Knots and structures of truss systems. Fasteners of the nodes of the truss system

The roof system is the outside part of the roof structure that is supported by the load-bearing structure. It includes a crate and a truss system. The triangle underlying this system should be a rigid and most economical structural element that contains the attachment points for the roof rafters.

The main characteristics of the nodes of the truss system

The main attachment points of the roof truss system are shown in fig. 1. They imply the presence of a rafter leg (Mauerlat - 1), a rafter leg (ridge run - 2), a rack (puffs - 3). The design of the truss system is the main load-bearing element of the roof.

All roof fasteners must be of sufficient strength to eliminate a significant degree of risk associated with roof collapse. The consequences of a mistake made when connecting elements can be the most unpredictable.

Figure 1. The main attachment points of the roof truss system: 1 - mauerlat, 2 - ridge run, 3 - puffs.

First, the rafters are installed on the Mauerlat if the building has brick walls. Similar nodes are provided for concrete blocks, then it is necessary to create a reinforced concrete stiffening belt, and it is necessary to insert studs into its design. Their location should be at a distance of 1 to 1.5 m from each other, and their diameter should be more than 14 mm. The top of the studs must be equipped with a special thread.

Mauerlat is drilled, making holes that are necessary for attaching elements to it. Each of the holes should be the same size as the diameter of the stud, and its pitch should correspond to the distance between the studs. A nut is put on each protruding end of the stud and tightened, which ensures the strength of the connection between the Mauerlat and the wall. The rafters should be connected to the Mauerlat in such a way that their bearing capacity is not weakened.

Description of the main fasteners for mounting the truss system

If during the construction of the house a rounded log or timber was used, then it is not necessary to create an armored belt. produced on the upper beam or on the log of the wall. For this purpose, connecting the Mauerlat with the rafters uses different methods of cutting (tie-ins).

What fasteners fasten metal rafters:

1. Plates.
2. Fasteners LK.
3. corners.
4. Brackets WW.
5. Self-tapping screws.
6. Varieties of the corner of the KR.
7. Wire ties.
8. Mounting tape perforated TM.
9. Bolts with nuts.
10. Brackets WW.

If brackets are used when connecting the rafters to the Mauerlat, then they are not cut into the rafters, which helps to strengthen the bearing capacity. Metal brackets are usually produced, and the metal is galvanized and has a thickness of 0.2 cm. The brackets are strengthened with nails, anchor bolts or screws.

You can use the LK fastener by creating attachment points not only for rafters with a Mauerlat, but also for various other elements that make up the roof structure. The LK fastener is fixed to the wood, as are the brackets, with the exception of the use of anchor types of bolts.

Mounting perforated tape allows you to strengthen the connecting nodes in the construction of roofing systems. It is used not only to create stronger nodes, but also to strengthen elements for additional use in order to give rigidity or strength to the system as a whole. They fix the perforated mounting tape with screws or nails, so it is used to strengthen the structure of the rafter system of any roof, the integrity of which will not be violated.

With the use of KR corners and their various modifications, the attachment points are reinforced so that they can effectively participate in the connection of the Mauerlat and rafters. Ensuring appropriate strength to the roof units is permissible when using corners, which improves the load-bearing characteristics of the roof structure.

The use of connecting elements made of metal is not associated with inserting corners into the roof system. This will not cause a decrease in the bearing capacity of the roofing system. You can use corners for connection using screws or nails, the protrusions of which resemble a ruff.

How are the knots connected in the ridge part?

There are three main types of fastening in the ridge parts of the roofing system:

1. Butt connection.
2. Mounting on the basis of a ridge run.
3. Ridge joint overlap.

For the purpose of fastening, in the first way, the ridge part is cut from the upper edge at an angle that is the same as the angle of the roof slope. Then it rests on the necessary rafter, which should also be cut at an angle, but on the opposite side of the roof. A special template is sometimes used to trim corners.

Nails for connecting the rafters under the ridge should be 150 mm or more in size, two of them will be needed. Each nail is driven into the rafters at the top of the rafters at the appropriate angle. The sharp end of the nail usually enters the cut of the rafter from opposite sides. Strengthening the ridge joint can be achieved by applying a metal plate to it on the side or a wooden lining so that it is enough to pull it with bolts or nails.

The connection in the second way, that is, through the ridge run, is associated with the strengthening of the rafters on the ridge beam. The run is one of the additional support beams or beams, which is a support for the rafters. It is located parallel to the ridge or Mauerlat. The method differs from the previous one in that a ridge beam is laid between the rafters, which are sawn at an angle, which is a laborious process, so this method is used less frequently.

A more common method is similar to the first, but it differs in that the fastening is overlapped, and the joint method is not used. The rafters should be in contact with the ends, and not the side surfaces. The rafters should be pulled together with a bolt or hairpin, nails. This connection is used by many masters in practice.

In general, it is possible to install rafters on a Mauerlat by creating structures for roof truss systems that are expansion or non-expansion. This determines the choice of the appropriate method of connecting the power plate and rafters, which can be similarly strengthened to the ridge.

The main shortcomings in the installation of attachment points for the truss system

The problem of choosing the method of attaching the truss system to the structure of the building is very important when creating attachment points. Often, when creating nodes, the Mauerlat serves as a support for the rafters. The fastening of the Mauerlat beam is carried out “tightly” with the help of anchor bolts to the reinforced stiffening belt.

A possible drawback is an unanchored stiffening belt, which can lead to the overturning of the Mauerlat beam and the stability of the roof truss system. There is a loosening of the roof, and the roof is sliding down. Due to erroneous placement of anchor bolts or incorrectly made holes, the fastening is no longer effective.

If the nuts are screwed onto the bolts with an overtightening, then the fastening assembly becomes fragile and is subject to rapid destruction. In this case, wire twisting is sometimes used to create an attachment point.

During the construction of the truss system, the safety of the joints should be observed.

For example, if the truss structure is combined with the floor without taking into account the bearing capacity of the attic floor, then this is the most dangerous moment that can lead to the destruction of the building.

If the tightening is changed to a precast concrete floor beam intended for bending, then the use of precast concrete beams should be effective due to their rigid fixation in the reinforced floor stiffener, which is arranged using a reinforcing cage. Its axis must go in the same direction as the acting forces.

At the same time, the presence of shortcomings in the process of creating a truss system, which is a load-bearing wooden floor structure, often arises due to a misunderstanding of the functions performed by the puff and crossbar in the entire roofing system. The puff differs from the crossbar in that it is a longitudinal, and the crossbar is a transverse beam.

The construction of the truss system is associated with the creation of a spacer system that works on the principle of divergence at the bottom of the planes, which occurs under the influence of not only their own weight, but also the load that falls on the line of intersection of the planes, which should be prevented by the transverse beam, that is, tightening.

Getting to the roof device, you should find out all the points associated with the assumption of certain errors that occur when performing work on the installation of the truss system. The device of the roof of the house is associated with possible difficulties and shortcomings that do not allow achieving the goal.

In any building, the main elements on which the maximum load falls are the foundation, walls and roof. The quality of the roof installation largely depends on whether the truss system is installed correctly or not. If the attachment points of the truss system do not meet certain requirements, then such a roof will not last even a minimum operational period without repair work.

Requirements for the truss system

The truss system of any roof must meet such important requirements as:

• Maximum rigidity. Any frame node must withstand loads without being subjected to deformation or displacement. The triangle obtained during the arrangement of the truss system must ensure the rigidity of the structure and its maximum stability;
• Optimal weight. Depending on the roofing material, the material used for the rafters is selected. Usually a wooden beam is chosen, but metal can also be used for heavy roofs.

  Important! To prevent damage to the rafters, their rotting and the formation of fungus on wood, it is treated with an antiseptic, and metal structures with anti-corrosion compounds.

• The high quality of the materials used. The wood used as rafter legs should not have cracks and chips.

Varieties of truss systems

The roof can be equipped with one of the types of truss system, of which there are only two:

• Hanging rafters;
• Overhead rafters.

Hanging truss system

Such a system is optimal in the case of a gable roof, when the span between the walls is no more than 6 meters, but when installing additional elements, it is also applicable for wider openings. The Mauerlat serves as the lower basis for the support, while the upper part of the structure abuts against each other. This design also contains a puff - necessary to relieve the load from the walls, by reducing the expansion of the rafters. Beam puffs are installed below the rafter legs and can serve as floor beams.

Attention! The role of the tightening may not necessarily be performed by a wooden beam, it can also be a ceiling made of reinforced concrete structures, which in some houses is equipped with an upper floor.

If the puff is located above the bottom of the truss system, then it is called crossbar. Important points in the arrangement of this type of truss system include:

• The roof overhang should not be allowed to rest on the lower part of the rafter legs, which extend beyond the wall. In such a situation, it is best to use a filly (the width of the overhang is set within one meter). With this arrangement, the rafter will be based on the Mauerlat. The cross section of the beam for the filly is chosen smaller than for the rafters;
• To give the roof additional rigidity, and prevent it from staggering and being destroyed by strong gusts of wind, a wind board is nailed on the slope, to the Mauerlat from the ridge;
• If the moisture content of the material used to equip the truss system is more than 18%, unsteadiness should be foreseen, which will cause gradual drying of the wood. That is why fastening should be done with bolts or screws, not nails.

Layered truss system

This arrangement is applicable for roofs with distances between walls from 10 meters (maximum 16 meters). The slope can be made at any angle, and inside the building there are load-bearing walls or supporting columns. From above, for the rafters, the ridge run serves as the main support, and from below this function is performed by the Mauerlat. The inner purlin is supported either by the inner wall or by the studs. Due to the presence of only vertical type of loads, there is no need to install the tightening.

With a 16-meter span, the replacement of the ridge run is carried out by two side structures, the support for which will be the racks.

Important! The absence of bends in the rafter legs is ensured by such nodes as struts and crossbars.

Particular attention to the arrangement of the roof using a layered truss system should be paid to such nuances:

Features of calculations of the gable roof truss system are shown in the video:

The main nodes of the truss systems

The main nodes of the roof truss system include:

• Rafter. They perform the function of a skeleton, supporting the internal and external elements of the roof, and also serve as the basis for laying communications;
• Mauerlat. This is a kind of roofing foundation, which is a beam on which the entire structure is installed. It performs an important function - uniform distribution of the load of the entire structure;
• Run. Designed for fastening rafter legs together and can be located both on top and on the side;
• Puff. Serves for fixing the rafters in the lower part of the structure;
• Struts and racks. Provide the most stable location of the rafter beams;
• Skate. The junction of the roof slopes;
• Filly. These are continuations of the rafter legs, which are sometimes equipped;
• Rigel. It is necessary for high-quality and reliable support of load-bearing elements;
• Sill. Cross bar necessary to distribute the load.

In addition to the listed elements, the design includes attachment points for the roof truss system. When performing them, it is necessary to follow certain rules.

Important! It is absolutely not worth performing a simple fastening of the base to the crossbar, as this can lead to the complete destruction of the truss system.

The following types of fasteners should be used:

• With emphasis on the end of the crossbar;
• Teeth point-blank;
• Teeth in a spike.

The number of teeth should be chosen depending on the slope of the slope, and additional structural reliability can be created using metal corners.

The main attachment points of the truss structure include:

• Beam knot;
• Mauerlat knot;
• Skate knot.

beam knot

With such a node for connecting elements, an insertion of teeth into a spike is made into the rafter, and a recess corresponding to the teeth is made in the transverse section of the crossbar. Such a recess or nest should not exceed 30% of the timber thickness.

Fastening is carried out with special hardware with metal corners, or wooden beams, spikes and overlays.

Attention! If the roof is made of materials with low weight, and the slope of its slope does not exceed 35º, then the bases of the supports should be placed so that they rest against an area larger than the beam itself.

Mauerlat knot

Such fastening can be performed both by rigid technology and by sliding technology. Rigid technology involves the installation of a strong connection between the Mauerlat and the rafters, preventing the possibility of slipping, deflecting or popping out. For this, special support corners with bars are used. The resulting knot is fastened with wire using hardware. At the same time, nails should be hammered obliquely so that they enter the wood crosswise. The last nail must be driven in vertically.

In the case of a sliding fastening, alignment is performed using a special mechanism that allows you to move the rafter leg in the required direction. To do this, a tie-in is made on the supports, on which the Mauerlat is then laid. The structure is fixed, as in the previous case, with crossed nails. This method of arranging the connection nodes allows all nodes of the truss structure to move within certain limits.

Attention! Rigid fastening with inexperience of builders can lead to damage to the walls of the building.

ridge knot

In this case, the fastening can also be made in two types - butt and overlap. With a butt joint, the top of the supports is cut with a bevel, as is the angle of the roof. They rest on the same undercut opposite supports. Fastening is carried out using nails, in the amount of two pieces. They are hammered from above at a certain angle. The seams that form between the supports are connected with metal plates or plates. In the second case, fastening is carried out by overlapping, not by end parts, but by lateral sections and fixed with bolts.

Conclusion

When performing work on the installation of the roof, the arrangement of the truss system should be given careful attention, avoiding disruption of technological processes. This will provide the structure with strength, durability and reliability.

Layered truss system - a structure used in the construction of roofs of buildings with intermediate load-bearing walls, supporting pillars or columns. It relies not only on the walls outside, but also on the inner central support (in some cases, two).

If we talk about use, then layered rafters are the most common for residential private houses, which, as a rule, have internal partition walls.

The constituent elements of the layered system: two rafter legs, the lower edges of which are supported and fixed on the outer walls (Mauerlat), and the upper ones - on a horizontal ridge run. The run, in turn, is held by vertical posts resting against an intermediate wall.

This is a classic layout of a layered system, suitable for a gable roof. With a shed roof, the same rules can be traced, but with a different implementation. The rafters included in the rafter system are laid with support on opposite load-bearing walls (it turns out that only on two supports). An internal partition is not needed here. In fact, its function is performed by a higher wall.

To increase the bearing capacity of the truss structure, struts are introduced into the system. Their presence allows you to increase the length of overlapped spans.

For shed roofs, it is possible to use layered rafters without the introduction of struts for spans up to 4.5 m. The presence of a strut increases this possible length to 6 m. A similar trend can be traced with gable roofs. A gable structure with one intermediate support is used for spans up to 9 m. Installing struts increases the maximum span length to 10 m. And the combination of struts with a scrum (a horizontal beam connecting a pair of rafter legs) - up to 14 m.

There are several options for the implementation of layered systems, among which there are non-thrust and spacer structures with additional supporting struts, scrambles, and rafter beams.

Consider the basic designs of layered rafters.

Bezrasporny rafters without struts

This type of layered rafters does not spread on external walls. Leveling of bursting loads occurs due to a special combination of fasteners. One edge of the rafter is always fixed rigidly, and the second - on a sliding support. This gives the absence of thrust.

Rigid fastening can mean that the node is fixed, but the beam can be rotated in the hinge (one degree of freedom). There is also a rigid pinching of the rafter beam, in which any displacement is impossible (zero degree of freedom).

More freedom is given by a sliding mount, which allows the rafter leg not only to turn, but also to move horizontally (two degrees of freedom).

The non-thrust design is characterized by the fact that it always has both a rigid and a sliding fastening. Due to this, under the influence of the load, the rafters bend without transferring the thrust to the walls.

Options for fixing rafter legs

The bottom of the rafter is fixed rigidly, the top is loose (sliding support)

The lower edge of the rafter is fixed rigidly to the Mauerlat (one degree of freedom), by cutting with a tooth. In another case, apply washed down with fixation with a support bar.

At the upper end of the rafter, a horizontal cut is made with a bevel. If cutting is not possible, then the edge of the rafter leg is hemmed from below with a trim of the beam and fastened on both sides with mounting plates. The fastening of the upper edge of the rafter to the run is performed according to the type of sliding support. At the same time, opposite rafters are laid on the ridge one by one, without fastenings between them. Therefore, a gable roof made according to this scheme can be perceived as two shed roofs adjacent to each other.

The complexity of the scheme is that any error in the implementation of the ridge knot turns the non-thrust structure into a spacer. Therefore, this option is rarely used for gable roofs, more often for shed roofs.

The bottom of the rafter leg is fixed freely, the top is rigid

The most common scheme for private houses.

The lower edge of the rafter is fixed to the Mauerlat on a slider (metal bracket), so that it can move and bend under load. So that the rafter could not “leave” in the lateral direction, it is fixed on both sides with metal corners or bars.

The top of the rafter legs is fixed on a hinge with a rotation tolerance (one degree of freedom). At the same time, the ridge knots of this type of layered rafters are performed as follows: the edges of the rafters are whipped together and connected with a bolt or nails. Or, the ends pre-cut at an angle are joined, and then they are tied with metal or wooden linings.

The bottom of the rafter leg is fixed freely, the top is rigidly pinched

This scheme differs from the previous one in that the connection of the rafters in the ridge knot is carried out with a rigid pinch. The rafters are supported by beveled ends against each other, and then they are connected to each other and to the ridge run with two crossbars-puffs. It turns out a knot with pinching.

The bottom of the rafter legs is connected to the Mauerlat freely, on a slider.

This mounting option is characterized by increased bearing capacity, which allows it to be used in regions with an increased level of snowfall.

Ways to increase the stability of non-thrust systems

All three considered truss systems show themselves as stable under uneven loads only in the case of rigid fixation of the ridge run. That is, when its ends are brought out to the gables or supported with additional slanting rafters.

If the ridge run rests only on the racks, the roof may lose stability. In the second and third options considered (the bottom of the rafter leg on the slider, the top is rigidly fixed), with an increase in the load on one of the slopes, the roof will shift towards the increased load. The first option will keep its shape, but only with perfectly vertical racks (under the run).

So that, despite the non-rigid fixation of the run and uneven loads, the layered rafter system remains stable, it is supplemented with a horizontal scrum. The scrum is a beam, usually with the same section as the rafters.

It is fastened to the rafters with nails or bolts. The intersection of fights and racks is fixed with a nail fight. The work of the fight can be described as emergency. In the event of an uneven heavy load on the slopes, the fight is included in the work and protects the system from skew.

You can strengthen the system with a rigidly fixed top and a free bottom (second and third options) with the help of a slight transformation of the lower node. Rafter legs are taken out of the edge of the walls. At the same time, the fastening itself remains sliding, like a slider.

Another option for increasing stability is to rigidly mount the bottom of the racks on which the horizontal ridge run is held. To do this, they are cut into the bed and fixed to the floors, for example, using linings from boards or bars.

Spacer rafters without struts

In this case, the rafters rest on the load-bearing walls and transfer the thrust to them. Therefore, such systems cannot be used for houses whose walls are built of aerated concrete. Aerated concrete blocks do not resist bending at all and collapse under expansion loads. And other materials, such as brick or concrete panels, easily withstand such loads and do not deform.

The spacer rafter system requires a rigidly fixed Mauerlat. Moreover, in order to withstand the thrust, the strength of the walls must be high. Or an inextricable reinforced concrete belt should go along the top of the walls.

For spacer rafters, the same fastening options discussed above for non-thrust systems are used. But with one caveat: all existing sliding mounts (sliders) are replaced with hinged ones with the ability to rotate. To do this, a support beam is nailed to the bottom of the rafter or a cut is made with a tooth in the Mauerlat. Hinged fastening in the ridge knot is performed by laying the rafters on top of each other and fastening them with a nail or bolt.

The spacer structure is a cross between layered non-spacer and hanging systems. The ridge run is still used in them, but it no longer plays a significant role. After all, the rafters are rested with their lower edges against the walls, and with their upper edges against each other. When the walls subside or the ridge run sags under its own weight, the run stops working altogether. In essence, such rafters become hanging.

To increase the stability of the system, a contraction is included in it, which works in compression. It partially, albeit to a small extent, removes the pressure on the walls. In order for the fight to remove the thrust completely, it must connect the lower edges of the rafter legs. But then it will no longer be a fight, but a puff.

The installation of a rigidly fixed ridge run also reduces the thrust.

Rafters with struts

Such systems can be arranged both according to spacer and non-spacer schemes. Their difference from the options already considered is the presence of a third supporting part under the rafter leg - a strut (rafter leg).

The brace changes the system. The rafter from a single-span beam turns into a two-span continuous one. This allows you to increase the overlapped span, up to 14 m. And also - to reduce the cross section of the rafters.

The brace is connected to the rafter in such a way as to prevent its displacement. This is done as follows: the strut is brought under the rafter and fixed with wooden plates on the sides and bottom.

Laying system with rafter beams

This rafter design is suitable for buildings with two longitudinal load-bearing walls or intermediate transverse walls. Racks in this case are located not under the ridge, but under the rafters. There is no ridge run.

The rafter legs in the scheme are supported by two rafter beams (through girders), which, in turn, are laid along the roof slopes and rest on vertical racks. Racks are fixed to the bearing intermediate walls through the beds.

Through runs may not be included in the scheme. Then the racks will have to be brought directly under each rafter and secured with a puff with a nail fight.

From above, the rafter legs are joined together and connected with overlays made of metal or wood on both sides.

The absence of a ridge run automatically means that the rafter system forms a spacer. In order to neutralize it in the non-spacer version of the system, a puff is fixed below the through runs. Under load, it will stretch and eliminate unwanted thrust. To maintain stability in the system, a scrum is used, fixed at the bottom of the hasty legs. Also, the structure will be protected from folding by special jointing, which is fixed crosswise between the uprights.

In the spacer system, the scrum is set above the through runs. Then the contraction under load will shrink and, in fact, turn into a crossbar.

The installation of racks under the rafters or through runs (and the absence of central racks!) makes it possible to use this type of layered rafters for arranging spacious attic spaces. Other schemes are only suitable for attics and attics with partitions.

Key points for installing layered rafters

Having on hand a calculated scheme of the device, you can proceed with the installation of the truss system. Installation is carried out in several stages, the main ones are:

1. A Mauerlat is laid on top of the outer walls - a board or timber. To prevent the Mauerlat from rotting, a waterproofing material is laid between it and the wall - roofing material, roofing felt, etc.

2. A bed is laid on top of the intermediate wall, which is necessary for attaching vertical racks.

3. Racks are fixed on the bed with a step of 3-6m.

4. From above, on the racks, install a ridge run.

5. Expose the rafters in increments of 0.6-1.2 m. From below, the rafter leg is attached to the Mauerlat in accordance with the selected mounting scheme (on a hinge or on a slider). From above, the rafter legs are either laid out separately on the ridge run, or they connect the upper edges to each other, resting on the ridge.

6. If the scheme provides, the rafter legs are connected by horizontal contractions.

7. Again, at the request of the scheme, struts, support elements are set.

When performing work on the installation of rafters, missteps should not be allowed. It should be remembered that the truss system is a roof frame that must withstand all possible loads. An incorrectly calculated or mounted system can easily lead to skew and even destruction of the entire roof.

Rafters and lathing - the roof frame, which bears the brunt of the roofing pie, thermal insulation, snow load, so they must have a high bearing capacity, strength. The idea of ​​using metal rafters to make the roof structure more rigid is not new, but in the past it was mainly used to cover industrial or utility structures. Now the rafter frame and the metal profile crate are considered a real alternative to wooden roof elements if the slope length exceeds 10 meters.

The design of the roof truss frame consists of many interconnected elements that form trusses. The step, the size of the section between the rafters and other supports determines the calculation of the loads to which they are subjected during operation. The roof frame performs the following functions:

1. Load distribution. Interconnected nodes, reinforced with corners, evenly distribute the weight of the roof, which can reach up to 500-600 kg, taking into account the snow load. The larger the cross section of the rafters and the smaller the pitch between them, the greater the bearing capacity of the structure.
2. Giving slope and shape. The rafters, located at an angle to the base of the roof, form an inclined plane of slopes, so that snow and water do not accumulate on the roof surface.
3. Formation of the base for fixing the roofing material. The top coat of the roofing pie is fastened to the roof frame. The crate acts as a basis for fixing the coating, distributing its weight evenly over the rafters.

Note! What should be all the nodes, rafters and lathing of the roof structure determines the engineering calculation. To determine the required bearing capacity of the frame, it is necessary to calculate the total load to which it will be subjected. To do this, add up the weight of the roofing material, insulation, waterproofing, the maximum snow load with the weight of the truss system.

Types of truss systems

The most common material from which the crate and rafters of the roof frame are made is wood. However, if the weight of the roofing material is large enough, and the length of the slope is more than 6 meters, then the structure is too massive. Builders have to reduce the step between the rafter legs, increase their cross section, which is why the roof units acquire a lot of weight, increasing the load on the foundation. You can unload the load-bearing walls and the base of the building using stronger, but lighter metal rafters. According to the type of material used, the following types of truss systems are distinguished:

• Wooden. Rafters and lathing made of wood are used for the construction of roofs, the slope length of which does not exceed 7-10 meters. The fastening of the frame elements to each other occurs with the help of self-tapping screws, nails or movable metal elements. The step between the legs is usually in the range of 50-80 cm.
• Metal. Metal roof frames are made of zinc-coated steel profile, which is not afraid of moisture. The rafters and the crate made of this material are light, durable, so the step between them can be increased to 1.5-2 meters. The fastening of the metal profile is carried out by welding or fasteners. Metal roof units are used with a slope length of 10 meters or more.
• Combined. The truss frame, combining metal and wooden knots, is called combined. The combination of wooden and galvanized steel support elements allows for a cheaper structure with a high load-bearing capacity by increasing the spacing between the rafters.

Please note that metal and wooden frame elements cannot be connected to each other without a gasket made of waterproofing material or treated with an antiseptic preparation. Since the metal has a high thermal conductivity, its proximity to wood leads to condensation and rotting of the rafters.

Fastening methods

Metal rafters are assembled into triangular, trapezoidal or arched trusses. Internal stiffeners are attached to the frame beams, which form corners that significantly increase the load-bearing capacity of the frame. Such a system allows you to take a step between the rafters more, taking into account the support capabilities of each truss. The fastening of the metal elements of the roof frame is carried out by one of the following methods:

1. Fastening by welding. If you weld the details of the truss system using a welding machine, you can get a rigid structure with high strength and bearing capacity. If you correctly calculate the roof, you can lighten the frame and reduce the load on the foundation of the structure. The disadvantage of this method is that only a professional with the help of special equipment can perform welding.
2. Fastening with bolts. Fixing the rafters with fasteners allows for less rigid fastening. This method of assembling a truss frame based on a metal profile is used in private housing construction, where the length of the slopes does not exceed 10 meters. Rejection of welding allows you to speed up the installation of the roof.

Experienced craftsmen rely on the fact that the rafters can withstand more weight than wooden ones, so you can increase the step between them and reduce the thickness of the section of the elements. Moreover, ready-made roof trusses are sold in hardware stores, the fastening of which was carried out by welding, suitable for overlapping buildings of standard width.

Advantages

The metal truss frame is used for the construction of roofs of any shape, any slope with a slope of 1-2 degrees. As the material from which the rafters and lathing are made, steel corners, pipes of round and rectangular cross-section, and tauri are used. In order to correctly select the thickness of the frame elements and choose the step between them, the calculation of the roof structure is performed, taking into account the permanent and temporary loads that are transferred to the rafter beams during operation. The advantages of a truss system made of this material are:

• Fire safety. Unlike wooden beams, metal frame beams are not flammable, which increases the fire safety of the building.
• Ease of maintenance. Zinc galvanized coating, which covers the steel corners, protects the roof frame from corrosion during the entire service life. They, unlike wooden ones, do not require annual antiseptic treatment.
• Wastelessness. Although metal structures are quite expensive, it is considered cost-effective, since the fastening is carried out by welding, and there is practically no waste left.
• Long service life. If you correctly calculate the loads, then the metal roof frame will last more than 100 years, which exceeds the life of even the most stable roofing.

Professional roofers believe that it is advisable to use welded metal structures for the manufacture of a roof truss frame with a slope length of 10-12 meters. In this case, the main task is to correctly calculate the loads, and then determine the step between the legs in accordance with the climatic characteristics and properties of the roofing material.

disadvantages

Despite the obvious advantages, metal truss systems are not the most popular design solution in private housing construction. Even a large step between the rafters and a sparse crate at a high cost of metal cannot make the structure cheaper than wood. The disadvantages of metal rafters are:

1. High thermal conductivity. The metal has a high coefficient of thermal conductivity, so the rafters and form bridges are cold. From the point of view of energy efficiency, a metal profile frame is not the best solution.
2. Difficulty in transportation and installation. When choosing a metal rafter system, take into account that transporting, lifting and securing long and heavy elements is more difficult than standard length lumber rafters.
3. Difficulty of installation. To rise to a height and fix the trusses, special equipment is used, the rental or purchase of which is expensive.
4. Deformation under high temperature. Although metal is considered a non-combustible material, during a fire it is highly deformed, which usually leads to the collapse of the roof.

To determine whether it makes sense to use more expensive metal profile truss elements, you need to calculate the roof frame. If the slope length exceeds 10 meters, and the load is more than 450-600 kg, then the installation of a metal frame is economically feasible.

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