The distance between the rafters: principles and examples of calculating the pitch of the truss system. The distance between the rafters: learning to choose the right one How to determine the pitch of the rafters
It is pointless to argue about the importance of the roof for any building. It is not for nothing that over a dozen different types of roofs have been invented in the history of mankind, from simple to quite complex in design and construction. An important element in planning the construction of a roof is the step between the rafters - strong bars that are the basis of the structure. This will be discussed in this article.
The distance between the base of the roof slopes is not a constant value and depends on the following components:
- type of roof;
- slope angle;
- the type of roofing material to be installed;
- rafter section dimensions.
Before proceeding with the process of erecting the upper structure of the house, a calculation should be performed by determining the optimal distance between the rafters.
Gable roof rafter pitch
The most widespread in our country are gable roofs. They are a structure that has two parallel planes, with an angle of inclination relative to the horizon from 20 to 50 degrees.
With an insufficient slope of the gable roof in snowy areas, there is a danger of accumulation of large snow masses, which can lead to the destruction of the structure. An increase in the slope angle in regions with a predominance of strong winds is also fraught with a high load and the risk of breaking not only the roof, but the entire structure as a whole.
Mansard roof truss system
Most private houses have an exploited under-roof space called an attic. This design is characterized by an increased height of the slope, which is caused by the need to create a living space of a comfortable height. As a rule, the slopes of the mansard roof are broken, having a varying angle of slope. For their installation, a double truss system is used.
The steepness of the lower slopes of the mansard roof significantly exceeds the slope of their upper extensions. The plane load perceived by them is not great. Thanks to this, the rafters in the lower part can be installed with a maximum pitch. The upper ridge slopes are recommended to be mounted with a reduced gap from each other.
Rafters in a shed roof
For outbuildings and some private houses, roofs with one slope are used. Due to the limited angle of inclination, high pressure is exerted on them. Experts recommend using lumber of increased cross-section for the rafters of a single-pitched roof, with the installation of a minimum step from each other.
When calculating the distances at which roof beams are installed, special attention should be paid to the amount of snow load in a particular area. With a small slope, this characteristic is of great importance. Roofing material for such roofs is best chosen with a minimum dead weight, which will reduce the bending load.
Hip roof truss system
The hip roof truss system is considered the most difficult in construction. This type is called four-pitched, since the roof is formed not only by side, but also by additional end slopes, where the installation of rafters is carried out not on the ridge, but on the corner bowstrings. This places special demands on the organization of the roof frame.
Under the hip roof, the attic is not often arranged. This is due to the small angle of inclination of the rafters and the roof as a whole. In the case of an increase in the angle of the slopes to the horizon, the distance between the rafters is increased, with a decrease, vice versa. An additional aspect of the calculation is the roofing material used.
The dependence of the pitch of the rafters on the roofing material
In addition to variable snow and wind loads, a constant (static) one also acts on the roof, the force of which depends on the roofing material used. It is no secret that different types of roofs have their own weight, which can differ by 10 or more times.
The right choice of material affects not only the top, but also all other parts of the structure of a residential building and other buildings. Not without reason, when designing the foundation, it is necessary to decide in advance on the choice of the roof.
Roofing from profiled sheet
Currently, one of the most common roofing materials is a profiled sheet, which is produced galvanized or followed by a polymer coating. The distinguishing features of the profiled sheet include the following parameters:
- High corrosion resistance;
- As a result, a long (more than 15 years) service life;
- Easy installation even without the necessary qualification;
- Small sheet weight (weight 1 m 2 is 4-5 kg).
Since this roofing material does not exert a high load on the truss system, the distance between the elements is chosen as much as possible for a particular angle of inclination. In addition, the profiled sheet does not require high strength characteristics from the roof lathing. All this together allows minimizing the total load on the foundation and walls.
Roofing from a metal tile
The second common type of steel roofing materials is metal tiles. This type of profiled sheet, which successfully imitates natural clay material, but with a lower mass (10 or more). A feature of the rafters under the metal tile is the smaller section size.
When choosing at what distance to install the rafters, first of all, you should be guided by the dynamic load. Like a profiled sheet, a metal tile is not demanding on the size of the rafter legs and is well mounted on a crate made of an inch coniferous board. All this makes a metal roof low-cost.
Rafter system for ondulin
In the 21st century, corrugated sheet materials were replaced by a more durable and lightweight analogue - ondulin. Among others - the most lightweight material. The weight of the sheet does not exceed 6 kg.
The small thickness of ondulin sheets at slope angles of less than 15 ° requires the construction of a continuous crate of plywood sheets, for example, which will require an appropriate rafter pitch. This should be taken into account in the calculations.
slate roof
Not so long ago, a wavy material from an asbestos-cement mixture, called slate, was widespread. High mass and fragility are the main disadvantages, however, even today he finds his fans in the construction of various outbuildings.
A high mass, comparable to the weight of a clay tile, will not allow the use of the same truss system as under a metal tile. Building codes define a minimum angle of slope for a slate roof of 22 degrees or more. Otherwise, the load from the material itself and the truss system with the crate exceeds the permissible parameters. The step of the inclined bars, as well as their cross section, are selected individually in each case.
Polycarbonate roof
In recent years, more and more often on the roofs of verandas and gazebos, an artificial polymer material, polycarbonate, has been used. Produced in two versions - monolithic and honeycomb. The first in its properties is similar to ordinary quartz glass, but significantly exceeds it in strength. The second one has less mechanical properties, but high thermal insulation and light transmission.
Cellular polycarbonate is standardly much lighter than a monolithic counterpart. It is used as a roof without the use of lathing, provided that the step does not exceed ½ of the width of the sheet of material. The high strength of the monolithic analogue also makes it possible to avoid elements transverse to the rafters. Sufficient flexibility allows you to cover semi-circular roofs on a metal frame, the pitch of which does not exceed 0.9 meters.
Thematic material:
Rafters under a soft roof
The original pattern can be obtained through the use of soft roofing materials, which are spread with an adhesive layer. They are installed on a continuous crate made of plywood or OSB. The step of the rafters should allow you to fix the sheets, so it is chosen as a multiple of ½ of the width. Given the standard dimensions of plywood 1520x1520 mm, the center distance between the rafters will be: 1520: 3 = 506 mm.
Rafter step for insulation
The installation of residential under-roof spaces is often combined with the laying of insulation sheets in the rafter gap. The most common plates with dimensions of 600x1000mm. These parameters are used as starting points.
Rafter pitch calculation scheme
According to building codes, the pitch of the roof rafters is in the range of 0.6 - 1 meter. Its final calculation is performed according to a simple formula, depending on the total length of the roof. For the calculation, you must perform the following list of actions:
- determine how much distance should be between the rafters for your specific building conditions. According to the reference book, the magnitude of wind and snow loads in the area is determined.
- the length of the roof is divided by the desired distance by adding one. The result will be equal to the number of rafter legs that are installed on one roof slope. If the value is not an integer, it is rounded off.
- the length of the roof is divided by the number of rafters calculated above, we get the final step in meters.
For example, with a slope of 30 degrees, the maximum distance between the rafters of a gable roof under a metal tile is 0.6 measure. The length is assumed to be 16 meters. Hence:
- 16:0,6+1=27,66;
- rounding the result, we get 28 rafters per slope;
- 16:28 \u003d 0.57 meters - the center gap of the rafter legs for these specific conditions.
As you can see, the calculation technology is not complicated, but this is just an approximate scheme. Taking into account many other parameters mentioned above can make certain adjustments.
The construction of the roof of a building is an important stage in the construction of a house, while it is necessary to calculate the pitch of the rafters as accurately as possible, since the reliability and service life of the roof will depend on this.
If the master does not pay attention to the calculations and installs the rafters, focusing only on his own eye, then the frame will turn out to be either too heavy, moreover, expensive, or fragile.
General calculation rules
The step of the rafters is the distance from one to the other rafter leg. It is believed that such a distance is set in the range from 60 cm to 100 - 120 cm (determining the optimal width, you need to take into account the dimensions of waterproofing materials and insulation).
The exact installation step of the rafters for a gable roof can be calculated according to the following scheme:
- measure the length of the slope along the cornice of the roof;
- we divide the measurement data by a certain step (we determined a step of 1 m - we divide by 1, if the step is 0.8 m - we divide by 0.8, etc.);
- add 1 to the resulting number;
- the result is rounded up (in the direction of increase) - we see the number of rafters for installation on one slope of a gable roof;
- we divide the length of the slope by the resulting number and get the exact step between the rafters.
For example, the length of the roof slope is 30.5 meters, given the width of the insulation and waterproofing rolls, a step of 1 meter is selected.
30.5 m / 1 m = 30.5 + 1 = 31.5. Rounding up gives a result of 32. 32 rafters are required for the frame for the roof slope.
30.5 m / 32 pcs. \u003d 0.95 cm - the distance (step) between the centers of the rafters.
However, this formula does not take into account the nuances of certain roofing materials, therefore, in the future, the article will discuss how to calculate the correct pitch of the rafters for the most popular coatings.
It is especially worth highlighting the rafter system during the construction of a hip roof (envelope). The construction of such a roof begins to be built precisely from the hip legs, into which the rest of the rafters then crash.
The hip, which has a length of over 6 m, is often reinforced by sewing it together from two elements. Otherwise, the installation and calculation of the pitch of the rafters for a hip roof are similar to the installation and calculation of a gable roof.
Rafter system for ceramic and metal tiles
The use of metal tiles in the construction of private sector houses is the most common phenomenon, which is facilitated by a number of its positive qualities.
Firstly, this is a small weight of the coating, which facilitates installation and helps to reduce loads on supporting structures.
Secondly, sheets of metal tiles can quickly cover large areas of the roof, which is very convenient, especially if you choose a gable version of the roof.
The step of the rafters under the metal tile is set in the range from 60 to 95 cm, since the relatively small weight of the roof does not require increased resistance. Due to the low weight of the coating, the thickness of the rafters can also be reduced.
As for the thickness of the heat-insulating material used to insulate metal roofs, experts believe that when installing an attic under it, for greater confidence in reliability, thermal insulation of 20 cm can be used, in other cases 15 cm is enough.
A feature of roofs covered with metal tiles is the possibility of condensation.
This can be avoided by organizing ventilation during the installation of the rafters: for this, several holes (about 10 mm in diameter) must be drilled in the rafters at a short distance from the top.
As for the installation of a frame made of rafters under a coating such as natural (ceramic) tiles, it is necessary to remember its large weight.
In comparison with other modern materials, tiles are 10 times heavier or more. Given this feature, experts recommend using exclusively dry wood for it in order to avoid sagging after drying.
For rafters in this case, you need to use a beam with a section of 50 - 60 mm * 150 - 180 mm; the step width of the rafter legs is set in the range of 80 - 130 cm, while the greater the degree of roof slope, the greater the distance between the axes of the rafters.
The pitch of the rafters is also dependent on their length: the greatest length will require the smallest distance, and vice versa. Otherwise, the roof frame will lose the necessary stability.
Given the size of the tile (as a rule, its length does not exceed 40 cm), it is necessary to correctly calculate the width of the step of the crate stuffed onto the rafters.
From the length of the slope, you need to subtract the length of the bottom row and the number of centimeters between the crate and the last bar.
Frame for ondulin, corrugated board and soft roof
Relatively new on the market of materials are such types of roofing as corrugated board and ondulin. At the same time, they are successfully used both in the construction of a gable and hip roof.
The weight of these materials is almost equal to the weight of the metal tile, so the calculation of the width of the rafter pitch will be similar to the calculation when laying metal sheets. The pitch of the rafters for corrugated board is determined from the range of 60 - 90 cm, for ondulin - 60 - 100 cm.
The sufficient thickness of the rafters themselves when covering the roof with ondulin and corrugated board is 50 * 200 mm.
If it is necessary to increase the distance between the centers of the rafters, then you can increase the strength of the transverse crate.
The rule for installing a batten under a corrugated board is similar to installing a batten for a metal tile: the last board must be taken wider than the rest (usually 15 cm thicker).
The crate for ondulin must be made solid to increase the resistance of the material to various weather conditions.
Speaking about the device of the truss system for a soft roof, it is worth remembering the types of such material presented on the markets: this is a roll type of roofing and bituminous tiles.
The name itself suggests that a soft roof requires a very strong base that will withstand heavy snow, gusts of wind and heavy rains.
The main requirements for roof frames for soft roofing are that they must be strong and even.
Strength must be calculated for the thickness of the snow typical for the region in which the structure is being built, and for the average weight of an adult male, so that troubleshooting is possible.
A flat roof should be for the reason that to exclude the possibility of sagging under the influence of the above weather phenomena.
Between the axes of the rafters for a roof frame with a soft roof, the step should be small, no more than 50 cm.
If the step is still larger, you will need to make a rough crate, on which the main, solid one will already be mounted.
The counter-lattice will help the soft roof not to bend and increase its service life by an order of magnitude. For a continuous crate, a 30 mm board or a 20 mm OSB board is more often used.
If a soft roof is provided as a covering for a hip roof with the organization of an attic space, then it is necessary to think about its insulation by thinking over a filing for laying insulation, hydro and vapor barrier.
These are all the subtleties of the roof frame device for a soft roof.
Rafter for slate roof
Despite the richness and variety of modern materials, the good old slate remains quite popular. The use of expensive roofing is far from always justified, and it is then that an asbestos-cement sheet comes to the rescue.
The weight of slate is quite impressive, while its installation does not require any kind of grandiose crate. This is because such a roof in itself is quite strong.
For the construction of the roof frame, in this case, rafters with a section of 150 * 40 mm and a bar with a section of 35 * 35 mm for the crate are required.
Lumber for the construction of the frame should be stacked and kept for some time to give the same level of moisture. 
After the drying process is completed, the bend must be determined. Rafter legs are recommended to be installed with the concave part (tray) up - so if water gets on the rafters, it will slide down, bypassing the attic space.
Before starting the installation, the beams are checked for the content of knots in them and rejected, since the knotted material may not withstand the weight of the roof.
Rafter fasteners in this case are produced with nails, the optimal length of which is 15 - 20 cm.
Nails should pierce the timber through, however, the peeking ends should never be bent, since it is necessary to maintain the mobility of the structure.
With drying and deformation of the tree, this technique will prevent cracking of the slate sheets.
The lathing for the device of a slate roof can be continuous or sparse. A solid one is made from a sheet of OSB or plywood and is more often used when laying flat slate.
Sparse is typical for laying wave slate on it. For a sheet with a standard size of 1.75 m, the crate pitch is about 80 cm. There is no point in making the crate more frequent, since the slate has a sufficient margin of safety.
After analyzing the materials of the article, you can independently engage in the construction of a gable and hip roof frame, determining for yourself their features and properties of roofing materials.
It would not be superfluous to recall that the calculation of the rafter pitch is a very responsible task, and it is necessary to approach its markup responsibly, since this will avoid mistakes and speed up the progress of work in the future.
A gable roof is one of the most common and versatile roof structures for buildings for various purposes. They can be made both cold for non-residential attic spaces, and insulated for attic rooms.
Important. The house has two main architectural elements that play a decisive role in the durability and safety of operation: the foundation and the roof. During their design, it is necessary to strictly comply with all the requirements of building codes and regulations.
Only professionals can deal with the design and construction of the truss system. They must have deep theoretical knowledge and extensive practical experience in performing such work, only practice allows you to make the best decisions during construction.
Each house has its own individual characteristics, each batch of lumber is different in strength, each load-bearing unit can be made and fixed in different ways. All this affects the stability of the rafter system, increases or reduces the estimated cost of the roof, etc. It is necessary to achieve such an option that the truss system is as simple as possible to build and at the same time reliable and cheap.
There are so many different opinions from inexperienced developers about how to choose the distance between the rafters. Some in all seriousness give advice on choosing this parameter for each type of roofing material: natural or artificial piece tiles, metal tiles and corrugated board, soft bitumen or slate coatings. In fact, all this is not so, architects never put in the initial data when calculating the step rafter system type of roof.
The physical properties of roofing materials, together with other factors, do not affect the distance between the rafters, but their dimensions and additional structural elements of the truss system to increase the stability of the structure, including:
- vertical supports;
- horizontal runs;
- corner supports;
- crossbars and other special elements.
There are many different elements in the construction of the roof of a wooden house, each of which performs its function and is fixed in a certain way. To find out in detail what elements the roof of a wooden house consists of,. You will find not only the description of the elements, but also the best practical tips!
Before starting calculations, engineers have initial data (technical specifications) for the entire system, taking into account these values, other parameters are calculated. Among the initial data there is also a rafter step, it is known before the start of design and does not change in the final project. What exactly influences this parameter?
| Factors affecting the distance between the rafters | Short description |
|---|---|
| This factor has an impact only if it is planned to make the roof insulated. The terms of reference for the design should indicate the type and size of the heaters used, and they are different. For example, the standard width of foam plastic and pressed mineral wool is 60 cm. In order to eliminate the formation of cold bridges, facilitate and speed up the process of installing insulation and minimize the amount of unproductive waste, the span between the rafters should be within 56–58 cm. Rolled mineral wool may have width from 120 cm to 100 cm. Accordingly, their installation requires a different step of the rafter legs. |
| The greater the distance, the more load each rafter leg takes. This affects its dimensions and the total amount of lumber for the roof. Currently, the tree belongs to a very expensive category of building materials, it is necessary to achieve a reduction in consumption. This is done both by using additional stops of the rafter system for optimal load distribution, and by adjusting the number of rafter legs, which allows to reduce the cross section of the roof elements and save expensive boards. |
| Each house has its own architectural features. This refers to the location and number of chimneys and ventilation outlets, the layout of attic spaces, materials for the manufacture of load-bearing walls, the presence of a wooden Mauerlat or a concrete reinforcing belt. Rafters cannot be located above chimneys and ventilation pipes, interfere with the installation of skylights, etc. Such nuances must be thought out during the design of the building, they also affect the distance between the rafters. |
Important. The step of the rafter legs is measured between the axes, when choosing the final parameter, the thickness of the boards must be taken into account. For the installation of insulation, the distance between the side planes, and not the axes of the rafters, is important.
What effect does the type of roofing material have on the distance of the rafters
It is necessary to dwell on this issue in detail, quite a lot of developers do not fully understand the problem. To answer, you need to know the fundamental differences between materials and their effect on the distance and calculation of rafters. We emphasize that we do not mean the performance characteristics of roofing or their design appearance, but structural and physical differences.
- Linear dimensions. Most of all, the dimensions of metal coatings can reach eight meters.
All these materials have fundamentally different methods of fixing to the truss system. But they do not have any effect on the step of the rafters.
- Bending strength. There is an erroneous opinion that for flexible roofing materials you need to reduce the pitch, this is not so. Not a single roof covering is fixed directly to the rafter legs; for this, a crate is made, and when it is arranged, fastening methods are taken into account. Moreover, for some types of roofing, it is necessary to control very accurately - the materials have precisely fixed places provided during manufacture.
- The weight. The calculation of the truss system is influenced only by heavy coatings: piece tiles and asbestos-cement slate. All other types of roofs have such an insignificant mass that it is not taken into account when designing a structure.
Prices for different types of tiles
Roof tiles
Algorithm for calculating the truss system
As mentioned above, the distance between the rafters is set at the initial stage and depends on the characteristics of the heaters. They affect another important parameter - the width of the boards.
It should take into account the minimum thickness of the insulation layer, taking into account the climatic zone of the building. If in cold regions the insulation should have a thickness of 20 cm or more, then for a warmer climate, 10 cm of insulation is enough. Accordingly, the width of the board for rafters is from 20 cm to 10 cm.
Practical advice. You always need to consider the cost of lumber. There are options when it is much more profitable for rafter legs to use boards 10 cm wide, and to increase the depth of the niche for insulation by building up ordinary thin low-quality ones. But in all cases, the main size criterion is the ability to hold the maximum design loads.
The calculation of rafters is carried out in several stages.
Determination of roof forces
Several types of loads act on the roof slope, they have different meanings and their own characteristics of influence on the strength of the system.
- Constant loads. This refers to the weight of building materials for the truss system and the mass of roofing. If heavy types of materials are used as coatings, then their mass must be taken into account.
As for light metal sheets, this is optional.
The fact is that the roof is one of the most important structural element of the building, and they have a safety margin of at least 140%. This means that the structure can withstand loads almost one and a half times more than the calculated ones. The maximum load on the roof is created by snow and wind. The values of these efforts are measured in hundreds of kilograms, and the mass of metal sheets is only a few kilograms per square meter. The safety factor fully covers the possible increase in effort.
- Variable loads. These include snow and wind forces, they are placed in the tables available in building codes and regulations. This takes into account the location of the building (in a city or in an open area), the number of storeys, the shape of the roof, etc. It must be borne in mind that recently the climate has been changing rapidly, and in the tables the information is fifty years old. It is not very correct to use them, it is better to take data from the reports of the hydrometeorological center of your region.
The maximum permanent and temporary loads are summed up and increased by approximately 40% to create a margin of safety for all load-bearing elements. The safety factor can also be taken into account by another method. After performing all engineering calculations, the linear parameters of the rafters are determined, in the final version they are multiplied by a factor of 1.4, the results are used during the creation of working drawings of the truss system. It does not matter which method to use, the main thing is to observe the accuracy of mathematical calculations, and only a specialist with a special technical education can perform them.
The methodology is prescribed in SNiP 2.01.07-85, it has been amended by some formulas adopted in 2008. Before taking into account the distance between the rafters, you should know all the loads acting on them.
Snow loads
Prices for snow guards
Snow guard
Engineers use the formula
Formula 1. Determination of snow load
We have already mentioned that the normative load can differ significantly from the actual one, in this regard, it is recommended to use more modern data. As for the angle of inclination of the roof α, this parameter is set in the initial specifications for the design of the truss system. The coefficient µ is determined by the formula
Formula 2. Definition of µ
One component of several efforts on the rafters has been determined, now you should move on to other types of loads.
Important. Please note that snow loads, depending on the climatic region, range from 120–180 kg/m2. Now it should be clear why the weight of light roofs can be ignored, their efforts are approximately 5–7 kg / m2, this is within the limits of a mathematical error. In addition, a safety factor is applied. 40% of 180 kg is 72 kg, this value is much larger than the mass of metal roofs and has already been taken into account when calculating the strength of the rafters.
Wind loads
These efforts can reach significant values and must be taken into account when calculating the parameters of the rafter legs. There are two types of wind loads. When the slopes are more than 30 °, the wind tries to overturn them, with great effort it presses on the leeward side of the roof. If the slope is small, then due to differences in the speed of air flow, a lifting force appears, tearing the roof from the Mauerlat. Wind loads are determined by the formula
The wind pressure coefficient by height includes several factors. All of them have a complex method of calculations, which are performed by competent thermodynamic engineers.
To facilitate calculations in regulatory documents, there is a ready-made table, a specific coefficient is selected depending on:
- building height;
- terrain type (open or closed);
- urban building heights.
The aerodynamic coefficient may be greater than one or less than one. In the first case, the wind load increases, in the second it decreases slightly. For most buildings, simplified calculations are made for wind loads, it is assumed that the coefficient is 0.8.
The mass of elements of the truss system and roofing
Taking into account the peculiarities of the crate and materials for the manufacture of rafter legs, their total mass can increase the values of the loads on the system in the range of 30–50 kg / m2. As already mentioned, this parameter can be ignored. A large safety factor makes the roofs universal, they can be covered with any kind of roofing materials.
Calculation of rafter legs
The distance between them is available in the design specification, is a stable value and is specified in the design specification. Next, you should find out the linear dimensions of the rafters so that they can withstand the maximum possible effort during operation. The distributed load per linear meter of the leg is determined by the formula
We have all the initial data for calculating the distributed load.
Now you can proceed to the selection of the optimal section of the rafter leg. In this case, one should be guided by the GOST 24454-80 table, which indicates the standard dimensions of lumber (thickness and width).
Nominal dimensions for thickness and width of edged sawn timber with parallel edges and thicknesses of unedged and edged sawn timber with non-parallel edges
Be sure to familiarize yourself with the table, this is necessary to understand the methodology for choosing boards. For example, with a thickness of 16 mm, the maximum board width is 150 mm, and with a thickness of 75 mm, the maximum width increases to 275 mm.
It is necessary to set the width of the board section and, taking into account this parameter, calculate the height. Formula used
It is suitable for cases where the roof slope α< 30°.
If the slope angle α > 30°, then you need to use the formula
- H- the desired height of the board for the rafters;
- Lmax- the distance between the most distant points of the abutment of the rafter. For small slopes, it is equal to the distance from the ridge to the Mauerlat; in other cases, it is necessary to install various types of stops and measure the distance, taking into account their location;
- QR- distributed load on the rafter leg, it was calculated earlier;
- B- the thickness of the board, is chosen arbitrarily, taking into account the individual characteristics of the truss system;
- Rizg- normative indicators of wood resistance to bending.
They depend on the quality of lumber and the type of wood, taken from the tables of state standards. The quality of lumber plays a decisive role in the bending resistance of lumber.
For example, if for the first grade of pine Rizg = 140 kg/cm2, then for the third grade this parameter is reduced to 85 kg/cm2. The standards strictly regulate the bending radius of the roof, if it is too small, then there are high risks of leakage due to a violation of the integrity of the roofing. For all roof elements, the deflection cannot exceed L (length of the working section) / 200.
In SNiP there is a formula for checking the condition under which the deflection does not exceed the established norms
If the sum exceeds one, then it is necessary to increase the thickness or width of the rafter leg.
Calculation example
The number of rafters is known, this value is always determined taking into account the required distance between them. In our case, the pitch is 80 cm, the slope angle is 35 °, the length of the working area is 280 cm. The truss system is made of pine, the bending radius of this material of the first grade is 140 kg / cm2. Piece cement-sand tiles will be used as roofing material. This is a very heavy material, its weight is recommended to be taken into account. The mass of a square meter of a tile reaches 50 kg. Now all the initial data are known, you can start the calculations.
Taking into account the climatic zone, the total wind and snow load is 253 kg / m2, the weight of the tiles should be added to them, for a total of 303 kg / m2. The distributed load on the rafter is calculated by the formula and in our case is 242 kg / m2. It is planned to make rafters 5 cm thick, you need to find their width.
Applying the formula
It is this formula that is used due to the fact that the angle of inclination of the slope is more than thirty degrees. Now it remains to check whether the maximum allowable deflection radius of the rafter will not be exceeded. If the value is less than one, everything is normal. If it is more than one, then it is necessary to increase the linear dimensions of the boards.
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When you need to calculate the distance between the rafter legs
Such a need arises very rarely and mainly concerns non-residential premises. For example, the developer already has boards for the manufacture of the truss system, he needs to know at what distance to fix the rafters so that the roof can withstand the calculated loads. That is, you need to do the reverse calculation. If in the standard situation the distance is known and the dimensions of the boards are selected taking into account these parameters, then in the second case the opposite is true. The dimensions of the rafter boards are known, it is necessary to determine the pitch of the rafters. It is done in this order.
Knowing the total load on the roof and the maximum load on one rafter, we determine the number of rafter legs with a simple arithmetic operation. Of course, all roundings are done upwards; an excess margin of safety will never hurt the truss system. The last stage - the length of the roof slope is divided by the minimum number of rafters and the distance between them is obtained. Rounding should be done in the direction of decreasing the step.
Video - Choosing the distance between the rafters
The rafter is one of the individual elements of the supporting structure of the roof, with the help of which its slope is formed. In the design, the rafter is attached with its upper end to the ridge, and the lower end rests on the Mauerlat, with a straight slope, or on a rack (with a sloping roof). Rafters are made from edged boards with a section of 150x60 mm or a beam of 150x100 mm. The distance between the rafters at which they are located from each other after installation is called a step, which can be in the range of 600 - 1200 mm.
Rafter systems their types
Depending on the type of installation of rafter legs, systems are divided into 3 types:
What may determine the installation step of the rafters
The step of the roof rafters depends on such factors, as:
Taking into account all these indicators when calculating, you can get a solid and reliable roof structure, which for a long time will be a high-quality support for the roof.
It is the calculation, subject to the basics of regulatory loads, allows you to choose the right distance between the rafters. As a rule, the standard values can be determined from the Sheaves, and the calculated ones are derived on their basis separately for each structure.
At the same time, it is customary to use rafter legs with a section of 150x50 mm with an optimal distance between the rafters within 0.8 - 1.8 m / p. but it should be remembered that when the slope of the roof slope changes, the distance between the rafters also changes.
Calculation of the truss structure
The strength and stability of the entire roof directly depends on the quality of the calculation for the selection of the section of the rafter legs and the distance between them. Regardless of which type of coating is chosen: profiled sheet, slate or metal tile, the initial calculation should remain unchanged. After all, each calculation is based on structural strength under mechanical loads, and other effects are no longer important.
When calculating the choice of the optimal distance between the rafters for the roof, are based on the following parameters:
- Type of roofing.
- View of the truss system and design features of the roof.
- feasibility and economy.
For a small house, the design calculation for the roof can be done by yourself. True, the method of calculating the elements for the roof truss system is rather complicated, and it is recommended to do it with the help of programs specially created for this. Especially if you need to calculate a complex sloping roof with a large area, most likely, you will not be able to do without a specialist. At the same time, the calculation of the step of the rafters will also be based on the standards - the minimum step is 0.6 m, the maximum step is 1.2 m.
Method of calculation
Produced in this way.
- The length of the building is measured along the eaves.
- The resulting length is divided by the estimated distance between the rafters. For example, the estimated pitch of the rafters will be 0.8 m / p. (mean distance is considered to be 950 mm).
- After performing this action, one should be added to the result obtained, and the resulting amount should be rounded. Thus, the required number of rafters is obtained on one side of the slope. After that, the length of the building must be divided by the number of rafters received, and as a result, the exact axial pitch of the rafters is determined.
Example, - building length 26.5 m / p. the distance between the rafters is supposed to be 0.8 m. So:
- 26.5 m ˸ 0.8 m = 33.1 33.1+1 = 34.1. As a result, after rounding, it turns out that 34 rafters need to be installed on one slope.
26.5 m/s ˸ 34 art. = 0.77 m - this value is the distance between the rafters along their central axes.
But this is only a general calculation method, which does not take into account the peculiarity of the planned roofing. Therefore, it is recommended by experts to calculate the step between the rafters for a certain roofing material and insulation, for example, for the most popular roofing metal tile today.
Roof structure for metal roofing
metal tile visually imitates ceramic roof tiles. It is made from thin sheet steel by cold stamping. Due to the polymer coating, it has a high resistance to weathering and a rather attractive visual appearance, it is not afraid of sudden temperature changes.
The advantage of metal tiles
Often used in the construction of most mansard roofs.
The cross section of wooden rafters under a metal roof is usually standard 150-50 mm, but the distance between them can be from 600 mm, but not exceed 900 mm (depending on their angle of inclination, which can vary between 22 - 45 degrees). This limitation of the pitch of the rafters is due to the fact that the crate under the metal tile is mounted with a distance from each other within 300 mm. The standard timber, which is used for the crate, has a section of 30x50 mm or 50x50 mm. And this means that each rafter is subjected to additional load.
The stability of the roof structure against various mechanical loads depends on four factors:
An important factor in the calculation of the truss structure, is the estimated maximum load on the roof, the formation of which includes:
- The weight of the entire truss structure.
- The weight of the crate under the cover.
- Weight of insulation and roofing.
- Snow load (determined according to a special, unique for each region, reference book).
- Wind load (also according to a special guide for the region).
- The weight of a person with a tool (repair work, estimated weight - 175 kg / m²).
When installing the rafter system, the distance of the rafter legs should not exceed an average value of 0.9 m / p. except for certain, foreseen in advance cases.
If, when calculating the loads, any inaccuracy is made in the selection of material for the roof and the location of the rafters, it may occur its deformation and destruction of the roofing. A reliable roof structure will be guaranteed only with the correct calculation of the cross-section of the rafters and their installation step.
It should be remembered. There is no universal value for the calculation of truss structures. Each building requires an individual calculation.
Rafters for a shed roof
Shed roofs can often be found on small outbuildings. They can also be used in private residential buildings, but quite rarely. In such cases, the angle of inclination of the roof is rather small, and with such an arrangement of the load-bearing floor beams, they are under a lot of pressure, especially in winter.
Therefore, for a shed roof, load-bearing floor beams are installed from a beam of large section from 60x150 to 100x220 mm, depending on the width of the overlapped span. At the same time, the distance at which the rafters are laid from each other should be within 400 - 800 mm, depending on the angle of the roof.
For pitched roof no complex rafter construction required, they can simply be laid on the walls, without even using a Mauerlat. In regions where winters are quite snowy with a lot of snow, it is recommended to build a roof slope at a maximum angle of 35 ⁰ and place the roof in the “wind”. This reduces windage and leads to its self-cleaning.
Gable roof
Represents a structure assembled from rafters interconnected in the form of a triangle. The upper part, which rests on the ridge, and the lower part on the Mauerlats, located parallel to each other on opposite walls. In simple words, this is a roof consisting of two opposite slopes, connected by a ridge.
Double pitched roof structure, depending on its area, is mounted from separate rigid elements that enhance the strength of the roof. These include racks that support rafters, puffs that connect rafters to each other, jibs, girders, support beams, etc.
For a gable roof, often the rafters are mounted taking into account the insulation in increments of 0.9 - 1.2 m / p. In this case, the strength of the mounted structure will be the highest if the resulting triangle is isosceles. In regions with strong winds, it is recommended to mount the rafters with a slope of approximately 20⁰, and in snowy areas, the optimal angle should be 45⁰.
Although the gable roof is considered classic, it has several alternative "related" types.
mansard roof
For mansard roof, for the calculated parameter for determining the step between the rafters and their number, a load is taken in the range of 40-60 kg for every 1 m / p. rafters, and the maximum deflection from its length is 1/250. Usually, with a properly selected section, this distance along the centers of the rafters is, as for a gable roof, 0.6 - 1.2 m / p.
It should be noted that the average load on the attic is approximately 200 kg / m2. So, with the standard calculation of the section of the rafters, it is recommended to add a small percentage of the margin of safety.
hip roof
Among all roof structures, it is considered one of the most difficult . It's almost a hipped roof., while the rafters of the end slopes are attached with their upper ends to the corner bowstrings, and not to the ridge. Therefore, certain requirements may be imposed on this type of roof during construction. In this case, the rafters are installed similarly to a gable roof at a distance of 60 cm - 1.2 m / p.
Attic rooms under such a roof are made in rare cases, since its slopes “eat up” some area of the attic room, especially in height.
The roof structure is one of the main enclosing elements of the building, the quality characteristics of which are subject to rather stringent requirements.
One of the most common roof sheathing materials is metal shingles, which are made from thin sheets of steel, aluminum, or copper.
From above, the elements are equipped with a polymer coating, which protects metal from aggressive external influences.
Externally, the metal tile is similar to ceramic, but it is more durable. This material is used to cover pitched roofs, the slope of which must be at least 14 degrees.
This is the national team roof frame structure, consisting of many wooden or metal parts. She is rests on load-bearing walls, which are a reliable basis for all overlying elements. The rafter system serves as a kind of skeleton, on the basis of which it is made, - and the roof, as well as laying the roofing finish roofing layer.
truss system
The constituent elements of the truss roof, and their main characteristics:
- Mauerlat. A softwood beam, which is a connecting element between the rafters and the underlying structures. It has a square cross section with a side of 100 or 150 mm. Mauerlat is laid along the load-bearing wall along its entire length. With the help of the Mauerlat, the loads from the roof are evenly distributed throughout the building.
- Sill. A beam having a square section similar to a Mauerlat. It is laid transversely to the bearing walls, as it serves to redistribute the load from the roof racks.
- Rafter leg. From these elements, the main triangular roof structure is created, which experiences the full severity of external atmospheric influences (rain, wind, snow, hail, etc.).
- Rack. Vertical connecting elements that distribute compressive loads from the ridge assembly over the entire area of bearing walls. They are made of square bars, the length of the edge of which is determined by calculation.
- Puff. It is the final horizontal element of the triangle of rafter legs, which does not allow them to crawl under the pressure of external loads and the own weight of the roof. It is used in systems with hanging rafters.
- Struts. Perceive and redistribute bending loads from the ridge assembly.
- Crate. It consists of boards, bars or plywood sheets (in the case of subsequent laying of bituminous tiles), which are located at a right angle relative to the rafter legs, while being an additional rigid element.
- . The junction of two roof slopes.
- Overhang. A roofing element protruding from the load-bearing wall structures at a distance of about 0.4 m. Its purpose is to limit the penetration of moisture to the walls.
- Filly. These elements are attached to the ends of the rafters if they are not long enough to organize an overhang.
Varieties of pitched roofs
Depending on the number of inclined planes, roof structures can be divided into:
In private housing construction, the most commonly used option gable roof, since he has a number of advantages. These include:
- Practicality. The gable roof has a significant angle of inclination, thanks to which rainwater does not accumulate on its surface, and snow and wind loads are distributed most optimally.
- Ease of device and operation. The assembly and joining of two pitched elements is much easier than with complex roof structures. In addition, the repair of such a roof will also be simple.
- Aesthetics. A roof with a gable structure is organically written into the surrounding infrastructure.
- Reliability(if done correctly).
- Democratic price constituent materials.
Types of pitched roofs
Gable roof - truss system for metal tiles
Frame from rafters under a gable roof from a metal tile has no significant differences from structures with other covering roofing materials.
But, in view of the fact that metal thin sheets have a low specific gravity, the rafters will experience less constant load.
This allows you to reduce the value of their cross section, due to which can save a lot on the purchase of wood materials.
Ideal for metal roofing the angle of inclination must be at least 14 degrees.
For a roof with two pitched elements, the following apply: frame options:
Laminated rafters under a metal tile.
In this case, 2 load-bearing rafter legs are fastened together using lying down(horizontally) and racks(vertically). The bed is laid parallel to the Mauerlat element, while taking on some of the force effects. The rafter system under the metal tile is taken over only bending loads, which significantly affects the selection of the calculated cross section. Such a system can be used for buildings with large and small spans.
Types of rafters
Hanging rafters.
Unlike layered systems, in this embodiment, two rafter legs fastened together only in the ridge knot. In this case, significant bursting forces arise on the bearing elements, which limits the use of hanging rafters only for buildings with a span of no more than 6 m.
They can be made of wood or metal, as well as installed at the bottom (acting as a supporting beam) or at the top of a triangular structure. It is worth considering that the higher the puff is located, the greater the effort it will take.
NOTE!
To ensure the quality of the tightening, care must be taken on the reliability of fastening with load-bearing rafter legs.
Combined variant
Used to create an original roof structure. Includes elements of both hanging and layered systems.
How to calculate the angle of inclination of the rafters?
To implement a gable roof, you need to know a few geometric values of the building, namely:
- Half span - L;
- The distance from the load-bearing wall to the roof ridge (or the height of the support post) - H.
Standard formula: α = arctg(L/H)
Where α is the desired angle of inclination of the roof.
Knowing this value, you can calculate the length of the supporting rafter leg:
l = H/sinα.
Where l is the length of the truss element.
Rafter Angle
How to calculate the load?
To carry out the correct selection of parts of the roof frame, it is necessary calculate live and permanent load values acting on its structural elements. 
The constant load includes the weight of all elements, as well as the mass of the load-bearing elements themselves and the crate.
The composition of temporary loading options includes force effects from wind, snow cover, rain masses, as well as the weight of a person (to take into account options for subsequent repairs).
Dead Load Calculation
Roofing cake weight.
It is determined by adding the masses of all its elements, namely steam, hydro and thermal insulation, as well as roofing from metal tiles. In this case, the weight of one linear meter (can be found in the regulatory documentation) is multiplied by the value of its length.
The weight of the truss system.
It is determined by adding the weight values of the crate, rough flooring, as well as the supporting frame. The mass of each element is calculated by the formula:
M=V*p,
Where V is the volume of the element, calculated depending on the geometric characteristics of the cross section and the length of the element;
P - The density of the wood used (depending on the species).
Total permanent load \u003d weight of the rafter system + weight of the roofing pie.
Calculation of live loading
Conducted in accordance with regulatory documents ( SNiP 2.01.07-85 "Loads and impacts" or Eurocode "Actions on structures" part 1-4).
To determine the value of the wind effect, the roof structure is conventionally divided by height into several parts. For each of them, the value of the wind load is calculated. To obtain the total wind pressure, they must be summed.
Formula for calculation:
Wm=Wo×k×c,
Where Wm is the value of the wind load;
Wo is the normative value of wind pressure determined from zoning maps;
k - wind pressure coefficient (determined depending on the height according to the regulatory documentation);
c - aerodynamic coefficient (for a gable roof - 0.8).
Determined by the formula:
S = µ×So;
Where So is the normative value of the snow load, determined from the zoning map.
µ is a coefficient that is determined depending on the angle of the roof:
- For α≤30 deg. — µ=1
- For α≥60deg. -µ=0
- For 30≤α≤60 deg. – µ=0.033×(60-α)
Snow load areas
How to choose a beam and calculate the pitch of the rafters under the metal tile?
Determining the value of the cross section of the beam of the truss element is carried out in several stages.
Calculation of the load distributed on each linear meter of the structure:
Qр = L×Q;
L - Step of the rafters.
The L value is calculated as follows:
The length of the roof slope is divided by the estimated step of the structures (for convenience, it is most often taken equal to 1). Then 1 is added to the resulting value. The resulting value reflects the number of rafters that need to be installed on one pitched roof surface. At the last stage, the value of the axial distance between the rafter elements is determined by dividing the length of the roof slope by the number of rafters.
The distance between the rafters under the metal tile - the standard step is 0.6-0.95 m.
Rafter pitch
Then we determine the maximum working area of \u200b\u200bthe rafter leg (Lmax). We proceed to the calculation of the cross section. To do this, we find its height using the formula:
H ≥ 8.6*lmax * sqrt(Qp/(b*r)), with roof slope α<30 град;
H ≥ 9.5*lmax * sqrt(Qp/(b*r)), with a roof slope α≥30 degrees;
Where b is the width of the cross section,
r - the value of the normative resistance of wood to bending loads (determined according to regulatory documentation, depending on the type of wood).
To simplify the calculations, you need to use the standardization table for truss elements (GOST 24454-80 “Softwood lumber. dimensions").
If the inequality is not observed, it is necessary to increase the value of the geometric characteristics of the section and repeat the calculation.
What is the difference between the truss system for cold and warm roofs?
The main difference between these two roofs is the support system of the truss elements. In the case of a warm attic, the main supporting element is the Mauerlat, as well as the supporting beam system. In a cold roof, rafters are installed directly on load-bearing walls.
Installation of rafters under the metal tile
All installation work on the installation of the roof is carried out at a sufficiently high height. To minimize the risk of falls, as well as greatly simplify work at height, you can assemble the frame of the supporting truss system on the ground.
To do this, you need to create a template from the boards, according to which further assembly will be carried out.
It is made in several stages:
- The boards are raised above the walls of the building, leveled, and then fasten together with the help of a nail.
- Align the angle of the boards in accordance with the project, by lowering and raising them. The elements are fixed.
- The result should be a structure that resembles in shape the future truss system, made in accordance with the estimated geometric dimensions of the roof.
- The template is lowered to the ground, in accordance with it, the finishing elements are fixed to each other. More details in the video below.
Then you should take care of installing the support element - Mauerlat. As mentioned earlier, it is laid on load-bearing walls in the longitudinal direction. Fastening is carried out using studs (on an armored belt or masonry) or using a wire rod (for buildings with a small roof height).
CAREFULLY!
When using a hairpin connection, connecting elements do not need to be tightly sealed into the wall. They should protrude from the wall by 30-40 mm, as the nut will be screwed onto the studs.
The next step is to create ridge run, serving as a supporting part for the entire structure of a gable roof. It is made from timber or hewn logs. If the span of the building is not more than 6 m, it can be supported without additional supporting elements. Otherwise, construction trusses must be used for installation.
Mounting. Part 1
After installing these elements, it is possible to carry out the lifting and installation of the main truss element, assembled according to the template. Fastening with a Mauerlat can be carried out in 2 ways:
Rigid connection. It is carried out with the help of corners and beams. Less often, fastening is used by means of washing down on rafter legs, followed by fixation with nails or staples.
Features: in addition to the main connection, it is necessary to tie the rafters to the wall using anchors or a wire structure.
Sliding. It is based on the creation of a swivel joint. It is made by joining elements using cuts. The elements are connected with a metal embedded part with holes for bolts, or with 2 nails that need to be hammered in at an angle.
It is necessary to carry out the installation of wooden trusses in a certain sequence. First, extreme trusses are installed, located at the ends of the building. Then a cord or rope is pulled between them, with the help of which the verticality of their installation is checked. Further, under the cord, further installation of truss structures is carried out in accordance with the specified design step.
Mounting. Part 2
Creating a roof from a metal tile is a rather laborious process that requires certain skills and a full hand. Therefore, for proper installation, you must at least work under the supervision of a competent specialist.
Useful video
Video instruction for self-installation of rafter legs: