The distance between the rafters: principles and examples of calculating the pitch of the truss system. The distance between the rafters of a gable roof The pitch of the rafters of a gable roof with slate

The roofs of modern private houses can have a different configuration. The most popular type of roofing is gable. The owners of suburban areas consider reliability, rather attractive appearance and economy to be the undoubted advantages of such a roof. The design of this type of roof is extremely simple, and therefore it will not be difficult to build it, including with your own hands.

Lightweight and durable sheet materials are what a gable roof is most often sheathed with. Decking, for example, is just perfect for such a design. The roof of this variety protects the interior space of the house well, lasts a long time and is inexpensive. Of course, the procedure for erecting a truss system under corrugated board, as well as under any other material, has some of its own characteristics.

Where to begin

How is a gable roof assembled? under the corrugated board it will turn out to be reliable, and the roof itself will be neat only if a detailed design project is first drawn up. When developing the latter, you need:

determine the angle of inclination of the slopes;

determine the type of materials needed for construction;

make detailed drawings of the frame indicating the method of connecting all nodes.

If all these operations are performed without errors, you will end up with a solid gable roof. The truss system for corrugated board, a drawing of which can be made using, for example, special software on a computer, will last as long as possible.

Load calculation

This step when drafting a roof should not be skipped in any case. Properly performed calculations are a guarantee that in the end you will get a durable gable roof. The rafter system for corrugated board (a photo of the assembly of such a frame can be seen below) is mounted taking into account the following parameters:

the weight of all used for materials;

The values ​​of the last two indicators can be found in special tables developed specifically for each specific region.

All figures obtained as a result of calculations must be added and multiplied by a reliability factor of 1.1. Having performed the calculation of the gable roof truss system in this way, it will be possible to determine, first of all, the type of materials necessary for assembly. Also, taking into account the final load indicator, the optimal angle of the roof frame supports is chosen.

Ramp angle

In addition to the load, when choosing this indicator, the features of the roofing material itself should also be taken into account. The angle of inclination of the slopes can be any, but not less than 12 degrees. If you make the roof more gentle, in the future it will leak. At the same time, the sheets themselves will begin to sag under the weight of snow in winter. That is, the roof will have to be constantly repaired. And this, of course, additional costs.

In Central Russia, taking into account wind and snow loads, it is almost universally allowed to build roofs with a slope angle of 30-45 degrees. This option is ideal for corrugated board. In the southern regions, the angle of inclination of the roof slopes of houses may be smaller, and in the northern regions, on the contrary, it may be larger.

Among other things, when choosing a method for installing rafters, it is worth considering how exactly the attic will be used in the future. If it is supposed to be insulated and equipped for living quarters, it is better to make the slopes more abruptly. But of course, in this case, the construction of the roof will cost a little more, since you will have to use more materials.

What to build from

From what specific materials can such a gable roof be built? with their own hands, which, in any case, is a responsible matter, should, of course, be as reliable as possible. But one of the advantages of corrugated board is that it is light in weight. Therefore, it is not necessary to use any special, very durable, materials for assembling the frame for it. For rafters, such a roof is quite suitable for a standard beam of 150x100 mm. It is advisable to use lumber of a larger section only if the attic is supposed to be used as a living space. Rafters for such a roof are usually made of timber 200x100 mm.

The crate can be assembled from edged boards 30x100-150 mm. Too wide lumber in this case cannot be used. After a while, such boards will simply dry out and become very warped, which will have an extremely negative effect on the reliability of the roof. Under the Mauerlat, it is better to take a thicker beam - 200x150 mm.

Step between structural elements

Too often, frame supports under light metal sheets are also not necessary. The optimal distance between the rafters of a gable roof for corrugated board is 60-80 cm. As for the crate, the step between its elements is selected taking into account the variety of sheets used. So, for a thick material of 0.6-0.7 mm with a profile height of 3.5 cm, boards can be packed at a distance of up to 1.5 m from each other.

For a standard H grade corrugated board, the crate pitch is usually 60-70 cm. The same distance is suitable for CH material. A thin sheet C is mounted on a crate stuffed in 10 cm increments or knocked down with a solid sheet of 12 mm plywood or OSB sheets.

How a gable roof is assembled. Rafter system for corrugated board

The roof frame of the house is mounted in a few steps:

Mauerlat is installed;

rafters are mounted;

if necessary, a heater is installed;

the rafters are covered with a waterproofing;

crate is stuffed.

Mauerlat installation

They begin to assemble such a structure as a gable roof made of corrugated board, with their own hands from the installation of this particular element. Mauerlat is mounted on top of the building box. As a basis for a rafter system, this element is used only on brick, monolithic or block walls. In cobbled and chopped, its role is played by the upper crown. Fasten the timber to the walls using anchor bolts. You can also use steel dowels, galvanized wire from 5 mm thick or staples.



Ways of fastening rafters

The supports themselves can be attached to the Mauerlat in two ways. For brick, block and monolithic houses, rigid fixation technology is used. In this case, they are attached to the Mauerlat using galvanized steel corners and self-tapping screws or a “knot” of three nails (one in the upper plane, two on the sides).

On chopped and cobbled walls, the rafters are fixed using a sliding method. The fact is that such houses for the first time after construction give a strong shrinkage. Therefore, a rigidly fixed roof frame in the future can simply lead. With the sliding method of installing the rafters, special fixing elements, “sleds”, are used to ensure the mobility of the supports in a small range.

How to install

The support elements of the frame must be mounted correctly. In this case, you get a neat symmetrical gable roof. The rafter system for corrugated board, as well as for any other material, must be assembled using a template. In this case, all legs will have the same length. For gable roofs from corrugated board, they can use both simple straight rafters (with an edge cut at the required angle), and a variant with landing nests under the Mauerlat. Between themselves at the top of the frame supports are usually fixed by means of special thick steel plates.

The end trusses are always installed first. Further, a cord is pulled between their highest points. Then, focusing on it, intermediate farms are installed. On large roofs, a ridge run on supports is pre-mounted between the extreme paired rafters. The latter are securely fixed on the floor beams.

Installation of insulation and waterproofing

If the attic is supposed to be made residential, it will, of course, have to be insulated during the construction of the roof. On corrugated roofs, mineral wool is most often used for this purpose. As a support under it, from the side of the attic, a wire is pulled onto the rafters. The mats themselves are installed by surprise.

The waterproofing agent on the rafters is sewn with a slight sag (2 cm). It is impossible to stretch the film strongly. Otherwise, when the frame moves, it can simply break. The waterproofing strips are mounted horizontally from the bottom up with an overlap of at least 10-15 cm.



Lathing installation

Support boards for corrugated boards are fixed to the rafters with nails with a diameter of about 3-3.5 mm. The length of the fasteners in this case should exceed the thickness of the fixed elements themselves twice. The assembly starts from the eaves. Boards should be fastened to each rafter with two nails. The last two rows at the ridge are stuffed without a gap.

After the crate is assembled, you can proceed to sheathing the frame with the corrugated board itself. At the final stage, the gables of the roof are upholstered with a board.

What you need to know

Wood is a material that is very easy to process and at the same time quite durable. That is why the do-it-yourself truss system of a gable roof is most often assembled from timber and boards. However, unfortunately, lumber cannot boast of too long a service life. Sooner or later, as the roof of the building begins to dry out or rot. Therefore, before assembling it, the timber and boards must be treated with special compounds that increase their moisture resistance.

Another disadvantage of wood is flammability. To minimize the risk of fire, the lumber used to build the frame of the building should, among other things, be carefully treated with a fire retardant.



This is how the gable roof is assembled. The rafter system for corrugated board, as you can see, is mounted elementarily. Nevertheless, the installation of a roof frame is a very responsible matter. If any technologies are violated, the owners of the house will subsequently have problems with leaks, the need to replace individual elements, etc. Therefore, it is worthwhile to approach the assembly of the roof truss system with maximum responsibility.

The roof frame must be reliable and durable. But without well-conducted calculations, this is difficult to achieve. In the process of carrying out the calculations, it is determined at what distance to put the rafters on the roof.

What can lead to an incorrect or inaccurate calculation of the loads that the truss structure will undergo? To the most negative consequences, ranging from deformation of the rafter legs and damage to the roof covering and ending with the collapse of the base of the roof frame. Therefore, when designing buildings, the list of mandatory calculations contains data on what should be the distance between the roof rafters. There is a certain technique that allows you to calculate this value.

Method for calculating the distance between the rafters

The distance between the rafters on the roof is called the pitch of the rafters. As a rule, the step of the rafters in the roof structure usually exceeds one meter, and the minimum gap varies within 60 centimeters.

The calculation of the required number of rafters for a roof of a certain length and the pitch of the rafters is carried out as follows:

Roof structure for metal roofing

During the construction of suburban private real estate, metal roofing can be found most often. This roofing material is similar to the flooring made of clay tiles, but in comparison with it has a number of advantages. Sheet metal tiles are easy to install, so you can build a roof in a shorter time, the rafter system for metal tiles is also not difficult.

Metal tiles are lighter than ceramic products, the difference in weight sometimes reaches 35 kilograms per square meter, depending on the thickness of the products (read also: ""). Due to a significant reduction in the weight of the roofing, it becomes possible to reduce the thickness of the elements of the truss structure and the cross-sectional dimensions of the lathing bars, and increase the installation step of the rafters.

Under the metal-tiled coating, the rafter legs are mounted with a distance of 600 to 950 millimeters, while the cross section of the construction material is 150 by 50 millimeters. According to experts, in this case, if a heater 150 millimeters thick is placed between the rafters, then such thermal insulation will create comfortable conditions for staying in the attic room. At the same time, for greater reliability, it is advisable to choose a 200 mm insulation.

When installing rafters, to ensure ventilation of the space that is filled with insulation, holes with a diameter of 10-12 millimeters are drilled in the rafters near the upper roof.

The technology for creating a truss system for metal tiles does not differ significantly from structures for other types of roofing materials. The only feature is that at the rafters, the upper support is mounted on the ridge run from above, and not on the side of the ridge beam. The presence of a free zone between the rafters ensures air circulation under the roofing, and this, due to the use of metal material, reduces the risk of condensation.

Shed roof truss system, main advantages and characteristics ").

The distance between the rafters of a gable roof is made taking into account the size of the heat insulator that is laid between them. An approximate step between the rafter legs is 1-1.2 meters (read: ""). Rafters regulate the size of the roof overhang.

The quality and durability of the roof largely depend on the correct calculation of the pitch of the rafters. It affects the laying of insulation, the fastening of roofing material and the installation of additional parts. Pay attention only to the requirements for the distance under the roofing sheets, then there may be problems with the insulation boards. And vice versa, adjusting the size of the insulation can make the frame too weak, and in winter there will be a threat of collapse. How to correctly calculate the distance between the roof rafters? We will talk about this in our article.

As a rule, the pitch between the rafters depends on several factors. But usually the distance fluctuates between 0.6 and 1.2 meters. These figures are rounded, in practice the distance may differ by several centimeters downwards. For an accurate determination, the following calculations must be performed:

1. Determine the length of the slope by measuring along the eaves of the house. Suppose that it turned out 17.8 meters.
2. Divide the figure obtained by the planned step of the rafters. If it was decided that the distance between the beams would be 0.8 meters, then 17.8 / 0.8 = 22.25.
3. Further, we round the result up and add one: 23 + 1 = 24. At this stage, the number of rafters required is determined.
4. Now we calculate the distance between the axes of the beams. To do this, the length of the slope must be divided by the number of rafters: 17.8 / 24 \u003d 0.74 m.

So, you will determine the actual distance at which the rafters for a roof made of metal or some other material should stand.

Often, when planning a roof frame, specialized calculators are used. They enter basic data and get the finished result. Nevertheless, you should not completely trust the program, since only a person can take into account some conditions. When calculating, you can understand how the system works, what loads are acting. If necessary, adjustments are made to strengthen or facilitate the design. Self-calculation will help to more accurately determine the required pitch of the rafters of a gable and single-pitched roof.

Step calculation depending on the roofing material

For each coating, its own standards and dimensions are developed for the truss system. The main factors are the strength of the material, its weight and resistance to external loads. Consider the main types of coatings.

Decking

The step of the rafters under the corrugated board can vary from 0.6 to 0.9 m. The final result depends on additional conditions, which we will discuss below. If for some reason the step should be larger, a transverse board with a large section is added. In this case, usually the rafters themselves are chosen with dimensions of 50x100 or 50x150 mm.

In addition to the rafters, a crate of 30x100 millimeters is used. The gap between the boards should be about 0.5 meters. It can be more, it all depends on the type of corrugated board and the height of the wave. Sheathing boards protruding beyond the boundaries of the eaves should have a thickness of 1.5 cm more than usual. This is done based on the need to mount ventilation, a chimney or a drain.

Ceramic tiles

The main difficulty in designing a roof frame for ceramic tiles is the large weight of the material for the roof. It is 10 times heavier than corrugated board and accounts for 40 to 60 kg per square meter.

Under such a coating, dry beams are used, with a moisture content of only 15%. Their cross section should be either 50x150 or 60x180 mm. With such indicators, the maximum allowable distance between the rafters can be 1.3 meters. The minimum allowable step through which rafters can be placed is 0.8 m. The roof slope affects the final decision: at 15 °, the step is 0.8 m, at 75 ° - 1.3 m.

The length of the rafter leg is also taken into account, the shorter it is, the larger the gap, and vice versa. In addition, if the slope is less than 45 °, and movement is planned on the surface, the beams are placed in increments of no more than 0.85 m.

The distance between the boards of the crate is selected so that each intersection of the tiles has its own base. The standard length of the material is 400 mm, and the amount of overlap during installation varies between 55 and 90 mm. It turns out that the distance between the central axes of the crate can be either 310 or 345 millimeters.

metal tile

In recent years, the question has been particularly acute: how to determine the distance between the rafters under the metal tile? It imitates more expensive ceramic material, but at the same time it has the positive properties of corrugated board. Installation of such a coating is simple and does not require large financial costs. All this makes the material attractive for the roofs of private houses.

The mass of metal tiles is less than ceramic, so a simpler frame is required. The section of the beams is reduced to 50x150 mm and the step between the crate is increased. The step of the rafters under the metal tile varies between 0.6 and 0.95 m.

Ondulin

The allowable step for covering from ondulin is 0.6-0.9 meters. The standard section of the rafters is 50x200 millimeters. Such dimensions will allow to withstand the load of the roofing pie on the gable roof.

On top of the counter-lattice, a crate of bars measuring 40x50 millimeters is attached. The pitch between the central axis is 600 mm.

Additional factors

When calculating the pitch of the rafters, attention is paid not only to the type of roofing. Many other points are taken into account, data on which can be found in reference books and SNiPs. Here are some of the factors:

1. Snow and wind loads. The more snow falls in winter and the stronger the winds blow, the smaller the distance between the beams should be. But, if you increase the angle of the roof slope over 45 °, then it is allowed to increase the step.
2. Choice of heater. To reduce the cost of insulation boards, it is recommended to take into account their standard sizes. They produce mats with a width of 600, 800 and 1200 mm. If this condition is neglected, then a large number of undercuts will appear, cold bridges and construction will be delayed.
3. lumber quality. It takes into account the type of wood, grade and section. As mentioned above, strength is affected by how well the beam was dried. When buying wood, pay attention to evenness and the presence of defects in the form of knots and traces of pests.
4. Floor beams and puffs. If you equip the attic floor under a gable roof, then the maximum distance between the rafters should be 0.75 meters.

The calculation of the distance between the beams for a gable and single-pitched roof is different. Even if he makes several slopes, for each it is necessary to make a calculation individually. This is especially true for buildings with different lengths of cornices.

Taking into account all these factors, it is possible to determine how far the rafters should be installed. Although all calculations can be done independently, since reference data is freely available, it is better to turn to professionals. They have experience in designing, they will quickly determine the required distance between the rafters. So, you will avoid possible mistakes and be sure of safety.

When carrying out installation work, it is very important to correctly determine the pitch of the rafters under the metal tile. It is important to understand that it is quite difficult to make such calculations, and it is on how correctly everything will be done that the level of roof strength directly depends. Many experts argue that the operational life of not only the roof, but also the structure itself depends on the level of reliability of attaching the metal tile to the rafters. As a result of the fact that the metal tile has a rather large weight, the truss system must be strong. In addition, beginners will not be able to lay roofing material. In order to properly carry out installation work, it is recommended to responsibly approach the calculation of the step of the truss system.

The device of the truss system under the metal tile

Do-it-yourself rafter system for metal tiles is not as difficult to perform as it is to make the appropriate calculations. It is important to understand that the type of system used depends entirely on the type of roof. To date, the truss system for a gable roof under a metal tile is considered the most popular option. Such truss systems, as practice shows, are great for installing metal tiles on the roof of a dwelling. To equip the rafters, wooden blocks of different lengths are used - the choice, as a rule, directly depends on the length of the legs. Many experts recommend paying attention to the fact that it is necessary to choose the right angle of inclination, which can vary from 15 to 30 degrees - this value is considered optimal.

Calculation of the distance between the rafters for metal tiles

The step of the rafter under the metal tile is the distance from one rafter leg to the other, to put it differently, then this distance is called the step of the rafter system by roofers. This distance directly depends on the type of roofing chosen. As practice shows, for all roofing materials, including metal tiles, this size varies from 0.6 to 1.2 m. However, it is important to understand that it is best to carry out accurate distance calculations for each material separately - it must be individual approach.

Before proceeding with the installation work, it is necessary not only to determine the distance between the rafters, but also to understand how much material is required for the legs. To find out the pitch of the rafters of a gable roof under a metal tile or a shed, it is recommended to adhere to the following recommendations:

1. First of all, calculate the length of the roof slope from the metal tile. Measurements, as a rule, are carried out along the eaves of the roof.
2. After that, it is possible to choose an arbitrary step. It is important to take into account the fact that the step limits for metal tiles can vary from 0.6 to 1 m. For example, initially it became necessary to adhere to a distance of 0.8 m. The length of the slope should be divided by the chosen value for the step of the truss system. Let's say that the length of the slope is 12 m, it turns out that 12 / 0.8 and it turns out 15. After that, you need to add 1 to the resulting value. In the case when the number turns out to be fractional, then it should definitely be rounded up. Thus, it turns out 15+1=16. This value shows exactly how many rafter legs should be installed on the roof. Then you need to take the length of the slope and divide it by the resulting number of legs. It turns out that 12/16 and it turns out 0.75 m.

Many experts recommend entrusting these works to professionals, which will allow you to get the desired result in the future. It is important to understand that there is a high probability that mistakes will be made, which will ultimately negatively affect the finished roof structure.

Attention! As practice shows, such calculations can be used for any roofing materials.

Installation of rafters under the metal tile

Since installation work is carried out at a great height, if necessary, it is possible to assemble the frame for the supporting truss system on the ground, and then lift and fix it on the roof. For these purposes, it is best to create a special template or scheme of the truss system for metal tiles from boards, according to which the installation will be carried out.

The whole process is carried out in stages:

1. First of all, the boards are raised above the wall of the building, leveled and fixed with nails.
   
   
2. According to the project, the boards must be aligned at a certain angle of inclination, after which all elements are securely fixed.
   
   
3. Ultimately, a structure should be obtained that, in appearance and shape, resembles a truss system. Manufacturing must be carried out taking into account the calculations made.
   
   
4. Then you need to lower the template to the ground and install all the finishing elements.

Do not forget about the installation of supporting elements, namely, Mauerlat. Laying is carried out on load-bearing walls in a longitudinal position. As fasteners, special studs or wire rods are used.

The next step is the installation of the ridge run, which is used as a supporting part for all roofs. For work, you will need to use a beam or a hewn log. In cases where the length of the slope does not exceed 6 m, then additional supporting elements can be dispensed with.

When the installation of all elements has come to an end, you can proceed with the installation and further installation work on the arrangement of the truss system according to the attached template.

Mauerlat can be fixed in several ways:

• rigid connection- in this case, a beam or corners should be used, nails and staples are used much less often, a feature is the fact that it will be necessary to additionally tie the rafters to the wall, using anchors for this purpose;
• sliding way- in such cases, an articulated connection must be created, it can be done by joining the elements with cuts.

During operation, the step distance of the truss system must be observed.

When choosing a rafter pitch for a metal tile, you should pay attention that the distance should not be less than the minimum allowable value and more than the maximum allowable value, that is, it can vary from 0.6 m to 1.2 m.

If it was decided to choose a distance from 0.6 to 1 m, then the length of the rafters in this case should not be more than 6 m. In the event that it is reduced, it is allowed to make a run up to 1.2 m.

If the legs of the truss system are installed more often than after 0.6 m, then this will only contribute to a significant weighting of the roof and, as a rule, an increase in costs. If necessary, the distance can be 1.2 m, but it is important to understand that in this case the level of bearing capacity will be significantly reduced, which will negatively affect the structure of the metal tile.

If necessary, you can increase the level of strength, then for this you will need to perform a continuous crate of wood boards. In such situations, the step can vary from 0.2 to 0.3 m. This option is used extremely rarely, since its implementation requires much more investment.

An interval of 1.2 m is almost never used during installation work. This is due to the fact that despite the rather high level of strength, the metal tile will begin to sag under the weight of snow in the winter or with strong gusts of wind.

An important value is the cross section of the timber used for arranging the truss system. It depends on him what distance will be chosen. The best option is the choice of wooden bars with a cross section of 150 * 150 mm, while the crate can be installed at intervals of 4 to 7 cm. In this case, it is recommended to take into account the step of the transverse wave.

Conclusion

The pitch of the rafters under the metal tile can only be determined after the appropriate calculations have been made. It is important to understand that if the distance is quite small, this can significantly increase the material consumption and project budget, and if the distance is made too large, then the roofing material will begin to sag during operation, which will lead to deformation of the metal tile.

The gable roof is formed on the basis of a frame that combines the elementary nature of the device and unsurpassed reliability. But the backbone of the roof in two rectangular slopes can boast of these advantages only in the case of a careful selection of rafter legs.

Parameters of the gable roof truss system

It is worth starting the calculations if you understand that the truss system of a gable roof is a complex of triangles, the most rigid elements of the frame. They are assembled from boards, the size of which plays a special role.

Rafter length

The formula will help determine the length of durable boards for the truss systema²+b²=c², derived by Pythagoras.

The length of the rafter can be found by knowing the width of the house and the height of the roof

The parameter "a" denotes the height and is self-selected. It depends on whether the under-roof space will be residential, and also has certain recommendations if an attic is planned.

Behind the letter "b" is the width of the building, divided in two. And "c" represents the hypotenuse of the triangle, that is, the length of the rafter legs.

Let's say that the width of half of the house is three meters, and it was decided to make the roof two meters high. In this case, the length of the rafter legs will reach 3.6 m (c=√a²+b²=4+√9=√13≈3.6).

To the figure obtained from the Pythagorean formula, 60–70 cm should be added. Extra centimeters will be needed to take the rafter leg out of the wall and make the necessary cuts.

The six-meter rafter is the longest, therefore it is suitable as a rafter leg

The maximum length of a beam used as a rafter leg is 6 m. If a strong board of greater length is required, then they resort to the method of fusion - nailing a segment from another beam to the rafter leg.

Section of rafter legs

For various elements of the rafter system, there are standard sizes:

• 10x10 or 15x15 cm - for Mauerlat timber;
• 10x15 or 10x20 cm - for the rafter leg;
• 5x15 or 5x20 cm - for running and brace;
• 10x10 or 10x15 cm - for the rack;
• 5x10 or 5x15 cm - for lying down;
• 2x10, 2.5x15 cm - for purlins.

The thickness of each part of the supporting structure of the roof is determined by the load that it will experience.

A beam with a section of 10x20 cm is ideal for creating a rafter leg

The section of the rafter legs of a gable roof is affected by:

The pitch of the rafters affects the cross section of the rafter legs most significantly. Increasing the distance between the beams entails increased pressure on the supporting structure of the roof, and this obliges the builder to use thick rafter legs.

Table: cross-section of rafters depending on length and pitch

Variable impact on the truss system

The pressure on the rafter legs is constant and variable.

From time to time and with varying intensity, wind, snow and precipitation affect the supporting structure of the roof. In general, the slope of the roof is comparable to a sail, which can break under the pressure of natural phenomena.

The wind tends to overturn or raise the roof, so it is important to make all the calculations correctly.

The variable wind load on the rafters is determined by the formula W \u003d Wo × k x c, where W is the wind load indicator, Wo is the value of the wind load characteristic of a certain area of ​​\u200b\u200bRussia, k is a correction factor determined by the height of the structure and the nature of the terrain, and c is the aerodynamic coefficient.

The aerodynamic coefficient can range from -1.8 to +0.8. A minus value is typical for a rising roof, and a positive value is for a roof that is being pressed by the wind. In a simplified calculation with a focus on improving strength, the aerodynamic coefficient is considered equal to 0.8.

Calculation of wind pressure on the roof is based on the location of the house

The standard value of wind pressure is recognized from map 3 of Appendix 5 in SNiP 2.01.07–85 and a special table. The coefficient that takes into account the change in wind pressure with height is also standardized.

Table: standard value of wind pressure

Table: value of coefficient k

The wind load is not only affected by the terrain. The housing area is of great importance. Behind the wall of tall buildings, the house is almost in no danger, but in open space the wind can become a serious enemy for it.

The snow load on the rafter system is calculated by the formula S = Sg × µ, that is, the weight of the snow mass per 1 m² is multiplied by a correction factor, the value of which reflects the degree of slope of the roof.

The weight of the snow layer is indicated in the SNiP "Truss Systems" and is determined by the type of area where the building was built.

Snow load on the roof depends on where the house is located

The correction factor, if the roof slopes heel less than 25 °, is equal to one. And in the case of a roof slope of 25–60 °, this figure decreases to 0.7.

When the roof is tilted more than 60 degrees, the snow load is discounted. Still, snow rolls off a steep roof quickly, without having time to have a negative impact on the rafters.

Permanent loads

Loads that act continuously are considered to be the weight of the roofing pie, including the lathing, insulation, films and finishing materials for arranging the attic.

Roofing cake creates constant pressure on the rafters

The weight of a roof is the sum of the weights of all the materials used in the construction of the roof. On average, it is 40–45 kg / sq.m. According to the rules, 1 m² of the truss system should not account for more than 50 kg of the weight of roofing materials.

So that there is no doubt about the strength of the rafter system, 10% should be added to the calculation of the load on the rafter legs.

Table: weight of roofing materials per 1 m²

Number of bars

How many rafters will be needed to equip the frame of a gable roof is set by dividing the width of the roof by a step between the bars and adding one to the resulting value. It indicates an additional rafter that will need to be placed on the edge of the roof.

Suppose it is decided to leave 60 cm between the rafters, and the length of the roof is 6 m (600 cm). It turns out that 11 rafters are needed (taking into account the additional timber).

The gable roof truss system is a construction of a certain number of rafters

The step of the beams of the supporting structure of the roof

To determine the distance between the beams of the supporting structure of the roof, you should pay close attention to such points as:

• weight of roofing materials;
• the length and thickness of the beam - the future rafter leg;
• degree of slope of the roof;
• level of wind and snow loads.

After 90-100 cm, it is customary to place the rafters in case of choosing a light roofing material

A step of 60–120 cm is considered normal for rafter legs. The choice in favor of 60 or 80 cm is made in the case of the construction of a roof inclined by 45˚. An equally small step should be if you want to cover the wooden frame of the roof with heavy materials like ceramic tiles, asbestos-cement slate and cement-sand tiles.

Table: rafter pitch depending on length and section

Formulas for calculating the truss system of a gable roof

The calculation of the truss system comes down to setting the pressure on each beam and determining the optimal section.

When calculating the truss system of a gable roof, they act as follows:

1. According to the formula Qr \u003d AxQ, they find out what is the load per linear meter of each rafter leg. Qr is the distributed load per linear meter of the rafter leg, expressed in kg/m, A is the distance between the rafters in meters, and Q is the total load in kg/m².
2. They proceed to the determination of the minimum cross-section of the beam-rafter. To do this, study the data of the table listed in GOST 24454–80 “Softwood lumber. Dimensions".
3. Focusing on the standard parameters, choose the width of the section. And the height of the section is calculated using the formula H ≥ 8.6 Lmax sqrt (Qr / (B Rbend)) if the roof slope α< 30°, или формулу H ≥ 9,5·Lmax·sqrt(Qr/(B·Rизг)), когда уклон крыши α >30°. H is the height of the section in cm, Lmax is the working section of the rafter leg of maximum length in meters, Qr is the distributed load per linear meter of the rafter leg in kg / m, B is the width of the section, cm, Rizg is the resistance of wood to bending, kg / cm². If the material is made from pine or spruce, then Rizg can be equal to 140 kg / cm² (wood grade 1), 130 kg / cm² (grade 2) or 85 kg / cm² (grade 3). Sqrt is the square root.
4. Check whether the deflection value complies with the standards. It should not be more than the figure that results from dividing L by 200. L is the length of the working area. The compliance of the deflection value with the L / 200 ratio is feasible only if the inequality 3.125 Qr (Lmax)³ / (B H³) ≤ 1 is true. Qr indicates the distributed load per linear meter of the rafter leg (kg / m), Lmax is the working section of the rafter leg maximum length (m), B is the width of the section (cm), and H is the height of the section (cm).
5. When the above inequality is violated, the indicators B and H increase.

Table: nominal dimensions of thickness and width of lumber (mm)

An example of the calculation of the supporting structure

Assume that α (roof pitch) = 36°, A (rafter spacing) = 0.8 m, and Lmax (maximum rafter foot length) = 2.8 m. , which means that Rizg \u003d 140 kg / cm².

Cement-sand tiles were chosen for roofing, and therefore the weight of the roof is 50 kg/m². The total load (Q) experienced by each square meter is 303 kg/m². And for the construction of the truss system, bars 5 cm thick are used.

From this follow the following computational steps:

1. Qr=A·Q= 0.8·303=242 kg/m - distributed load per linear meter of rafter beam.
2. H ≥ 9.5 Lmax sqrt(Qr/B Rbend).
3. H ≥ 9.5 2.8 sqrt(242/5 140).
4. 3.125 Qr (Lmax)³/B H³ ≤ 1.
5. 3.125 242 (2.8)³ / 5 (17.5)³ = 0.61.
6. H ≥ (approximate height of the rafter section).

In the table of standard sizes, you need to find the height of the rafter section, close to 15.6 cm. A suitable parameter is 17.5 cm (with a section width of 5 cm).

This value is quite consistent with the deflection index in the regulatory documents, and this is proved by the inequality 3.125 Qr (Lmax)³ / B H³ ≤ 1. Substituting into it the values ​​(3.125 242 (2.8)³ / 5 (17, 5)³), it turns out that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.

Video: detailed calculation of the truss system

The calculation of the gable roof truss system is a whole complex of calculations. In order for the bars to cope with the task assigned to them, the builder needs to accurately determine the length, quantity and cross section of the material, find out the load on it and find out what the step between the rafters should be.