the ends do not come into contact with the walls.

As a result, when building a house, 2 tasks are solved at once. First, the end area becomes larger. Secondly, the processes of moisture exchange are normalized.

Then they calculate the dimensions of the ridge beam, which must be installed in the wall and pass through it, contact with the wall must be taken into account. Therefore, the end of the run must be well treated with an antiseptic and wrapped with rolled material. A similar design is used to make an unloading console.

With a properly selected section for a solid wood beam, it must be taken into account that the beam in the ridge can bend under its own weight at any time. Experienced builders recommend installing a construction truss so that the fixed ridge wooden beam does not break.

Calculation of the section of the ridge beam

The calculation of the cross section requires taking into account the following parameters, according to which the required size will be calculated:

deflection data;
fracture strength.

To determine the cross section, it is necessary to apply special formulas in which each indicator is important. A separate calculation determines such data as:

Internal stress (Σ = M:W).
Run deflection (according to the formula f \u003d 5qL³L: 384EJ).
The dimensions of the beam section are determined by the formula h = √¯(6W:b).

The data for each formula is shown below:

Σ = M:W (definition of internal stress), where Σ is the value to be found. M is the limiting bending moment, which is calculated in kg / m. W is the resistance to deflection of the established section.

The calculation of the deflection of the run is carried out using other data that must be substituted into the formula f = 5qL³L:384EJ. The letter J means the moment of inertia, to obtain which you need to know the dimensions of the section of the run (height and width, denoted by the letters h and b). Then the indicator h must be cubed and multiplied by b. The resulting value is divided by 12. The parameter E is the elasticity of the modulus, which is taken into account and is individual for each type of wood.

The bending moment must be calculated by the formula h \u003d √¯ (6W: b), where b is the width of the beam in centimeters, W is the bending resistance of the run. W can be obtained by dividing M (the largest bending moment) by 130.

The width and height values that are obtained after the calculation must be rounded up. If the builder is afraid to make a mistake, you need to turn to specialists who will calculate the parameters, determine what the fixed beam and run should be.

Installation of the ridge beam

Consider how to fix the ridge bars. They are made only from high-quality lumber, which is associated with the importance of the structure, which must perform the functions of long-term and reliable operation, bear the load, and be safe for the residents of the building. It is important that the run does not increase the weight of the roof, otherwise the strength of the structure will be in question. The rafters should serve for a long time, performing the assigned functions. For this purpose, pine lumber is often used for ridge timber, the section of which is 20x20 cm.

The fastening of the rafters to the ridge beam is selected depending on the type of building: residential or commercial. Depending on this, the material of the ridge, its cross section and dimensions will be selected. For example, well-dried larch is usually used for a bath, which is characterized by a heavier weight and resistance to stress. Larch also copes well with steam, retains heat and holds tiles. Residential buildings are built of pine, since it is customary to cover the roof with the so-called flexible tiles.

Larch for the manufacture of timber is used if the house will be covered with heavy tiles, which require a strong and strong building frame structure. It is important that the rafters hold not only the roof itself, but also do not become extra weight for the walls. They should perfectly hold the runs, not bend under them.

In order to make the rafters a central support, you need to install a beam. Its ends will rest against parallel load-bearing walls. Correct installation of such a design requires the calculation of data such as:

The average annual amount of precipitation that falls in a particular area.
Are there strong winds in the region or not.
Design width of the house.

The ridge beam allows you to avoid such processes in building a house as hammering nails, drilling with a drill. As a result, it is possible to avoid the formation of cracks, maintain the integrity of the beam and ensure the reliability of the entire rafter system.

A gable roof also requires the use of a ridge run, which subsequently acts as a roof ridge. In order to build a residential building measuring 6x6 m, it is recommended to take a run made of logs or solid timber. The run will rest on 2 gables, and no supports will be needed. If the length of the house is more than 6 m, then it is allowed to use construction trusses and a composite ridge run. It is important that the beam lies on the outer gables.

The fastening of the ridge beam is carried out by different methods, which allows you to connect the bars in the right way. The main goal of each connection is to make the structure strong and reliable. Modern technologies make it possible to connect the bars to each other so as not to use any additional materials for insulation. If the project documentation is drawn up correctly, then the house will not only be strong, able to hold the roof, but will also become environmentally friendly and reliable for housing.

Tree selection

When buying, check that all boards are even and without rot, the strength and durability of the structure depends on this.

Thanks to this, we will create an air space between each piece of lumber, they will be ventilated in the right conditions, which will allow us to avoid rotting and moisture accumulation.

We put the ridge beam

Elongated beam

Mauerlat

Don't forget to put the Mauerlat in short lengths behind the gables, where you have the roof peaks.

Device and installation of rafters

9 m / 1.3 m = 6.92(round up) \u003d 7 - the number of rafters;
9 m / 7 = 1.28 m- a step between the rafters.

We multiply the number of rafters by two and again by two, thanks to these calculations we will get the total number of boards that will need to be used to make the structure.

Similarly, we fasten 9 segments on each side to the Mauerlat, only the length of the board should be 20 cm, and it should be vertical, this knot will be used to attach the lower sides of the rafters.

Installation of bows

Using a standard set of construction tools, two people can build a similar roof in 5-6 working days.

Evgeny Ilyenko, rmnt.ru

Building a house from foundation to top is an amazing event! Especially if you do some of the work with your own hands, live and breathe the future nest. And you know that no matter how much fatigue accumulates before the finishing work, everything still needs to be done competently and thoroughly. Especially when it comes to the roof, where any mistakes are fraught with costly unpleasant repairs. And therefore, in order for the "umbrella" of your dream house to serve properly, correctly perform all structural components, especially splicing the rafters in the ridge area - this is the highest point! And we will help you deal with the types of connections and important technological nuances.

Useful video instruction:

So, first, let's understand a little about the concepts.

So, a run is an additional beam that is placed parallel to the roof ridge and the Mauerlat. In simple terms, this is the same Mauerlat, only raised in level. And in the end, the ridge should be located at a certain distance from the run - depending on which corner of the roof was chosen.

A ridge is a horizontal roof element that connects both roof slopes at the top.

And the main task of the connecting elements in the ridge is to create reliable rigidity and strength of the entire roof structure. What will be discussed now.

Types of splicing rafters in the ridge

There are three ways to do this in total:

Method number 1. overlap

This method differs from all the previous ones in that here the rafters are connected by side planes and pulled together with a pin or bolt. Quite popular technology today.

If the house is wooden, then the upper log or beam will be suitable as a support for this method, but you will have to put a Mauerlat on the blocks.

The most popular such fastening is the splicing of half-tree rafters:

Overlapping rafters in the ridge are most often connected with nails. Usually these are the roofs of gazebos, sheds, bathhouses and garages - there are no special requirements for the strength of the truss system.

Method number 2. Butt joint

For this you need:

Trim the edge of the rafter at an angle so that this angle is equal to the angle of the roof slope.
Make an emphasis on the rafter.
Apply fastener.

It is much easier to make such cuts according to the template - just make it in advance. So all the planes will fit each other tightly.

If you fasten the rafters with nails, take at least two of them. Hammer each of the nails into the upper cavity of the rafters at an angle so that the nail goes into the cut of the second rafter to be joined. Additionally, strengthen the splicing of the rafters in the ridge with a metal plate or a wooden lining.

Or partially back-to-back:

The essence of this design is that the edges of the two rafters fit so precisely that they evenly distribute the load placed on them with each other. But it will not be enough to fix this connection with one nail - metal or wooden nozzles are also needed. Take a board 30 mm thick, fix it on one (preferably two) sides of the knot and nail it.

Method number 3. Beam connection

In this method, we will mount the rafters directly on the ridge beam. This design is good in that the beam can be provided with central supports, and each rafter can be attached separately and at a convenient time. This method is indispensable if there is no time to make a template

The connection to the ridge beam is recommended in cases where the roof is wide enough - wider than 4.5 meters. This design is quite reliable, but sometimes it requires the installation of additional supports under it, which is why the functionality of the attic is reduced significantly. After all, there are now beams in the middle of the room! For small attic roofs, this, of course, does not matter, but in the attic you will have to beat it as an element of the interior. But no template is needed for this design, and small discrepancies are not terrible.

Variation:

You can, of course, use a metal fixing plate - but this is only a connection, not a tightening. The essence of the tightening is precisely that it is located below and takes on part of the load.

This is a combined splicing of rafters, because. it is performed end-to-end, in the same way as when focusing on the Mauerlat.

What to splice? Choice of fasteners

The rafter legs form the contour of the roof and transfer the point load from the roof to the mauerlat, and the mauerlat, in turn, evenly distributes it to the load-bearing walls.

Since ancient times, the following elements have been used to fasten rafters:

Overlays.
Bars.
Wooden pins.
Wedges.
Nagel.
Metal staples.

But the modern market offers more functional fasteners that allow you to splice the rafters in the ridge area much easier and more reliably. At any angle, the desired rigidity and strength are obtained. It:

Nail and perforated plates.
Self-tapping screws.
Bolts and screws.
And much more.

But the choice of one or another fastener no longer depends on how much it costs and how strong it will be, but on what the load on a particular ridge assembly is and what it requires.

So, here is how, for example, the rafters in the ridge are spliced with self-tapping screws:

And like this with nail and perforated plates:

But in order to apply these plates, you will have to work with the press:

And now - from simple to complex.

Splicing rafters in the ridge of a gable roof

When leaning on the ridge run of a gable roof, the rafter legs can either rest against each other with their beveled ends, or be apart.

If the rafters rest against each other, in other words, end-to-end, then their ends must be connected with overlays on nails or bolts.
If the ends of the rafter legs in the ridge knot are apart, then they are connected with angle brackets and bolts.
If the rafter legs rest on two runs at once, then the ends of the legs rest on each other. Naturally, a certain thrust arises, the tension of which is relieved with the help of horizontal crossbars.
If there is no run at all, then the junction of the rafter legs in the ridge knot is performed by focusing the beveled ends of the legs against each other. Additionally, such joints should be fixed with paired overlays, which are nailed to the legs or connected with bolts.
To fix the rafter leg with the crossbar, the joint is performed using wooden linings - side ones. They are nailed directly to the crossbar with nails or bolted - it all depends on the cross sections of the materials used. Next, a block is already placed under the crossbar - for the perception of transverse forces.
But the rafter legs made of logs with a crossbar are already attached without overlays. Only at the end of the crossbar itself is a recess made in ½ of the cross section of the sprengel. In order for the system to eventually turn out to be stable, in the transverse direction the rafter legs are reinforced with struts and crossbars-puffs. Especially when it comes to the width of the span between the outer load-bearing walls of 8 meters.
If strong winds are not uncommon in the area, it is extremely important to protect the roofing ridge from possible displacement. And for this purpose, the ends of the rafter layered legs are additionally connected to the ridge run with angle brackets. Plus, be sure to fix the rafter legs and masonry at home with wire.
If you are splicing a truss system from logs, roundwood in a ridge, then expect it to be quite heavy.

Note that with significant loads on the rafter system, it is generally not recommended to do a tie-in in the rafter leg - only use intermediate scarves.

Here are more details:

If the truss schemes are tilted, external loads are transferred by supports (Mauerlat, girders, uprights, struts and beds), while compressive and bending stress forces arise in the rods themselves. And the steeper the slope roof, i.e. the more vertically the rods are tilted, the bending is already less, but the horizontal loads, on the contrary, only increase.

Simply put, the steeper the roof, the more durable all horizontal structures should be, and the more sloped the slope, the stronger the vertical structures of the rafter system should be.

Splicing rafters in the hip roof ridge

The splicing of rafters at a hip roof occurs in a completely different scenario than that of a gable roof. So, there are already new elements here - slanted rafters that need to be installed according to a certain technology. And to the ridge beam, these parts must be fastened by cutting with additional fixation with upper ties and crossbars. The fact that at the hip roof the sloped slopes contain dormer windows and ventilation openings, which are often located directly under the ridge, adds to its difficulties.

If there is only one run in the hip roof, its diagonal rafter leg is supported on the run console. The consoles themselves need to be released 10-15 cm beyond the rafter frame. And do it in such a way as to saw off the excess, and not build up the missing.

If there are two runs, then in the ridge directly to the rafters you need to sew a short board, up to 5 cm thick - a surf. On it we will just rely on the rafters and diagonal rafter legs.

Now let's deal with the outer valley. The rafter legs that rest on it are also called oblique and diagonal. Moreover, the diagonal rafters are longer than the usual ones, and shortened rafters from the slopes, the sprigs, rely on them. In another way, they are also called rafter semi-legs. In this case, the rafters already carry a load that is one and a half times more than that of conventional rafters.

Such diagonal rafters themselves are longer than ordinary boards, and therefore they should be made paired. This solves three problems at once:

The double cross-section bears the double load.
The beam is long and not cut.
The dimensions of the parts used become unified.
For the installation of rafters, you can use the same boards as for ordinary ones.

Summing up and speaking in simple terms, the use of boards of the same height for a ridge knot greatly forgives all the design solutions of a hip roof.

We go further. To ensure multi-span, one or two supports should be installed under the sloping legs. After all, the rafters are inherently a ridge run that has bent and forked, its kind of continuation. Therefore, these boards need to be spliced along the length so that all joints are at a distance of 15 m from the center of the support. And select the length of the rafter leg depending on the length of the spans and how many supports.

Technically, this node runs like this:

A couple of technical points:

If you are making a rafter attachment point in the hip roof ridge directly above the dormer window, then the support of the diagonal rafter legs should fall on the side struts and the crossbar.
If the rafter legs with a hip roof are spliced directly above the ventilation outlet, then there is no need to make a central emphasis on the struts.
At the hip roof, be sure to make sure that the mating surfaces in the ridge nodes adjoin tightly, almost perfectly. And therefore, it is much easier to make the desired configuration of all ridge elements still on the ground, and only then mount each rafter leg separately on the roof.

Here is a visual tutorial:

Splicing rafters in the ridge of an arched roof

The arched roof has almost the same technologies as the gable roof, except that the angle of the rafter connection is slightly different:

Splicing rafters in the ridge of a round roof

And here is how they get out of the situation during the construction of unusual roofs of the same unusual buildings:

Hi all!
for sure there is some kind of typical (tested) solution for a long ridge run.

There is a house made of aerated concrete under a gable roof. Distance from wall to wall between slopes 8m, between gables 10m. The slope of the roof is ~41grd, the length of the rafters is ~5.5m, the height in the ridge is almost 5m.

There are no internal walls, pillars and props do not want to do. The task is to get a studio.

The question is - how and from what to build such a long ridge run?

So far, from what I have dug up, three solutions have been drawn:

one). make a run from an I-beam 35-40

2) generally do without a run, connect the rafters with puffs at a height of 3 m from the floor, put diagonal boards in the roof plane, thus tying the rafters and eliminating longitudinal movements.

3). run in the form of a truss from pipes 50-70mm

Building rafters in length: paired and composite rafters

For large houses, it is not uncommon to splice the rafters when creating the frame, since the maximum length of the rafters is 6 meters. The larger the section of the product, the greater the length. To achieve the optimal ratio of the thickness and length of the rafter legs, they resort to increasing the thickness of the rafters by connecting them with additional elements (timber, boards).

The choice of rafters is of no small importance. Only high-quality materials will help create a reliable truss system, and the roof will last a long time. Therefore, before choosing, it would be useful to study GOST rafters.

How to increase the length of the rafters

When starting to build a roof, many are interested in how to lengthen the rafters. For this, short structural elements are usually connected to each other: rafter boards. bars and so on - this is shown in the photo. It is rare to achieve bending rigidity in places where the rafters join - usually there are lamellar hinges there. In order to solve this problem, the joint is made where the bending possibilities approach zero.

When using a plate hinge, the distance from it to the support for the rafters is considered as 15% of the span length (rafter installation step), where the connection is located. Since the distance of spans between the intermediate support and the Mauerlat, the ridge and intermediate supports are different, when joining the rafters, an equal, and not an equal-bending scheme, which is used when joining the runs, is used. As for how to join the rafters, it is important to ensure the same strength, and not create an equal deflection. But in the ridge run, the main thing is to ensure an equal deflection so that the roof ridge remains at the same height.

When constructing hip roofs, rafters are used, directed to the inner or outer corners of the walls. In this case, the rafter legs are called rafters. They turn out to be longer than usual, and become a support for short slope rafters.

The rafter system is usually assembled from various wooden elements, such as rafters, timber, boards, logs. Bent rafters allow you to build a roof of an unusual shape: for example, rounded.

Ways to splice rafters:

butt joint;
oblique cut;
overlap connection.

When connecting end-to-end, in order for everything to be securely fixed, both rafters cut off the joined ends at a right angle. So that the junction of the rafters is not subjected to deflection, the cut of the end of each element must be made at an angle of exactly ninety degrees. The cut ends of the rafters are connected with a metal fastener or an overlay from the board and fixed. In order to cover the junction of the rafters on both sides, board linings are used, for fixing which they take metal nails for the rafter system. Nail them in a checkerboard pattern, through one.

If the bevel cutting method is used, the ends of the rafters in contact are cut at an angle of 45 degrees. Then the ends of the rafters are connected together, and fastened in the middle with a bolt having a diameter of 12 or 14 millimeters.

As for how to build up the overlapping rafters, the wooden elements overlap each other with an overlap of a meter or more; it is not necessary to observe the accuracy of the cut of the rafters. Then, as in the case of joining the rafters end-to-end, the nails are nailed over the entire area of contact of the spliced elements in a checkerboard pattern.

Instead of nails, you can also use studs, fixed on both sides with washers and nuts. The connection of the elements of the truss system should take place in such a way that the minimum load falls on the junction. To connect the rafters to the Mauerlat, rafter brackets are used.

Pairing rafters

Conjugation is the connection of parts in which they fully or partially enter into each other. The rafters are connected to the mauerlat or beams by tie-in or with a spiked tooth, creating knots.

The upper part of the rafter leg is laid on a ridge run with partial or complete connection with another rafter leg. A simple truss system assembled from boards is no less durable than one made using wooden beams and poles. The boards are built up or connected in a certain order, and in some cases their use is more beneficial than a heavy beam, both in terms of versatility and economy.

You can give examples of rafter systems from a board, such as a roof structure with an attic, which can be insulated and converted into an attic. To increase the length of the legs, rafters are sometimes used, connected using two boards with a clearance. The peculiarity of this design is that in the lower part of the system it is enough to attach single rafters, and in the upper part - paired elements.

In this way, you can save on building material, and the assembly of rafters with each other and with the crossbar is easier. Inserts made from leg trimmings are laid between the rafters so that the distance between them is no more than seven heights of the connected boards. In this case, the flexibility of the rafter paired between the liners is zero, and it can work as a solid element. In this case, the length of the liners should be two board heights or more (read also: “What is the distance between the rafters, the calculation method“).

Rafters from boards are of two types: composite and paired.

Twin rafters

Paired rafters are made up of at least two boards, which are applied close to each other with a wide side, leaving no gaps, and stitched along the entire length with nails in a checkerboard pattern, through one.

The extension of the rafters from paired boards occurs by the simultaneous connection of the parts butt and overlap to the second rafter board, due to which not only the length of the element increases, but also its strength. When choosing rafters, it is necessary to pay attention to the fact that the distance between the joints of the combined boards of the connection is more than one meter and is staggered along the product. Hinged joints should not be opposite each other, and each joint should be protected by a solid board.

Rafter rafters are the longest elements of truss systems, and the best material for their creation is a paired rafter board.

How to splice a bar along the length, look at the video:

Composite rafters

Elements such as composite rafters are never used as diagonal elements. To create them, two boards of the same length are laid on an edge and interconnected with an insert (third board). Then three boards are nailed in two rows. The length of the liner must exceed two board heights.

The installation step of the rafters between the liners should be less than the thickness of the connected boards, multiplied by the number seven. The first liner should be at the beginning of the rafters - in this case, the rafter leg will be equal to the thickness of three boards.

The upper part of the rafter is made from one board, it is attached between the side boards with nails like an insert and mounted on a ridge beam.

There is nothing difficult in how to cut the rafters. There are several ways to increase the length of the rafters. The main thing is to do everything right, taking into account the slightest nuances, so that the roof turns out to be strong and reliable, and the rafter structure does not need to be repaired for many years.

How to splice rafters along the length: analysis of options and technological rules

Often, during the construction of frames for roofs of complex configuration, there is a need to use elements of non-standard size. Typical examples include hip and half-hip structures, the diagonal ribs of which are significantly longer than ordinary rafter legs. Similar situations arise in the construction of systems with valleys. So that the created connections do not become the cause of the weakening of the structures, you need to know how the rafters are spliced along the length, in what way their strength is ensured.

Splicing the rafter legs allows you to unify the lumber purchased for the construction of the roof. Knowledge of the intricacies of the process makes it possible to almost completely build a truss frame from a bar or board of one section. The construction of a system of materials of the same size has a positive effect on the total cost.

In addition, a board and a bar of increased length, as a rule, are produced with a section larger than that of a material of standard sizes. Along with the cross section, the cost also increases. Such a margin of safety when constructing hip and valley ribs is most often not needed. But with proper splicing of the rafters, the elements of the system are provided with sufficient rigidity and reliability at the lowest cost.

Without knowledge of the technological nuances, it is quite difficult to make joints of lumber that are really rigid in bending. Rafter junctions belong to the category of plastic hinges that have only one degree of freedom - the ability to rotate in the connecting node when a vertical and compressive load is applied along the length.

In order to ensure uniform rigidity when a bending force is applied along the entire length of the element, the pairing of the two parts of the rafter leg is located in places with the smallest bending moment. On diagrams showing the magnitude of the bending moment, they are clearly visible. These are the points of intersection of the curve with the longitudinal axis of the rafter, at which the bending moment approaches zero values.

We take into account that during the construction of the truss frame, it is required to ensure that bending resistance is equal over the entire length of the element, and not the same opportunities to bend. Therefore, the junction points are arranged next to the supports.

As a support, both an intermediate rack installed in the span, and directly a Mauerlat or a trussed truss are taken. The ridge run can also be assessed as a possible support, but it is better to place the rafter legs joining areas lower along the slope, i.e. where the minimum load is placed on the system.

In addition to accurately determining the location for pairing the two parts of the system element, you need to know how the rafters are properly extended. The method of forming the connection depends on the lumber chosen for the construction:

Bars or log. They are built up with an oblique cut formed in the connection zone. For reinforcement and to prevent rotation, the edges of both parts of the rafters cut at an angle are fastened with a bolt.
Boards sewn in pairs. They are spliced with the location of the docking lines apart. The connection of two parts superimposed on each other is made with nails.
Single board. The priority is splicing with a frontal stop - by joining the trimmed parts of the rafter leg with the imposition of one or a pair of wooden or metal linings. Less commonly, due to the insufficient thickness of the material, an oblique cut is used with fastening with metal clamps or a traditional nail fight.

Let us consider in detail these methods in order to understand in depth the process of increasing the length of the rafters.

The method involves the formation of two inclined cuts or cuts, arranged from the side of the pairing of parts of the rafter leg. The cutting planes to be joined must be perfectly aligned without the slightest gaps, regardless of their size. In the connection area, the possibility of deformation must be excluded.

It is forbidden to fill cracks and leaks with wedges made of wood, plywood or metal plates. It will not work to fit and correct flaws. It is better to accurately measure and draw cutting lines in advance, according to the following standards:

The depth is determined by the formula 0.15 × h, where h is the height of the beam. This is the size of the area perpendicular to the longitudinal axis of the beam.
The interval within which the inclined sections of the cut are located is determined by the formula 2 × h.

The location for the docking area is found according to the formula 0.15 × L, which is valid for all types of truss frames, in which the value of L displays the size of the span covered by the rafters. The distance is measured from the center of the support.

Details from a bar, when making an oblique cut, are additionally fastened with a bolt passing through the center of the connection. The hole for its installation is drilled in advance, its Ø is equal to the Ø of the fastener rod. To prevent the wood from being crushed at the place where the fastener is installed, wide metal washers are placed under the nuts.

If a board is connected using an oblique cut, then additional fixation is carried out using clamps or nails.

In the case of using rallying technology, the center of the connected section is located directly above the support. The joining lines of the trimmed boards are located on both sides of the center of the support at a calculated distance equal to 0.21 × L, where L denotes the length of the overlapped span. Fixation is carried out with nails installed in a checkerboard pattern.

Backlash and gaps are also unacceptable, but they are easier to avoid by carefully trimming the board. This method is much simpler than the previous method in execution, but in order not to waste hardware and not to weaken the wood with extra holes, the number of points of fasteners to be installed should be calculated with accuracy.

Nails with a stem section up to 6 mm are installed without pre-drilling the corresponding holes. For fasteners larger than the specified size, it is necessary to drill so that when connecting, the board does not split along the fibers. An exception is cross-section hardware, which, regardless of size, can simply be hammered into wooden parts.

To ensure sufficient strength in the rally zone, the following conditions must be observed:

Fasteners are placed every 50 cm along both edges of the boards to be joined.
Nails are placed along the end joints in increments of 15 × d, where d is the diameter of the nail.
For rallying the board at the interface, smooth round, screw and threaded nails are suitable. However, threaded and screw versions are preferred because they have much higher pull-out strength.

Note that the connection of rafters by rallying is acceptable in the case of an element of two sewn boards. As a result, both joints are overlapped with a solid piece of lumber. The advantages of the method include the size of the overlapped span, which is impressive for private construction. Similarly, rafter legs can be increased if the distance from the top to the bottom support reaches 6.5 m.

The method of frontal extension of the rafters consists in the butt joining of the connected parts of the rafter leg with the fixation of the section with nails, dowels or bolts through the lining installed on both side planes.

To exclude backlash and deformation of the extended rafter leg, the following rules must be followed:

The edges of the board to be joined must be perfectly trimmed. Gaps of any size along the connection line must be excluded.
The length of the overlays is determined by the formula l = 3 × h, i.e. they must be no less than three board widths. Usually the length is calculated and selected based on the number of nails, the formula is given to determine the minimum length.
Overlays are made of material, the thickness of which is not less than 1/3 of the same size of the main board.

Nails are hammered into the overlays in two parallel rows with a checkerboard "scatter" of fixing points. In order not to damage the overlay, which is thin in relation to the main lumber, the number of attachment points is calculated based on the resistance of the nails to the transverse force acting on the legs of the hardware.

When the junction of the rafter parts is located directly above the support, there is no need for nailing calculations to fix the overlays. True, in this case, the docked leg will work as two separate beams for both deflection and compression, i.e. according to the normal scheme, it will be necessary to calculate the bearing capacity for each of the constituent parts.

If steel rod bolts or rods without thread, dowels are used as fasteners, when joining a thick board or beam, then the threat of deformation will be completely eliminated. In fact, even some gaps in the joining of the ends can be ignored, although such flaws are still best avoided.

When using screws or screws, holes are pre-drilled for their installation, the Ø of the holes is 2-3 mm less than the same size of the fastener leg.

In the production of frontal joints of rafters, it is necessary to strictly observe the estimated installation step, the number and diameter of fasteners. If the distance between fixing points is shortened, splitting of the wood may occur. If the holes for the fasteners are larger than the prescribed dimensions, the rafter will be deformed, and if less, the lumber will crack during the installation of the fasteners.

To connect and increase the length of the rafters, there is another very interesting way: building with two boards. They are sewn to the side planes of the single element being extended. Between the extended parts there is a gap equal to the width of the top board.

The clearance is filled with trimmings of equal thickness, set at intervals of not more than 7 × h, where h is the thickness of the board being extended. The length of the distance bars inserted into the clearance is at least 2 × h.

Extension using two stackable boards is suitable for the following situations:

The device of a layered system along two side runs, which serve as a support for the location of the docking area of the main board with the attached elements.
Installation of a diagonal rafter that defines an inclined edge of hip and half-hip structures.
Construction of sloping roofs. As a support for the connection, the strapping of the lower tier of rafters is used.

Calculation of fasteners, fixation of remote bars and connection of boards is carried out by analogy with the methods described above. For the manufacture of remote bars, trimming of the main lumber is suitable. As a result of the installation of these liners, the strength of the prefabricated rafters significantly increases. Despite significant material savings, it works like a solid beam.

Demonstration of the basic techniques for splicing the structural elements of the truss system:

A video with a step-by-step process for connecting rafter parts:

Video example of one of the ways to connect lumber:

Compliance with technological requirements, according to which the rafters are spliced along the length, guarantees trouble-free operation of the structure. Elongation methods allow you to reduce the cost of building roofs. You should not forget about preliminary calculations and preparation for making connections so that the result of the effort becomes ideal.

Rules for building rafters

Designers-designers, when drafting a house, necessarily carry out calculations of the expected loads on the truss system and determine which section and length of the rafters are necessary for a given roof.

The rafters are lengthened by cutting, followed by fixation with staples, nails, bolts, etc.

Rafters of non-standard sizes are often required, for example, for the construction of a hip roof, diagonal rafters of 9 meters are needed - this is much longer than standard sizes. And the point is not at all that trees do not grow over 6 m, as experienced installers of truss systems joke. You can try and get ready-made rafters of the right size, but it will be very expensive (manufacturing, delivery), which is completely impractical. Therefore, roofers use different ways to lengthen the rafter leg. How to build rafters yourself? Building rafters is a responsible business. Incorrectly executed connection nodes will disrupt the entire truss structure.

The cross section of the rafter directly depends on its length. If the length is increased by splicing, then the width should also be larger. It is necessary to achieve the correct ratio of all dimensional parameters, only then can the reliability of the truss structure be guaranteed.

Butt joint or front stop

In order to avoid critical deflection at the junction in the future, you need to follow a simple rule: make a connecting cut of the bars strictly at an angle of 90º. A tight and precise fit of the rafters in the frontal stop creates the prerequisites for a strong connection node. It remains to fix it with wooden plates with a section of 50 mm with a nail fight or studs located on one or both sides of the joint - this depends on the required power of the structure.

Ways of connecting rafters.

The fastening elements are driven in with a checkerboard arrangement. Their distribution is not accidental - an additional reinforcement is being created. The length of the wooden lining (at least 50 cm) is calculated according to the required number of nails. The number of fastening elements is determined by the holding factor of the transverse force directed to the shearing of the studs or nails (the bearing capacity of each nail is calculated).

Plank overlays can be replaced with newfangled nail (toothed) plates made of 3 mm steel. The teeth of the metal fasteners securely connect the rafters. When using metal elements in the truss system, do not forget that the metal quickly corrodes, which is why the entire wooden structure rots. It is easy to avoid negative consequences if the beams and rafters at the points of contact with the metal are treated with bituminous mastic, and the steel itself is painted with anti-corrosion paint. You can protect the tree from contact with metal in the old fashioned way - use pieces of roofing material as a cushioning material.

When assembling the roof and its rafter system, modern roofers use not only wooden, but also metal elements. Of the wooden fasteners, the most common are:

overlays for the formation of a wooden spike;
bars;
plates;
triangles;
pins.

Metal fasteners:

studs, bolts, nails;
steel corners;
taverns, crossbars, collars, brackets;
sliders (device for rafters);
nail or toothed plates;
perforated plates.

Rafter lap connection

Types of splicing and building rafters.

When the rafters are built up, a plastic hinge is inevitably obtained at the junction. It is extremely difficult to make a joint that is hard to bend. In order to still achieve the greatest rigidity of the structure, plastic hinges are located in places where the bending factor tends to zero. The connecting nodes must rest on the longitudinal axis of the rafter.

The plastic hinge is placed at a certain distance from the support - 0.15L. L is taken as the length of the span over which the docking joint is located. When splicing the rafters, an equal-strength scheme is used - this is due to the different distance from the ridge run to the intermediate support beam and from the support beam to the Mauerlat. After all, it is very important to ensure the strength of the entire length of the rafter leg.

When building rafter legs with an overlap, the wooden elements overlap each other. The overlap must be at least one meter. The entire area of contact of two wooden planes is nailed with a checkerboard arrangement. Instead of nails, you can use studs tightened on both sides with nuts. This splicing method does not require precision cuts of the ends of the rafters.

Rafter connection with an oblique cut

Methods for splicing wooden elements: 1 - half a tree; 2 - oblique cut; 3 - direct overhead lock.

The method of splicing with an oblique cut in half a tree is most often performed when the rafters are made from timber. Some difficulties in such a connection - to make an even cut at an angle of 45º. To achieve high-quality docking, two rafters should be cut at the same time. If, after cutting, there is still a gap or unevenness in the cuts, these shortcomings can be eliminated with a planer or angle grinder (popularly a grinder) and an emery cloth. When the bars are tightly joined (without gaps) into an even and beautiful connection, they are pulled together with two 14 mm bolts or studs. If splicing with an oblique cut is performed on a bend and rafters with a section of 100x200 mm are used, then two wooden linings with a nail fight are added to the above connection.

Splicing rafters from boards

The rafter system from boards has no less strength than from heavier lumber. Boards connected in a special way, in some cases, have advantages over heavy timber or poles, both for economic reasons and for versatility. Most often, boards are used in a rafter system for a roof with a cold attic, when it is not necessary to do roof insulation.

Composite plank rafters

Methods for building up wooden elements: 1 - end-to-end with a hidden spike and a through comb; 2 - half a tree on bolts; 3 - end-to-end with overlays on bolts; 4, 5 - half a tree with fastening with strip steel and clamps; 6 - oblique cut on collars.

The uniqueness of such an assembly lies in its structural simplicity, saving lumber and reliability. When assembling plank rafters, all joints are made on nails. In the upper part of the rafter system, where no large loads are expected, the rafters can be installed in one board, and the lower part can be made composite. Such an assembly system allows you to significantly save material, select the optimal cross-sectional size and easily resolve the design issue of connecting rafter assemblies both among themselves and with a grasping crossbar.

Composite rafters are assembled from two boards of the same length. Between the boards installed on the edge, insert liners (truss cuts) so that the clearance between them is no more than seven heights of the rafters being fused. In this case, the deflection between the liners is completely eliminated, and the rafter will work as a whole. The inserts are made of arbitrary length, but not less than 2 heights of the rafters being fused. The components are pierced with nails.

The first liner is placed at the beginning of the rafter to get the thickness of the rafter leg from 3 boards. The other end (upper) of the rafters can be made into one board. This board will be inserted between the side boards, like an insert, and stacked on the ridge run. Composite plank rafters cannot be used as layered (diagonal) rafters.

Rafters paired in two or three boards

Paired rafters are made up of several boards folded together with the wide side. The required number of boards - two or three - is determined from the required section of the rafters. Well-fitted boards (without gaps) are pierced with nails in a checkerboard pattern along the entire length.

The paired rafters are lengthened, simultaneously using extension techniques such as a frontal joint and an overlap (through one). In this case, the swivel joints will be located in a run-up (chess order), and each of their joints is reliably protected by a solid board. The distance between joints of adjacent boards should not be less than one meter. Only if this condition is met can the reliability of the structure be guaranteed.

This extension method allows you to get any length, whatever it may be. Plank beams made in this way are used in the construction of diagonal (layered) rafters.

A little about fasteners

For greater reliability, the docking nodes are additionally reinforced with bolts, metal corners, plates, brackets. The dimensions of the fasteners are determined based on the thickness of the rafters. Steel parts with existing holes are fixed with screws or self-tapping screws, the purchase of which is not worth saving. It is better to buy high-quality (factory) products of guaranteed strength, since overheated cheap self-tapping screws easily burst already when screwed in. It is worth remembering that nails have plasticity. If the nail bends and stretches, then the self-tapping screw immediately breaks under pressure. Today, ruffed nails are in great demand.

For bolts, holes are drilled in the connection details. The drill size is selected 1 mm less than the bolt section.

Which method of building rafters to choose depends on the loads and deformation that a particular truss structure will experience. For example, a half-tree oblique joint is used for compression joints, but not for tension and bending.

Sources:

In attic roofs, there is no need to use long and heavy purlins; shorter and lighter beams and boards can be used here.

The run is supported on racks. Racks are made of a wooden beam, which is supported by the lower end on a bed or a wooden lining, and they, in turn, are laid on brick posts. In buildings with prefabricated reinforced concrete floors, brick columns are a part and continuation of the internal load-bearing wall, but they can also be made directly on reinforced concrete floor slabs. The bed can also be laid without posts, directly on the inner wall or on the ceiling with horizontal alignment with wooden linings. The bottom of the bed is made at a height of no more than 400 mm from the top of the ceiling. Aligning the top of the bed to the horizon simplifies the installation of racks and girders. Racks sawn to the same height and installed on a horizontal beam automatically give the same height of the roof ridge. In all cases, under the bed: between it and the wall, between it and the brick columns or the ceiling, rolled waterproofing is laid.

Racks do not have to be placed directly under the rafters. Usually, the spacing of the rafters is from 60–80 cm to 1.2–1.5 m; it does not make sense to install racks that hold the run so often, so they are usually made along the length of the boards or timber going to make the run. The simplest under-rafter structure looks like a rectangular frame, consisting of an upper belt - a run, a lower belt - a lying position, a vertical filling - racks and several wind ties, which are made from a board 40–50 mm thick. For example, a sub-rafter structure 9 m long can be made from two bars 4.5 m long and three racks, joining the bars along the length on the middle rack. Or two beams and one rack, if it is possible to support the ends of the run on the walls of the gables. Such a run is called a split run, its parts are calculated for bending and deflection like ordinary single-span beams (Fig. 27). The beams of the run are joined on the supports with an oblique cut with a nail, screw or bolt connection or a longitudinal frontal stop. Both that, and another pairing, gives a hinged version of the connection of beams.

rice. 27. Variants of the device of sub-rafter structures with split runs

Racks are calculated as compressed elements according to the formula:

σ = N/F ≤ Rco, (4)

where σ is internal stress, kg/cm²; N - compression force directed along the axis of the rack, kg; F - cross-sectional area of the compressed element, for a rectangular rack F = b×a, cm²; Rszh - design resistance of wood to compression, kg / cm² (accepted according to the table SNiP II-25-80 "Wooden structures" or according to the table on the website page);

Increasing the number of racks reduces the size of the run section. Racks, even if their section will be accepted constructively, must be checked by compression calculation, and make sure that their number will be sufficient to hold the run. If, as a result of the calculation, too small cross-sectional dimensions of the racks are obtained, their cross-section is taken constructively, but not less than 10 × 10 cm. Such sections of the racks allow them to be accepted without calculating flexibility, since the flexibility of low racks is practically zero. If we take less than 10 × 10 cm, the section of the racks passing by the calculation for compressive strength, then they must also be checked by calculating the flexibility, the description of which is in SNiP II-25-80. Otherwise, a thin rack passing through compression will simply bend under load, and what good will it do for us from its sufficient bearing capacity? Timber racks of calculated or constructive section can be replaced with racks of boards knocked together closely or with wooden short stacks installed between the boards with a clearance of no more than 7h. Then the flexibility and strength of composite racks will be approximately equal to the same parameters of racks from a solid bar of the same section.

Split runs are easy to manufacture and install, but uneconomical. A more economical design is obtained if the girders are made cantilever, and single-span beams are inserted between them (Fig. 28). Such a run is called a cantilever beam (Gerber beam) and in fact remains the same split beam, in which cantilever and single-span beams are calculated separately. Single-span girders are placed between two cantilever purlins in such a way that at the junction the bending moment tends to zero (where the curve of the moment diagram crosses the horizontal axis of the purlin). These nodes splicing beams along the length are called plastic hinges. Splicing of runs is carried out with an oblique cut and tightening with a bolt with a diameter of 12–14 mm. The maximum length of overlapped spans is 5 m.

rice. 28. Cantilever-beam truss structure

There are two options for the device cantilever-beam run. With a distance from the support to the joint of 0.15L, a run is obtained with equal bending moments in all spans and on all supports, that is, the run is obtained in all sections with equal strength. If the emphasis is on the rigidity of the run, then it is made uniform. Plastic hinges (beam joints), in this case, are located at a distance of 0.21L from the support. In the end spans, single-span beams rest on one side on the console of the adjacent run, and on the other side on the gable wall or post.

In order not to disturb the harmony of the beam, it is necessary to make the end spans shorter than ordinary ones by about 20%, so the end span is set equal to L1 = 0.8L–0.85L. This statement is true for the actual length of the span, that is, the size of the "clearance", taking into account the depth of support of the run on the wall or rack, which is at least 10 cm.

There is another way to reduce the section of the runs: the device of a continuous run by rallying the boards (Fig. 29). In continuous runs from paired boards, plastic hinges are located apart, at a distance of 0.21L from the support. The run is obtained with equal deflections, but different bending moments. In a plastic hinge, each joint of two boards is covered with a solid board. Maximum flights for a continuous span of boards can reach 6.5 m, that is, the full length of the board according to the state standard.

rice. 29. Under-rafter construction with plank continuous runs

The run boards are sewn together along the length with nails staggered every 50 cm, and nails are placed at the joint according to the calculation. The calculation of the nail connection of the plastic hinge of the continuous run from the boards is done according to the formula:

n = Mop/2ХТгв,

where n is the required number of nails, pcs; Mop - bending moment on the support, kg×m; X - distance from the center of the support to the center of the nail field; Tgv - the bearing capacity of one nail in a single-cut connection.

The calculation of runs of any type is allowed to be carried out both for concentrated forces from the pressure of the rafters, and for a uniformly distributed load. Usually, the calculation for a uniformly distributed load is used, as it is faster and simpler. If girders with cantilever extensions beyond the wall will be installed on the racks (by analogy with Fig. 24.2), then the length of the consoles should be made equal to 0.21 or 0.15 spans (0.15L, 0.21L). Otherwise, the run must be recalculated taking into account the unloading action of the console. This calculation is quite complicated and should be done by specialists.

The cross section of the bed is taken constructively, most often, the same as the cross section of the run. For example, it can be a 10 × 15 cm beam if the bed rests only on brick posts. If the bed is laid on the ceiling or on the wall (all cases when many leveling wooden linings can be placed under it), the bed height can be reduced to 10 or even 5 cm. If the roof truss system is made without rafter legs (braces), from you can completely refuse, and constructively connect the bottoms of the racks by nailing contractions.

A prerequisite for the installation of layered rafters is to provide their upper part with a support. In shed roofs, this issue is solved simply: walls are built of different heights, Mauerlat beams are laid on them, on which rafters are in turn laid.

In a gable roof, you can do the same: build an inner wall to the required height and lay a Mauerlat on it. Then lay out the rafters on the low outer and high inner walls. However, this decision limits the layout options for the attic, which is increasingly being used as an attic. Yes, and for ordinary attic roofs, this option is not profitable, because. requires significant financial costs for the construction of a high internal capital wall. Therefore, in the attic, the inner wall is replaced with a horizontal beam mounted on supports or supported on opposing wall gables. A horizontal beam laid on the roof is called a run.

The name itself: run, says that this beam is “thrown” from wall to wall, although in fact, for example, in hip roofs it can be shorter. The simplest design solution for installing a ridge run is to lay a powerful beam on the gables of the walls without any additional supports (Figure 24.1).

rice. 24.1. An example of installing a ridge run, without additional supports, on the walls of the attic.

At the same time, in order to calculate the sections of the girders, the load acting on them must be collected from half of the horizontal projection of the roof area.

In buildings with large dimensions, the purlins are long and heavy, most likely they will have to be mounted with a crane. For the manufacture of a run, it is rather problematic to find an even beam made of solid wood with a length of more than 6 m, therefore it is better to use a glued beam or log for these purposes. In any case, the ends of the runs, walled up in the walls of the gables, must be treated with antiseptics and wrapped in a rolled waterproofing material. The ends of solid wood beams are beveled at an angle of approximately 60 ° and left open; in a niche, they should not rest against the wall material (Fig. 25). Beveling the end of the beam increases the end area and favors better moisture exchange of the entire beam. If the run passes through the wall, then at the place of support on the wall, it is also wrapped with waterproofing material. The beams are passed through the walls for architectural reasons, in order to provide an overhang of the roof over the gables, although this can also be achieved by moving the battens out of the wall. Runs passed through the wall form unloading consoles. The load pressing on the console tries to bend the run up, and the load acting on the span - down. Thus, the total deflection of the run in the middle of the span becomes smaller (Fig. 24.2).

Rice. 24. 2. Run with consoles.

If you use a log as a run, then it is not necessary to cut it into two edges, it is enough to cut it in the place where the rafters are supported and where the run is supported on the walls. It is not advisable to make long runs of solid wood, passing by the calculation for strength and deflection, they, however, can bend under their own weight. It is better to replace them with construction farms.

The cross section of the run is selected according to the calculation for the first and second limit states - for destruction and for deflection. A beam working in bending must meet the following conditions.

1. The internal stress that occurs in it when bending from the application of an external load should not exceed the design bending resistance of wood:

σ = M/W ≤ Rbend, (1)

where σ is internal stress, kg/cm²; M - maximum bending moment, kg×m (kg×100cm); W - moment of resistance of the section of the rafter leg to the bend W = bh² / 6, cm³; Rizg - design resistance of wood to bending, kg / cm² (accepted according to table SNiP II-25-80 "Wooden structures" or according to the table);

2. The value of the deflection of the beam should not exceed the normalized deflection:

f = 5qL³L/384EJ ≤ fnorm, (2)

where E is the modulus of elasticity of wood, for spruce and pine it is 100,000 kg / cm²; J - moment of inertia (measure of body inertia during bending), for a rectangular section equal to bh³ / 12 (b and h - width and height of the beam section), cm4; fnor - normalized deflection of the beam, for all roof elements (rafters, girders and lathing bars) it is L / 200 (1/200 of the length of the checked beam span L), see.

First, the bending moments M (kg×cm) are calculated. If several moments are shown on the design scheme, then all are calculated and the largest is selected. Further, by simple mathematical transformations of formula (1), which we omit, we obtain that the dimensions of the beam section can be found by setting one of its parameters. For example, arbitrarily setting the thickness of the beam from which the beam will be made, we find its height using the formula (3):

h = √¯(6W/b) , (3)

where b (cm) - beam section width; W (cm³) - the moment of resistance of the beam to bending, calculated by the formula: W \u003d M / Rbend (where M (kg × cm) is the maximum bending moment, and Rbend is the bending resistance of wood, for spruce and pine Rbend \u003d 130 kg / cm²) .

You can, and vice versa, arbitrarily set the height of the beam and find its width:

After that, the beam with the calculated width and height parameters according to the formula (2) is checked for deflection. Here you need to focus your attention: according to the bearing capacity, the rafter is calculated according to the highest stress, that is, according to the maximum bending moment, and the section that is located on the longest span, that is, in the area where the largest distance between supports is checked for deflection. Deflection for all: one-, two- and three-span beams is easiest to check using formula (2), that is, as for single-span beams. For two-span and three-span continuous beams, such a deflection test will give a slightly incorrect result (a little more than it actually is), but this will only increase the safety margin of the beam. For a more accurate calculation, you need to use the deflection formulas for the corresponding design scheme. For example, such a formula is shown in Figure 25. But we repeat once again that it is better to include a certain margin of safety in the calculation and calculate the deflection according to a simple formula (2) at a distance L equal to the largest span between the supports than to find a formula corresponding to the design loading scheme. And one more thing you need to pay attention to, according to the old SNiP 2.01.07-85, both calculations (for the bearing capacity and for the deflection) were carried out for the same load. The new SNiP 2.01.07-85 states that the snow load for calculating the deflection must be taken with a coefficient of 0.7.

rice. 25.1. An example of the location of purlins on a T-shaped roof

rice. 25.2. An example of the location of purlins on a T-shaped roof

rice. 26. Loads acting on the runs of a T-shaped roof.

If, after checking the beam for deflection, it will be no more than L / 200 in the longest section, then the section is left as it turned out. If the deflection is greater than the standard, we increase the height of the beam or bring additional supports under it, but the cross section must be recalculated according to the appropriate design scheme (taking into account the introduced supports).

If someone managed to read up to this point, then let's say that the most difficult thing in this calculation is not to get confused in units of measurement (in meters to centimeters), and everything else ... Multiply and divide several numbers on a calculator a lot of knowledge is not required.

In the end, only two digits will appear: required for a given load, which are rounded up to a whole number.

If a log is used instead of a beam (solid, glued or assembled at the MZP), then it should be taken into account that when bending, due to the preservation of the fibers, the load-bearing capacity of the log is higher than that of the beam and is 160 kg / cm². The moment of inertia and resistance of a circular section is determined by the formulas: J = 0.0491d³d; W = 0.0982d³, where d is the log diameter at the top, see. W \u003d 0.088d³, with a sloping width of d / 2.

The height of the girders and rafters, depending on the loads and the architectural solution of the roof, can be very diverse. In addition, the forces that press on the walls, especially the girders, reach large values, so the roof, like everything else, must be designed in advance, even before the construction of the house. For example, in the scheme of the house, you can enter an internal load-bearing wall and unload the girders, or make capitals on the gables of the walls, put slopes under the girders and thereby reduce their deflection. Otherwise, it will be quite difficult to join the runs of different heights among themselves and to coordinate the height marks with the gables of the walls.

When using long and heavy runs, the so-called "construction lift" can be used. This is the manufacture of a beam in the form of a rocker arm. The height of the "rocker arm" is made equal to the standard deflection of the run. The loaded beam will bend and become flat. The method came to us from our ancestors. In log houses, when laying mats and translations (beams), they hewed logs from below, along the entire length, making the undercut deeper in the middle part, and, if necessary, hemming the edges of the beams from above. Rocker beams eventually sagged under their own weight and became straight. This technological technique is used quite often, for example, prestressed reinforced concrete structures are made. In everyday life, you simply do not notice this, because the structures bend, and without that a small building rise becomes completely invisible to the eye. To reduce the deflection of the beam, you can also introduce additional struts under it. If it is impossible to install struts or make a “construction lift”, you can increase the rigidity of the beam by changing its section: to a T-beam, I-beam or lattice - a truss with parallel belts, or change the section by placing cantilever beams under the supports, that is, make its bottom in the form of an imperfect arch.

The support of the purlins on the wall is provided by a transverse side stop and must be designed for crushing the wood. In most cases, it is enough to provide the desired support depth and place a wooden lining under the bar on two layers of roofing material (hydroisol, etc.). However, it is still necessary to hold wood for crushing. If the support does not provide the required area at which collapse will not occur, the area of \u200b\u200bthe wooden lining must be increased, and its height must distribute the load at an angle of 45 °. Collapse stress is calculated by the formula:

N/Fcm ≤ Rc.90°,

where N is the pressure force on the support, kg; Fcm - crushing area, cm²; Rcm90 - calculated resistance to crushing of wood across the fibers (for pine and spruce Rcm90 = 30 kg / cm²).

It is necessary to pay special attention to the wall under the support of the ridge run. If a window is located below, then from the top of the lintel to the bottom of the run there must be at least 6 rows of reinforced masonry, otherwise reinforced concrete lintels must be laid above the window along the inside of the gable. If the layout of the house allows, ridge runs should not be made long and heavy, it is better to divide them into two single-span runs, or leave one and add support under it. For example, the layout of the house shown in Figure 25 implies the installation of a partition in the room under the second run. This means that it is possible to install a truss truss in the partition and unload the ridge run, and then hide the truss with sheathing, for example, with drywall.

Rice. 26.1. rafterless roof

Another way to unload ridge runs is that you can simply increase the number of stacked runs, for example, install one or two unloading runs along the roof slopes. With a significant increase in the number of beams, the question arises, why do we need rafters here at all, the crate can be made directly along the runs. It really is. Such roofs are called rafterless (Fig. 26.1). However, in mansard insulated roofs, the issue of drying the insulation is acute, so you still have to do something like rafters. To provide air ventilation, it will be necessary to fill wooden bars along the slopes (in the same direction as the rafters are laid), for example, 50 × 50 or 40 × 50 mm, thereby providing an airflow with a height of 50 or 40 mm.

Note. Earlier, here and further in the text, there are such absurdities in the formulas: d³d, this hurts the eyes a little, but from a mathematical point of view, this is the correct notation. It shows that the variable is in the 4th power. Since writing down the 4th degree in the language of the website “breaks” the beauty of the formula, one has to resort to such a notation. The same applies to the root expression: everything in brackets is included under the root sign.

An example of calculating the section of runs.

Given: country house 10.5 × 7.5 m. Estimated roof load for the first limit state Qp = 317 kg / m², for the second limit state Qн = 242 kg / m². Roof plan with dimensions indicated on.

1. We find the loads by the limit states acting on the first run:

qр = Qр×a = 317×3 = 951 kg/m
qн = Qн × a = 242×3 = 726 kg/m = 7.26 kg/cm

2. We calculate the maximum bending moment acting on this run (formula for):

M2 \u003d qp (L³1 + L³2) / 8L \u003d 951 (4.5³ + 3³) / 8 × 7.5 \u003d 1872 kg × m

3. We arbitrarily set the width of the run, b = 15 cm, and using the formula (3) we find its height:

h = √¯(6W/b) = √¯(6×1440/15) = 24 cm,
where W \u003d M / Rbend \u003d 187200/130 \u003d 1440 cm³

According to the assortment of lumber, the nearest suitable beam has dimensions of 150 × 250 mm. We select it for the subsequent calculation.

4. On the longest span, we check the run for deflection according to the formula (2).

First, we determine the standard deflection: fnor = L / 200 = 450/200 = 2.25 cm,
then calculated: f = 5qнL²L² / 384EJ = 5 × 7.26 × 450² × 450² / 384 × 100000 × 19531 = 2 cm,
where J = bh³/12 = 15×25³/12 = 19531 cmˆ4

Condition met 2 cm< 2,25 см, прогиб прогона получился меньше нормативно допустимого. Сечение первого прогона определили, будет применен брус размерами 150×250 мм. Если бы расчетный прогиб получился больше нормативного, то нужно увеличить сечение (лучше высоту) прогона.

5. We find the load acting on the second run.

From the calculated uniformly distributed for the first limit state, it will be equal to: qр = Qр×b = 317×3 = 951 kg/m;
for the second limit state qн = Qн×a = 242×3 = 726 kg/m = 7.26 kg/cm

At the connection point of the runs, from the action of the first run, a concentrated force P will be applied to the second run (formula on):

according to the first limit state Рр=RB = qр b/2 - M2/b = 951×3/2 + 1872/3 = 2051 kg
according to the second limit state Рн=RB = qн b/2 - Mн/b = 726×3/2 + 1429/3 = 1566 kg,
where Mn \u003d qn (L³1 + L³2) / 8L \u003d 726 (4.5³ + 3³) / 8 × 7.5 = 1429 kg × m

6. First you need to determine by what formula we will calculate the maximum bending moment on the second run, for this we find the ratio of the forces Р / qрL and the lengths of the application of the force c / b (see):

Рр/qрL = 2051/951×7.5 = 0.29; c/b = 4.5/3 = 1.5

c/b turned out to be more than p/qрL, so the maximum moment is calculated by the formula:

Мmax = ab(qрL + 2Pр)/2L = 4.5×3(951×7.5 + 2×2051)/2×7.5 =10112 kg×m

7. We arbitrarily set the width of the run, b = 20 cm, and using the formula (3) we find the height of the run:

h = √¯6W/b = √¯(6×7778/20) = 48 cm,
where W \u003d Mmax / Rbend \u003d 1011200/130 \u003d 7778 cm³

There are no beams of this height in the assortment of sawn timber, which means we decide to take two beams with dimensions of 200 × 250 mm, lay them on top of each other, twist them with studs and sew with MZP steel plates, or we will make a beam with wooden ties. Thus, we get a beam with a width of 200 and a height of 500 mm.

8. We check the composite beam for deflection according to the formula. First, we determine the standard deflection:

fnor = L/200 = 750/200 = 3.75 cm

Then the calculated one, in our case it is calculated as the sum of the deflections from the application of a uniform load and a concentrated force to the beam:

f = 5qнL²L²/384EJ + PнbL²(1 - b²/L²)√¯(3(1- b³/L³)/27EJ) = 5×7.26×750²×750²/384×100000×208333 + 1566×300×750² (1 - 300²/750²)√¯(3(1 - 300³/750³)/27×100000×208333) = 1.4 + 0.7 = 2.1 cm,
where J = bh³/12 = 20×503/12 = 208333 cmˆ4

Estimated deflection turned out to be less than the normative 2.1 cm< 3,75 см, значит составная балка удовлетворяет нашим требованиям. Таким образом, первый прогон принимаем из цельного бруса 150×250, второй - составным, общей высотой 500, а шириной 200 мм.

The calculation clearly shows that by introducing an additional support under the intersection of the runs, it would be possible to eliminate the concentrated force and reduce the cross section of the second run, and with the dimensions of the structure given in the example, make it equal to the first run.

An example of checking the bearing nodes of the runs for crushing.

We check the area of support of the girders on the walls so that there is no irreversible crushing of the wood or destruction of the wall material. Let's assume that the walls of the gables are made of gas silicate D500. The compressive strength of gas silicate D500 is 25 kg/cm², the compressive strength of pine wood in the supporting parts of structures at an angle of 90° to the fibers is 30 kg/cm². To prevent the destruction of the wall material and the irreversible collapse of wood, the following conditions must be met:

N/F ≤ Rszh - for wall material;
N/Fcm ≤ Rc.90° - for wood

In this example, it turned out that the wood has greater strength than the material of the walls. The calculation will be made to prevent the destruction of the wall material, i.e. the compressive stress must not exceed 25 kg/cm².

We find the value of the pressure of the first run on the walls (formulas for , load qp on the page of the run calculation example):

RA \u003d qr a / 2 - M2 / a \u003d 951 × 4.5 / 2 + 1872 / 4.5 \u003d 2556 kg
RC \u003d qp L / 2 + M2L / ab \u003d 951 × 7.5 / 2 - 1872 × 7.5 / 4.5 × 3 \u003d 2526 kg

We calculate the area of \u200b\u200bsupport for the ends of the first run:

F \u003d N / Rszh \u003d 2556/25 \u003d 103 cm
where N \u003d 2556 kg (the largest of the forces pressing on the wall), and Rszh \u003d 25 kg / cm².

It turns out that to support a 15 cm wide run, you need a “hook” on the wall equal to only 103/15 = 7 cm and at the same time there will be no irreversible collapse of the wood and destruction of the gas silicate blocks of the wall. Therefore, we will take the length of the support of the run on the wall constructively, for example, equal to 15 cm.

We find the pressure on the walls of the second run:

RD = qр L/2 + bPр/L =951×7.5/2 +4.5×2051/7.5 =4797 kg
RE = qр L/2 + aPр/L =951×7.5/2 +3×2051/7.5 =4387 kg

We calculate the area of \u200b\u200bsupporting the ends of the second run:

F \u003d N / Rszh \u003d 4797/25 \u003d 192 cm,
where N = 4797 kg (the largest of the forces pressing on the wall).

To support the second run with a width of 20 cm, you need a “hook” on the wall of at least 192/20 = 10 cm. And here we will take the length of the support of the run on the wall constructively, equal to 15 cm.

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Splicing rafters in the ridge area: we splice the rafters along the length and installation methods in the ridge area. Types of gable roof truss system: for small and large houses Why do you need a run under the ridge

Tree selection

We put the ridge beam

Elongated beam

Mauerlat

Device and installation of rafters

Installation of bows

Calculation of the section of the ridge beam

Installation of the ridge beam

Tree selection

We put the ridge beam

Elongated beam

Mauerlat

Device and installation of rafters

Installation of bows

Types of splicing rafters in the ridge

Method number 1. overlap

Method number 2. Butt joint

Method number 3. Beam connection

What to splice? Choice of fasteners

Splicing rafters in the ridge of a gable roof

Building rafters in length: paired and composite rafters

How to increase the length of the rafters

Pairing rafters

Rafters from boards are of two types: composite and paired.

Twin rafters

Composite rafters

How to splice rafters along the length: analysis of options and technological rules

Rules for building rafters

Butt joint or front stop

Rafter lap connection

Rafter connection with an oblique cut

Splicing rafters from boards

Composite plank rafters

Rafters paired in two or three boards

A little about fasteners

An example of calculating the section of runs.

An example of checking the bearing nodes of the runs for crushing.

What else to read

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