The rafter is one of the individual elements load-bearing structure 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 with its lower end it rests on the mauerlat, with a straight slope, or on the rack (with a sloping roof). Rafters are made from edged boards section 150x60 mm or timber 150x100 mm. The distance between the rafters at which they are located from each other after installation is called the pitch, 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 pitch of the rafters on the roof depends on such factors, How:
Taking into account all these indicators when calculating, you can get a strong and reliable rafter structure, which will serve as a high-quality support for the roof for a long time.
It is the calculation, subject to the basics of standard loads, allows you to correctly select the distance between the rafters. As a rule, standard values can be determined from Sheaves, and calculated values are derived on their basis separately for each structure.
In this case, it is accepted as a standard value to use rafter legs with a cross-section of 150x50 mm with an optimal distance between the rafters in the range of 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 truss structure
The strength and stability of the entire roof directly depends on the quality of the calculation for selecting the cross-section of the rafter legs and the distance between them. Regardless of what type of coating is chosen: profiled sheet, slate or metal tile, the initial calculation should remain the same. After all, each calculation is based on structural strength under mechanical loads, and other influences are no longer important.
When calculating the choice of the optimal distance between the rafters for the roof, The following parameters are taken as a basis:
- Type of roof covering.
- View rafter system And design features roofs.
- Expediency and savings.
For a small house, you can do the design calculations for the roof yourself. True, the method of calculating elements for a roof truss system is quite complicated, and it is recommended to do it using programs specially created for this. Especially if you need to calculate complex broken roof with a large area, most likely, it will not be possible to do without a specialist. In this case, the calculation of the pitch of the rafters will also be based on the standards - the minimum pitch is 0.6 m, the maximum is 1.2 m.
Calculation method
Produced this way.
— The length of the building is measured along the cornice.
— 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. (the average distance is considered to be 950 mm).
— After performing this action, you should add one to the result obtained and round the resulting amount. Thus, the required number of rafters is obtained on one side of the slope. After this, the length of the building must be divided by the number of rafters obtained, 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. This means:
– 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/p ˸ 34 art. = 0.77 m - this value is the distance between the rafters along their central axes.
But that's only general methodology calculation that does not take into account the features of the planned roofing. Therefore, experts recommend calculating the pitch between the rafters for a certain roofing material and insulation, for example, for the most popular metal roofing tiles today.
Rafter structure for metal tiles
Metal tiles visually imitates ceramic roofing tiles. Made from thin sheet steel by cold stamping. Thanks to polymer coating, has high resistance to atmospheric influences and quite attractive visual appearance, 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 tile 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 range from 22 to 45 degrees). This limitation of the pitch of the rafters is due to the fact that the sheathing under the metal tiles is mounted at a distance of 300 mm from each other. The standard timber used for sheathing has a cross-section of 30x50 mm or 50x50 mm. This means that each rafter is subject to additional load.
Sustainability roofing structure against various mechanical loads depends on four factors:
An important factor when calculating the truss structure is is the intended maximum load on the roof, the formation of which includes:
- The weight of the entire rafter structure.
- Weight of the sheathing under the covering.
- Weight of insulation and roofing.
- Snow load (determined using a special directory, unique for each individual region).
- Wind load (also according to a special directory for the region).
- Weight of a person with a tool ( renovation work, estimated weight - 175 kg/m²).
When installing the rafter system, the distance of the rafter legs should not exceed the average value of 0.9 m/p. except in certain cases provided for in advance.
If, when calculating the loads, there is any inaccuracy in the selection of material for the roof and the location of the rafters, its deformation and destruction of the roofing covering. Robust design the roof will be guaranteed only with the correct calculation of the cross-section of the rafters and their installation pitch.
It should be remembered. Universal meaning there is no calculation of rafter structures. When building each house, an individual calculation is required.
Rafters for a pitched roof
Shed roofs can often be found on small outbuildings. They can also be used on private residential buildings, but quite rarely. In such cases, the angle of inclination of the roof is quite small, and with this arrangement of the load-bearing floor beams, a lot of pressure is exerted on them, especially in winter.
Therefore, for a pitched roof, they are installed load-bearing beams ceilings made of large-section timber from 60x150 to 100x220 mm, depending on the width of the span to be covered. In this case, the distance at which the rafters are laid from each other should be in the range of 400 - 800 mm, depending on the angle of inclination of the roof.
For a pitched roof not required complex design rafters, they can simply be laid on the walls, without even using a Mauerlat. In regions where there are fairly snowy winters with a lot of snow, it is recommended to build the roof slope at a maximum angle of 35⁰ and position the roof in the direction of the “wind”. This reduces windage and leads to its self-cleaning.
Gable roof
It is a structure assembled from rafters connected to each other in the form of a triangle. Top part, which rests on the ridge, and the lower one 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.
Gable roof structure, depending on its area, is mounted from separate rigid elements that enhance the strength of the roof. These include racks that support the rafters, tie rods that connect the rafters to each other, jibs, purlins, support beams, etc.
For a gable roof, rafters are often installed 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 highest if the resulting triangle is isosceles. In regions with strong winds, it is recommended to install 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, the calculated parameter for determining the pitch between the rafters and their number is taken to be a load 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 correctly selected section, this distance at 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, when calculating the cross-section of rafters in a standard manner, it is recommended to add a small percentage of safety margin.
Hip roof
Among all truss structures, it is considered one of the most complex . It's practically a hipped roof., while the rafters of the end slopes are attached with their upper ends to the corner strings, and not to the ridge. Therefore, this type of roof may have its own specific requirements 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 spaces under such a roof are built in rare cases, since its slopes “eat up” some area of the attic space, especially in height.
The service life and final reliability of the roof lie in the correct preparatory work, which includes the correctly selected distance between the rafters. It is this element of the supporting frame that directly determines the distribution of the load on load-bearing frame, an error in the calculations of which can lead to deformation and collapse of the entire roof. So, calculating the total distance between rafter elements is one of the most important tasks when planning and calculating a load-bearing house.
Basic calculation technology
The rafter pitch is the exact distance between the two legs of one truss. Most often, in private construction, a distance between legs of about one meter is used, but exact numbers should be based on a detailed calculation of the load-bearing capacity of the selected type of roofing system. In order to independently perform such a calculation, you must adhere to the following scheme of actions:
- By measuring the eaves of the roof structure, it is necessary to determine the total length of the roof slope.
- The resulting value must be divided by the selected and most optimal rafter pitch. In other words, if the size of exactly one m is selected in the rafters, then the length of the slope must be divided by this unit of measurement.
- One is added to the resulting result, after which the finished value can be rounded. It is this method that allows you to determine the number of trusses on one roof slope.
- Now it’s worth dividing the length of the roof slope by the resulting number of trusses, which will allow you to determine the pitch of the rafters.
It is worth recalling that this method miscalculation of such building element, like the pitch of the rafters, cannot be considered extremely accurate, because in such calculations it is also necessary to take into account the features of the selected roofing material, which can be a problem in the absence of proper experience in this matter. It is for this reason that methods will be described below that will allow you to choose the correct rafter pitch for popular roofing materials.
Ceramic tiles and rafter installation
The main feature of the manufacture and installation of the rafter support system for tiles ceramic type lies in the extremely large weight of the roofing material, which in its totality is tens of times greater than the weight of any roofing material.
Usually, this type and the size of the roofing material provides up to 60 kg per square m of rafter supporting system. All this becomes the reason for using only dry lumber on the roof, which eliminates the possibility of further deformation during drying and pressure.
When using wooden beams on the roof, the cross-section of which is 150x50 mm, the pitch between can vary from 70 to 120 cm.
In addition, the distance between the rafter legs may depend on the overall slope of the roof.
Do not forget about the length of the rafter-type legs themselves. The longer this length is, the smaller the distance should be between the legs of the rafters, otherwise the load-bearing frame of the roof will be deprived of maximum strength and stability.
Experts recommend using no more than 80 centimeters of pitch between trusses if the roof slope is up to 45 degrees. Special attention should be paid to the lathing system when using ceramic roofing material on the roof.
To determine such a step, it is necessary to take into account the characteristics of each individual element of the roofing material, namely each tile. In order to correctly calculate the sheathing pitch for such material, it is necessary to subtract the length of the lowest row and the distance between the last bar and the row of sheathing from the pitched length.
As a rule, all varieties of such roofing material have a length of no more than 40 cm. When laying the tiles, they are mounted overlapping, which takes up to 9 cm from the bottom row. In other words, the correct step for this calculation is from 31 to 35 cm.
Metal tiles and installation methods
As a rule, the use of metal tiles in private construction is considered the most common, for which there is whole line reasons and positive characteristics of this roofing material on the roof.
If we compare such material roofing type with the one described above, several striking differences can be noted, each of which can safely be considered a positive feature of the material. Firstly, it is worth recalling the simplicity and speed of installation of such material, which is made in the sheet type, allowing you to cover a significant area of the roof at a time.
The second feature of metal roof tiles can be considered the extremely light weight of the material, which has a positive effect on installation, work on the roof and the load on the supporting frame of the roofing system.
This feature can be considered the most important, because it is responsible for lower labor and time costs when organizing a rafter support system. When using this roofing material, the pitch of the rafter load-bearing legs is used from 60 to 95 cm, because the reduced weight and size of the roof does not require high load resistance parameters.
With such indicators, it is worth using thermal insulation material, the thickness of which is at least 15 cm, which is laid between the rafters. When organizing, the thickness of the insulating material should or can be greater, because this is necessary for arranging a residential floor in the attic. This also affects such an element as the thickness of the material used to make the rafter load-bearing legs.
All this indicates the absence of significant differences in the rafter system for metal tiles on the roof. This roofing material requires standard knowledge about the structure of rafter and roofing systems, the installation of which is simply necessary. The main difference between such a system may be the attachment of the upper type support to ridge run, and not through the ridge-type beam itself.
Obtained through installation of rafter legs free space The top of the system is ideal for ventilation of the entire under-roof space, which metal roofing materials are often covered with condensation.
In the event that changes are made to the roof of an old wooden house, it is necessary to completely remove the Mauerlat, replacing it with a common crown of the upper type. In such a crown it is necessary to put marks and make selections with the required step, into which the elements of the rafter load-bearing legs will be installed. This allows you to more accurately calculate the pitch of the rafter load-bearing legs and install them.
Corrugated sheeting and determining the distance between racks
When using a roofing material such as corrugated sheeting, the calculation of the correct pitch of the rafter load-bearing legs is 60-90 cm, which practically coincides with all the described types of structures and roofing materials.
If it is necessary to set a larger distance between the elements of the rafter legs, it is necessary to secure the transverse elements of the sheathing with a larger thickness and cross-section. This will strengthen the entire supporting structure, which will extend the service life of the entire roof. The step between is about 50 centimeters, when using a board whose size is at least 10 centimeters.
When it comes to such a moment as calculating the sheathing on the rafter system, you should not forget that last board should be 1.5 centimeters thicker than all other elements of the system. Before installing the sheathing elements, it is worth remembering the presence of a chimney and a special system that should be on each roof. This will save time on calculating and altering the rafter system and the sheathing in general.
Ondulin and construction installation methods
As in the cases described above, ondulin provides for the calculation of the pitch of the rafter load-bearing legs from 60 to 100 cm, which makes such dimensions standard when using the most popular roofing materials.
Sami roof trusses are made of lumber with a cross-section of 200x50 millimeters, which is quite enough to ensure that the strength and reliability of the rafter system is at the maximum level, and it is easy to install them. It is immediately worth noting the fact that the described roofing material requires a continuous lathing system, which will allow the material to qualitatively resist loads from snow layers, as well as solar exposure.
In some cases, you can use a thinned type of sheathing, the calculation of which is quite simple if a wooden beam is used for this, the step between the elements is no more than 30 cm. As a rule, this option requires more costs than is necessary for installing a continuous sheathing. In other words, using a thinned type of system in this case is irrelevant.
Slate and rafter structure for it
As a rule, slate coating is considered the most common in our country. The reason for this indicator can be considered the extremely low cost of the material, the simplicity and speed of installation, as well as the possibility of replacing individual sheets if certain sections of the roof are damaged.
The distance between the rafters for this type of roof should be 80 cm. This distance is considered the most optimal solution when combining the load from the weight of slate sheets.
The lathing in such a system needs a thinned type using a board or timber of any type, but of a certain section, which should not be less than 30 mm. This thickness of the board or beam is needed for high-quality and correct distribution of weight from the roofing material itself, as well as from winter precipitation.
Do not forget about a constant margin of safety when organizing a rafter system, which may be required in unforeseen circumstances associated with mechanical loads and bad weather. All other features of the rafter and lathing system do not differ much from the options listed above. In other words, for all other types of coverings we determine the distance in the same way as for the above roofing.
The reliability of the roof and its service life mainly depend on how correctly the calculations are made.
One of the most important parameters rafter system is how far apart the rafters are located.
After all, the distribution of the load on the frame depends precisely on this desired parameter.
If the calculation is made incorrectly, deformation may occur and the roof will collapse.
Therefore, it is extremely important to accurately calculate the interval between the rafters when drawing up a roof project.
Basic principles of calculation
The pitch of the rafters is the distance from one rafter leg to another.
When constructing roofs of private houses, this value is 1 meter.
But this figure is approximate.
To obtain Exact size interval, it is necessary to calculate the load-bearing capacity of the rafter system of the selected type.
For execution correct calculation the following scheme should be used:
- first you need to determine the length of the roof slope;
- Now the resulting figure should be divided by the value of the approximately selected rafter pitch. If you previously selected its value as 1 meter, then you need to divide by 1. If you chose 0.8, then you should divide by 0.8, etc.;
- after that, one is added to the result obtained and the resulting value is rounded up. This action is necessary in order to receive exact amount rafters that will need to be installed on one slope;
- the total length of the slope must be divided by the number of rafter legs obtained in the previous calculation. And we get the exact distance between them.
To understand this better, let's look at an example.
Having measured the length of the roof slope, we got 27.5 meters.
We choose a step of 1 meter to make it more convenient to count.
Further calculations will look like this:
Now we add the unit 27.5 m +1 = 28.5 m.
Round up to the nearest value and get the number 29.
That is, 29 rafter legs must be installed on one slope of our roof.
Now we divide the length of the roof by their number: 27.5/29 = 0.95 m.
This means that in our case, the pitch of the rafter legs should be 0.95 meters.
This is a general calculation.
In which the peculiarities of a particular roofing material are not taken into account.
And he can significantly change this parameter.
When you know what kind of material you will use on the roof, you will have to make some changes to the calculation procedure.
Optimal and minimum value
The step is a unit of calculation.
After all, it depends, first of all, on the load on the roof frame and the cross-section of the rafter legs.
You can take a thick board for their construction and make the interval between them 120 cm.
At this distance, the sheathing may begin to sag.
And how to install the insulation?
After all, most materials have a width of 1 meter.
So when calculating, they take an approximate step of 1 meter.
The minimum distance between rafter legs is 70 cm.
And to receive optimal value For each material it is necessary to perform calculations.
Determining the pitch for a pitched roof
The pitched roof is the simplest.
After all, in the truss of such a roof there are only rafter legs.
There are no struts, racks, or braces in its design.
Such a roof is usually installed on garages, outbuildings, and bathhouses.
In order to determine the pitch of the rafters on a pitched roof, you can use the table:
As can be seen from the table, rafter step depends on their length and cross-section.
Distance between rafters for a gable roof
A gable roof is much more common than a shed roof.
And it’s easy to explain such popularity: the design is very simple, but despite all its simplicity, the gable roof is distinguished by its reliability.
Its design can be easily adjusted to any climatic conditions and any roofing material.
When calculating the pitch of the rafters gable roof, everything is done according to common system calculation (see above).
If the slopes have the same length, then it is enough to perform calculations for one slope.
If the slopes are of different lengths, then the calculation is performed for each slope.
It should be remembered that limit values pitch of rafter legs 70 and 120 cm.
Ondulin
If ondulin is used as a roofing material, then the rafter system is constructed from pine boards with a cross-section of 50 x 200 mm, and the rafters should be located at a distance of at least 60 cm and no more than 90 cm from each other.
A sheathing of timber with a cross-section of 40 x 50 mm is laid on top.
Metal tiles
Metal roofing during construction country houses is used most often.
Because this material is much lighter than ceramic or cement tiles.
Although appearance very similar to her.
The low weight of metal tiles makes it possible to use boards of a smaller cross-section when creating a rafter system, and to use thinner bars for sheathing.
More about the counter-lattice for metal tiles.
ABOUT hip roof with hanging rafters at the link. Also about the design of hanging rafters.
About the price of Viking metal tiles here. Technical characteristics, photographs, reviews.
Reducing the size of the elements of the rafter leg system, in turn, reduces the load on the walls of the building and its foundation.
When constructing a rafter system under metal tiles, the rafters are mounted in increments of 60–95 cm.
The cross-section of the material is 50 x 150 mm.
According to experts, if insulation with a thickness of 150 mm is placed in the space between the rafters, then the most efficient conditions will be created in the attic space. comfortable conditions for accommodation.
To ensure ventilation of the space where the insulation is laid, when installing the rafter system, holes near the upper roof with a diameter of 12 mm are drilled.
The technology for manufacturing a rafter system for metal tiles does not differ significantly from the technology for manufacturing a frame for other roofing materials.
Its only difference is the installation at the top.
The upper support is mounted not on the side of the ridge beam, but on the ridge girder.
The presence of a free zone between the rafters allows air to fully circulate under the flooring, which helps prevent condensation from forming.
Ceramic tiles
The design of the system for ceramic tiles has its own characteristics.
After all, clay is used to make such roofing material.
And this is very heavy material.
If you compare metal tiles and ceramic tiles, the latter weighs 10 times more.
Accordingly, the rafter system is significantly different.
For 1 square meter of roof surface there is a load of 40 - 60 kg, depending on the manufacturer and brand of the product.
The rafters for such a frame system are made of wood that has undergone long-term drying.
Such wood should have a moisture content of no more than 15%.
A beam with a cross-section of 50 x 150 or 60 x 180 mm is used.
It's more reliable.
And the distance between the rafter legs can be 80 - 130 cm.
The exact value can be given if the angle of inclination of the slopes is known.
If the angle of inclination is 15 degrees, then the pitch of the rafters will be 80 cm.
And if the angle of inclination, for example, is 75 degrees, then the step may be larger - 130 cm.
The interval between rafters is not greater than 130 cm.
Also, when calculating the pitch of the rafters, their length is taken into account.
The larger the dyne, the smaller the distance between them.
The shorter the rafter legs, the greater the distance can be made.
If the angle of inclination is 45 degrees, then the roofer can safely move on the roof if the rafter pitch is 80 cm.
Corrugated sheet
When creating a rafter system for corrugated sheets minimum distance between the rafter legs is 60 cm.
The maximum size is 90 cm.
If for some reason the pitch of the rafters is more than 90 cm, then it becomes necessary to install cross-section boards of large cross-section.
The rafter legs themselves can have a cross-section of 50 x 100 or 50 x 150 mm.
Slate
Despite the emergence of a large number of new roofing materials, asbestos-cement slate remains one of the most popular.
If you plan to lay slate on the roof, then the rafters should have a cross-section of 50 x 100 or 50 x 150 mm.
The distance between them is made no less than 60 and no more than 80 cm.
The lathing is made of timber with a cross-section of 50 x 50 mm or boards with a cross-section of 25 x 100 mm.
When installing any building structure It should be remembered that there are still unforeseen situations.
And, therefore, when calculating the sections of rafters and the distance between them, it is necessary to provide a safety margin.
about the installation of the rafter system.
Sergey Novozhilov is an expert in roofing materials with 9 years of experience practical work in the field of engineering solutions in construction.
Source: http://ProRoofer.ru/montaz/stropilnye-sistemy/rasstoyanie-mezhdu-stropilami.html
The distance between the rafters of a gable roof and the pitch of the rafters
The quality and durability of the roof largely depend on the correct calculation of the rafter pitch. It affects the installation of insulation, fastening of roofing material and installation of additional parts.
Pay attention only to the requirements for the distance under the roofing sheets, then problems may arise with the insulation boards. Conversely, adjusting the insulation to fit the dimensions can make the frame too weak, and in winter there is a risk of collapse.
Basics of determining rafter pitch
As a rule, the pitch between the rafters depends on several factors. But usually the distance ranges between 0.6 and 1.2 meters. These figures are rounded; in practice, the distance may differ by several centimeters downward. For an accurate determination, the following calculations must be performed:
- Determine the length of the slope by measuring along the eaves of the house. Let's assume that it turns out to be 17.8 meters.
- Divide the resulting figure by the planned pitch 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.
- Next, round the result up and add one: 23 + 1 = 24. At this stage, the number of required rafters is determined.
- 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 = 0.74 m.
So, you will determine the actual distance at which the rafters should stand for a roof made of metal tiles or some other material.
Often, when planning a roof frame, specialized calculators are used. They enter basic data and get the finished result. However, you should not completely trust the program, since some conditions can only be taken into account by a person. When calculating, you can understand how the system works, what loads are applied. If necessary, adjustments are made to strengthen or lighten the structure. An independent calculation will help you more accurately determine the required pitch of the rafters of a gable and pitched roof.
Calculation of pitch depending on roofing material
For each coating, its own standards and dimensions for the rafter system are developed. The main factors are the strength of the material, its weight and resistance to external loads. Let's look at the main types of coatings.
Corrugated sheet
The pitch of the rafters under the corrugated sheet 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, add a cross-section board with a larger cross-section. In this case, the rafters themselves are usually chosen with dimensions of 50x100 or 50x150 mm.
In addition to the rafters, 30x100 mm sheathing 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 sheet and the height of the wave. The sheathing boards protruding beyond the boundaries of the cornice should be 1.5 cm thicker than usual. This is done based on the need to attach ventilation, chimney or drainage.
Ceramic tiles
The main difficulty of design rafter frame under ceramic tiles is a large weight of roofing material. It is 10 times heavier than corrugated sheeting and weighs from 40 to 60 kg per square meter.
Dry beams with a moisture content of only 15% are used for this coating. Their cross-section should be either 50x150 or 60x180 mm. With such indicators, the maximum permissible distance there can be 1.3 meters between the rafters. The minimum permissible step through which rafters can be placed is 0.8 m. The final decision is influenced by the slope of the roof: 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 on the surface is planned, the beams are placed in increments of no more than 0.85 m.
The distance between the sheathing boards is selected so that each intersection of the tiles has its own base. Standard length material is 400 mm, and the amount of overlap during installation ranges between 55 and 90 mm. It turns out that the distance between the central axes of the sheathing can be either 310 or 345 millimeters.
Metal tiles
IN last years A particularly pressing question is: how to determine the distance between the rafters for metal tiles? It imitates a more expensive one ceramic material, but at the same time has positive properties corrugated sheets 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 tiles, so a simpler frame is required. The cross-section of the beams is reduced to 50x150 mm and the pitch between the sheathing is increased. The pitch of the rafters for metal tiles varies between 0.6 and 0.95 m.
Ondulin
The permissible step for ondulin coating is 0.6-0.9 meters. The standard rafter section is 50x200 millimeters. These dimensions will allow you to withstand the created load of the roofing pie on the gable roof.
A lathing made of bars measuring 40x50 millimeters is attached on top of the counter-lattice. 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:
- 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 roof pitch above 45°, then you are allowed to increase the pitch.
- Choice of insulation. To reduce the cost of insulation boards, it is recommended to take into account their standard sizes. Mats are produced in widths of 600, 800 and 1200 mm. If we neglect this condition, then it appears a large number of undercuts, cold bridges and construction will be delayed.
- Quality of lumber. Here the type of wood, grade and section are taken into account. As mentioned above, strength is affected by how well the beam was dried. When purchasing wood, pay attention to the evenness and presence of defects in the form of knots and traces of pests.
- Floor beams and tie rods. If you are installing under a gable roof attic floor, then the maximum distance between the rafters should be 0.75 meters.
The calculation of the distance between the beams for a gable and pitched roof is different. Even if he makes several slopes, for each it is necessary to make the calculation individually. This especially applies to buildings with different lengths of eaves.
Taking into account all these factors, you can determine at what distance to install the rafters. Although all calculations can be done independently, since reference data is freely available, it is better to turn to professionals. They have experience in design and will quickly determine the required distance between the rafters. This way, you will avoid possible mistakes and be sure of safety.
Source: https://build-experts.ru/shag-stropil/
The distance between the rafters is one of the key parameters affecting the strength of the structure. Proper calculation of the installation pitch of the rafters allows you to build a roof that is resistant to high operating loads.
Roof loads and calculation of the rafter system
Development of a project for a single-pitched or gable roof begins with choosing the type of rafter system, the angle of inclination of the slopes (roof height) and materials for constructing the structure. Calculation of the distance between the rafter legs is carried out taking into account the loads that the roof will experience during operation. Constant loads include :
- the weight of the materials from which the rafter system is made;
- roofing weight;
- weight of roofing materials (waterproofing, vapor barrier, insulation);
- weight of finishing elements of a residential attic or attic.
In addition to permanent loads, the roof also experiences temporary loads, which include :
The pitch refers to the distance between the rafters of one slope. When calculating a single slope, gable or complex roof, usually follow the following scheme :
- the length of the future roof slope is measured;
- the resulting value is divided by the optimal numerical value of the rafter pitch;
- one is added to the resulting value, the result is rounded;
- the length of the roof slope is divided by the rounded result.
The final result will allow you to determine at what distance the rafter legs should be placed.
Determining the pitch cannot be extremely accurate, since it is necessary to take into account a number of additional factors, including the width of the insulation, features of installation of the sheathing under different kinds roofing material.
If a roof with a chimney is being designed, the pitch can be adjusted taking into account its location, so that you do not have to subsequently remove part of the rafters and install a supporting structure, such as a special rafter system.
Distance between rafters under slate
Slate is a traditional roofing material. Its advantages include characteristics such as resistance to external influences (excluding mechanical) and low cost. Slate allows you to create roofing, the repair of which can be reduced to replacing individual elements. Slate is heavy and requires the installation of a fairly powerful rafter system. Calculation of the distance at which it is necessary to place the rafters under the slate is carried out taking into account the cross-section of the beam for the manufacture of rafter legs.
The optimal solution is to install a system under slate, in which the gap between the rafters should be at least 800 mm. In order for the slate structure to withstand not only the weight of the material, but also increased external loads, the sheathing is made of timber or boards with a cross-section of at least 30 mm. When calculating a rafter system for slate, you should read that this material has quite large restrictions on the choice of the angle of inclination of the slopes.
Rafters for metal tiles
Metal tiles are actively used as a practical and aesthetic roofing material when arranging a pitched, gable, hip or complex roof. The frame for metal tiles is built according to standard principles.
To calculate at what distance it is better to install the rafters, it is necessary to take into account the loads and the angle of the roof. Metal tiles are characterized by their relatively low weight, thanks to which they can serve as a replacement for old slate or ceramic tile roofing.
In this case, the rafters will not need to be strengthened or the pitch of their installation changed.
The standard pitch of rafters for metal tiles is 600-900 mm. The cross-section of the elements can be 50-150 mm - this is enough to create a reliable frame for metal tiles.
But if you plan to use insulation, the layer of which in areas with low winter temperatures should be 200 mm; for rafters under metal tiles, it is recommended to use 200x50 timber, so as not to install an additional system that holds the insulation.
It is better to adjust the gap between the rafters for metal tiles to the width of the sheet or roll insulation.
Corrugated sheeting: rafters and sheathing
Corrugated sheeting is a lightweight and easy-to-use roofing material. Galvanized or coated with a decorative protective layer, corrugated sheeting can be used both for installing a single-pitched roof for a utility room or garage, and for a gable roof of a residential building. How to calculate the distance required to install rafters under corrugated sheeting?
To ensure the necessary structural rigidity, it is enough to install the rafters under the corrugated sheet in increments of 600-900 mm. In this case, you need to pay attention to the angle of the roof.
Calculations show that under high external loads, it is better to lay corrugated sheets on the system with a minimum step.
But if the distance between the rafters under the corrugated sheet is close to the maximum, and the roof slope angle is small, then the structure is strengthened by using more frequent sheathing. In this case, the pitch of the sheathing under the corrugated sheet should be about 50 mm, the width of the elements should be at least 100 mm.
Rafter system for soft roofing
Soft roofing includes bitumen and bitumen-polymer roll materials, roofing membranes, and soft tiles. Soft roofing is characterized by relatively low weight and the absence of the need to install a massive rafter structure.
The minimum distance between the rafter legs is 600 mm, the maximum is 1500 mm. When installing a support under soft roof it is important to take into account the angle of inclination of the slopes: the smaller the angle, the smaller the distance between the supports for a continuous sheathing should be. The choice of step is also influenced by the thickness of the material for the sheathing - the thicker the sheet of plywood or OSB, the larger the installation step of the rafter legs can be.
Ondulin: calculation of rafters
Ondulin (bitumen slate) is laid on a flat, continuous sheathing made of sheet material. This allows the roofing covering to successfully resist wind and snow loads. The sheathing for ondulin rests on the rafters, which should be located in increments of 600 - 1000 mm, depending on the angle of inclination of the gable or pitched roof.
Rafters for ondulin are made of timber with a section of 200×50 mm. When choosing at what distance to place the rafter legs for the structure under the ondulin, it is recommended to take into account the width insulation material to make it easier to install. This calculation allows you to reduce the financial costs of roofing.
Sandwich panel roof
Sandwich roofing is most often erected on houses made of sip panels or hangar-type buildings. A special feature of the sandwich is its bending rigidity, which makes it possible to dispense with the installation of traditional rafters. Small spans from the ridge of a gable roof to the top of the wall (or the distance between the supporting structures of a gable roof) allow the installation of a sandwich without additional supports.
If the span exceeds 4 meters, additional purlins must be installed. To install a sandwich roof on a residential building, a traditional rafter system is often installed, but in this case the rafters can be positioned big step– they serve as a support for the purlins. The distance between the rafter legs is selected based on the length of the available material for the purlins and the length load-bearing walls. Technical specifications sandwich allow the roof to withstand high operating loads .
Polycarbonate: construction of a supporting structure
Polycarbonate in Lately is actively used as a roofing material. First of all, polycarbonate is in demand in the construction of gazebos, canopies, winter gardens. The lathing and rafter system for polycarbonate can be made of wood or metal.
Polycarbonate varies in weight depending on the thickness of the sheet. It is recommended to perform lathing under polycarbonate in increments of 600-800 mm. The lathing (wooden or metal) is mounted on rafters, which can be straight or arched.
Typically, the gap between rafters for polycarbonate is from 1500 to 2300 mm.
Source: http://krovlya-mp.ru/shag-stropil-dvuskatnoj-kryshi.html
Distance between rafters: what step should be between rafters
The task of correctly calculating the distance between the rafters is a very important one. How seriously you begin to solve this problem will determine not only the reliability and durability of the roof, but also all subsequent work on it: laying insulation, installing roofing, installing additional elements. If you manipulate the pitch of the rafters under the roofing sheets, as many do, then it is not a fact that insulation will then fit between the rafters.
If you focus only on insulation, the very first winter with its abundant Russian snow will crush the rafter system. That’s why the whole point is to choose the optimal rafter pitch for all slopes, and this is the skill we’ll teach you now.
Here is a good video tutorial on how to independently calculate the distance between the rafters:
What determines the pitch of the rafters?
So, the distance between the rafters is determined by the following important factors:
- Roof shape (gable, single-slope or multi-slope).
- Roof angle.
- Parameters of the timber used to make rafters (width, thickness).
- The design of the rafter system (sloping, hanging or sliding).
- The totality of all loads on the roof (weight of covering, atmospheric phenomena, etc.).
- Lathing material (20x100 or 50x50) and its parameters (solid, with gaps of 10 cm, 20 cm or solid plywood)
And each of these parameters needs to be taken into account, which is exactly what this article is about.
Online calculators vs. notepad and pencil
To correctly calculate the cross-section of rafters and the pitch of their installation, today there are many complex formulas. But remember that such formulas were at one time developed more so that it would be possible not so much to perfectly calculate roof structures, but to study the operation of such elements.
For example, today simple online programs are popular that do a good job of calculating rafter parameters. But it’s ideal if you can install it yourself specific tasks and calculate everything you need.
It is important to understand down to the smallest detail what exactly happens in the rafter system during operation, what exact forces are exerted on it and what loads. But a computer program cannot always take into account everything that the human brain notices.
Decorative rafters: 0% load
First of all, decide on the most important point: the type of roof and its purpose. The fact is that in winter the roof of a residential building can withstand a large cap of snow, a constant wind at altitude, and it is often insulated from the inside, but completely different requirements are placed on the rafter system of a small gazebo hidden under the treetops.
For example, if you are building a pergola in its classical sense, then it does not matter at all what exactly the distance between the rafters will be - this is a purely aesthetic factor:
The illustration above shows that even such a building has its own pitch of rafters. After all, here it provides both the aesthetic factor and the rigidity of the structure itself. But they choose a step arbitrarily.
Functional rafters: detailed calculation
We come to the main question: what distance should be between the rafters of the roof of a residential building? Here, be patient and carefully study all the nuances.
Point 1. Wall length and choice of rafter spacing
The first step in installing rafters on the roof of a residential building is usually chosen structurally based on the size of the building, although taking into account many other factors.
For example, the easiest way is to install rafters in 1-meter increments, so for a wall 6 meters long, 7 rafters are installed as a standard. At the same time, you can save money by placing them at a distance of 1 and 2 meters, and you will get exactly 5 rafters. It can also be placed at a distance of 2 and 3 meters, but reinforced with lathing. But it is extremely undesirable to make the rafter spacing more than 2 meters.
Point 2. The influence of snow and wind loads on the shape of the roof
So, we settled on the fact that the average distance between the rafters of a regular roof is 1 meter. But, if the area has a significant snow or wind load, or the roof is more or less flat or simply heavy (for example, covered with clay tiles), then this distance must be reduced to 60-80 cm. But on a roof with a slope of more than 45 degrees it can be even increase to a distance of 1.2 m-1.4 m.
Why is this so important? Let's figure it out. The fact is that air flow On its way, it collides with a wall under the roof of a building, and turbulence occurs there, after which the wind hits the eaves of the roof. It turns out that the wind flow seems to bend around the slope of the roof, but at the same time trying to lift it. And at this moment forces arise in the roof that are ready to tear it off or overturn it - these are two windward sides and one lifting one.
There is another force that arises from wind pressure and acts perpendicular to the slope, trying to press the roof slope inward. And the greater the angle of inclination of the roof slope, the more important are the safe wind forces and the less tangential ones. And the greater the angle of the slope, the less often you need to install rafters.
This map of the average wind load will help you understand whether to make a high roof or a flat one:
The second point: in the Russian region, the standard roof of a house is constantly exposed to such atmospheric phenomena as snow. Here too, you need to take into account that the snow bag usually accumulates more on one side of the roof than on the other.
That is why in places where such a bag is possible, you need to insert paired rafter legs or make a continuous sheathing. The easiest way to identify such places is by the wind rose: single rafters are placed on the windward side, and paired rafters are placed on the leeward side.
If you are building a house for the first time, then you will not find your own worldview, but determine the average snow load for your area according to official data:
Point 3. The issue of insulation and standard width of mats
If you will be insulating the roof, then it is advisable to set the pitch of the rafters to the standard sizes of insulation boards, which are 60, 80 cm and 120 cm.
Modern insulation materials are sold today standard width, usually at the same standard rafter pitch. If you then take them and adjust them to existing parameters, then there will be a lot of waste, cracks, cold bridges and other problems.
Point 4. Quality and strength of the lumber used
It is also of great importance what kind of material you use to build the rafter system. So, for each type of wood there is its own normative documents, which concerns its load-bearing capacity:
Because For the manufacture of roof truss systems in Russia, pine and spruce are most often used; their bending strength and features of use have long been prescribed. If you use wood of other species, you can remove correction factor.
In addition, if the rafters have sections, notches or holes for bolts, in this place the load-bearing capacity of the beam must be calculated with a coefficient of 0.80.
Point 5. Distance between tie rods and floor beams
One more point: if the roof is built with interconnected trusses, and their lower chord is used simultaneously as floor beams, then the distance between the trusses should be within 60-75 cm in order to take into account the design of the future floor.
Point 6. Loads on rafters
So, here are the main loads that act on the roof truss system:
- Static, which includes the weight of the rafter system itself, the weight of the roof, the snow lying on the roof and additional elements.
- Dynamic, which includes wind force, unexpected damage to the roof, the weight of a person and equipment for repairs, and similar factors.
And all these factors are capable of influencing the roof simultaneously at a certain moment, and therefore there is such a thing as critical value. This is exactly the load value at which the roof cannot withstand and becomes deformed.
Therefore, if a building is being built with significant spans, then steel roof trusses must be used. The fact is that there is no longer tension in such rods, and the entire load falls on the nodes - here they are affected by compressive and tensile forces. And the distance between such trusses is calculated depending on the type of roof and the design of the roof itself.
Typically, a unified truss is installed with a span that is a multiple of six, and therefore a distance that is a multiple of one and a half meters is made between the nodes of the truss.
Point 7. Weight of the rafter system and roofing pie
Do not forget that the main purpose of the rafters is to support the entire roof, and its weight is crucial:
Point 8. Ease of installation of roofing
The distance between the rafters is also influenced by factors such as the selected roofing covering. The higher the roof slope, the more roofing materials will be used. And the heavier they are, the more often you will have to put rafters under them. But what about continuous lathing? The fact of the matter is that it also has its own weight:
Each type of roof has its own optimal rafter pitch. After all, many standard sheets at the edges need to be fastened directly into the rafters or sheathing, and it is important that they coincide. Otherwise, the work of covering the roof will easily turn into a living hell at altitude, believe me.
That is why, even before starting installation, you must make a layout and check everything several times. And know some important subtleties for each type of coating.
Determination of the totality of loads on the roof as a whole and the rafters separately
So, we have determined that, in addition to other design factors, a whole set of loads simultaneously acts on the roof rafter system: the weight of the rafter system, the snow cap, wind pressure. After you add all the loads together, be sure to multiply them by a factor of 1.1. So you will count on unexpected favorable conditions, that is, provide an additional 10% percentage strength.
Now all you have to do is divide the total load by the planned number of rafters and see if each of them can cope with its task. If it seems that the structure will be frail, feel free to add 1-2 rafters to total, and you will be calm about your home.
You need to make calculations for destruction, i.e. for the full load acting on the roof. All these loads are determined by technical specifications materials and SNiPs.
The standard roof structure consists of rafters and lattice purlins, and each of these elements responds only to the load that puts pressure on it, and not on the roof as a whole. Those.
Each individual rafter is subject to its own load, total, but divided by the number of rafter legs, and by changing the step of their location, you change the area where the load is collected on the rafters - reducing it or increasing it.
And, if changing the pitch of the rafters is inconvenient for you, then work with the parameters of the section of the rafter legs, and the overall load bearing capacity the roof will increase significantly:
When making this calculation, try to ensure that the longest rafter in your project is no more than six and a half meters, otherwise, splice along the length. Now let's explain in more detail.
So, on roofs with slopes up to 30 degrees, the rafters have so-called “bending elements”. Those. they work specifically for bending, and there are certain requirements for them.
And the possibility of rafter deflection is calculated using a special formula, and if the result exceeds the norm, then the rafters are increased in height and a new calculation is made again.
But on a roof with a warehouse slope of more than 30 degrees, which rafters are already considered “bending-compressed” elements. That is, they are affected not only evenly distributed load, which causes bending of the rafters, but also forces that are already acting along the axis of the rafters. Speaking in simple language, here the rafters not only bend a little under the weight of the roof, but are also compressed from the ridge to the mauerlat. In addition, the crossbar, which usually holds the two rafter legs, must also be checked for tension.
As you can see, even a person far from construction can handle such calculations. The main thing is to take everything into account, be attentive and ready to spend a little more time on design, so that then all the work goes like clockwork!
A gable roof is formed on the basis of a frame that combines the simplicity of the device and unsurpassed reliability. But the roof skeleton of two rectangular slopes can boast of these advantages only if the rafter legs are carefully selected.
Parameters of the gable roof rafter system
It’s worth starting the calculations if you understand that the rafter 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 rafter 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” indicates the height and is independently 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 assume that the width of half 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).
You should add 60–70 cm to the figure obtained from the Pythagorean formula. The extra centimeters will be needed to carry the rafter leg beyond the wall and make the necessary cuts.
The six-meter rafter is the longest, so it is suitable as a rafter leg
The maximum length of a beam used as a rafter leg is 6 m. If a durable board of greater length is required, then they resort to the fusion method - nailing a section from another beam to the rafter leg.
Section of rafter legs
For various elements rafter systems have their own standard sizes:
- 10x10 or 15x15 cm - for mauerlat timber;
- 10x15 or 10x20 cm - for the rafter leg;
- 5x15 or 5x20 cm - for purlin and bracing;
- 10x10 or 10x15 cm - for a stand;
- 5x10 or 5x15 cm - for a bed;
- 2x10, 2.5x15 cm - for laths.
The thickness of each part of the supporting roof structure 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 cross-section of the rafter legs of a gable roof is affected by:
The most significant effect on the cross-section of the rafter legs is the pitch of the rafters. Increasing the distance between the beams entails increased pressure on the supporting structure of the roof, and this obliges the builder to use thick rafters.
Table: rafter cross-section depending on length and pitch
Variable impact on the rafter system
The pressure on the rafter legs can be constant or variable.
From time to time and with varying intensity, the supporting structure of the roof is affected by wind, snow and precipitation. In general, the roof slope is comparable to a sail, which can break under the pressure of natural phenomena.
The wind tends to overturn or lift 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 = Wo × k x c, where W is the wind load indicator, Wo is the value of the wind load characteristic of a certain area Russia, 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 vary from -1.8 to +0.8. A negative value is typical for a rising roof, while a positive value is typical for a roof on which the wind presses. 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 determined from map 3 of Appendix 5 in SNiP 2.01.07–85 and a special table. The coefficient taking into account the change in wind pressure with height is also standardized.
Table: standard value of wind pressure
Table: k coefficient value
It's not just the terrain that affects wind loads. The location of the housing is of great importance. Behind a wall of tall buildings there is almost no threat to the house, but in an open space the wind can become a serious enemy for it.
The snow load on the rafter system is calculated using 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 roof slope.
The weight of the snow layer is indicated in SNiP “Rafter Systems” and is determined by the type of terrain where the building is built.
The snow load on the roof depends on where the house is located
The correction factor, if the roof slopes tilt 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 sloped 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.
Constant loads
Loads acting continuously are considered to be the weight of the roofing pie, including sheathing, insulation, films and Decoration Materials for arranging an attic.
The roofing pie creates constant pressure on the rafters
The weight of the roof is the sum of the weight of all materials used in the construction of the roof. On average it is 40–45 kg/sq.m. According to the rules, per 1 m² of rafter system there should not be more than 50 kg of roofing material weight.
To ensure that there is no doubt about the strength of the rafter system, it is worth adding 10% to the calculation of the load on the rafter legs.
Table: weight of roofing materials per 1 m²
| Type of roofing finish | Weight in kg per 1 m² |
| Rolled bitumen-polymer sheet | 4–8 |
| Bitumen-polymer soft tiles | 7–8 |
| Ondulin | 3–4 |
| Metal tiles | 4–6 |
| Corrugated sheeting, seam roofing, galvanized metal sheets | 4–6 |
| Cement-sand tiles | 40–50 |
| Ceramic tiles | 35–40 |
| Slate | 10–14 |
| Slate roofing | 40–50 |
| Copper | 8 |
| Green roof | 80–150 |
| Rough flooring | 18–20 |
| Lathing | 8–10 |
| The rafter system itself | 15–20 |
Number of beams
How many rafters will be needed to arrange the frame of a gable roof is determined by dividing the width of the roof by the pitch between the beams and adding one to the resulting value. It indicates an additional rafter that will need to be placed on the edge of the roof.
Let's say it was 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 (including the additional timber).
The rafter system of a gable roof is a structure made from a certain number of rafters
The pitch of the beams of the supporting roof structure
To determine the distance between the beams of the supporting roof structure, 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 roof slope;
- level of wind and snow loads.
It is customary to place rafters at 90–100 cm intervals when choosing a lightweight roofing material
A normal step for rafter legs is 60–120 cm. The choice in favor of 60 or 80 cm is made in the case of constructing a roof inclined at 45˚. The same small step should be taken if you want to cover wooden frame roofs heavy materials like ceramic tiles, asbestos-cement slate and cement-sand tiles.
Table: rafter pitch depending on length and cross-section
Formulas for calculating the rafter system of a gable roof
Calculation of the rafter system comes down to establishing the pressure on each beam and determining the optimal cross-section.
When calculating the rafter system of a gable roof, proceed as follows:
- Using the formula Qr = AxQ, they find out what the load per linear meter of each rafter leg is. Qr is the distributed load per linear meter of a rafter leg, expressed in kg/m, A is the distance between the rafters in meters, and Q is the total load in kg/m².
- Proceed to determining the minimum cross-section of the rafter beam. To do this, study the data from the table included in GOST 24454–80 “Lumber coniferous species. Dimensions".
- Focusing on standard parameters, select the section width. And the section height is calculated using the formula H ≥ 8.6 Lmax sqrt(Qr/(BRbend)), if the roof slope is α< 30°, или формулу H ≥ 9,5·Lmax·sqrt(Qr/(B·Rизг)), когда уклон крыши α >30°. H is the section height in cm, Lmax is the working section of the rafter leg maximum length in meters, Qr - distributed load per linear meter of rafter leg in kg/m, B - section width cm, Rbend - bending resistance of wood, kg/cm². If the material is made from pine or spruce, then Ri can be equal to 140 kg/cm² (grade 1 wood), 130 kg/cm² (grade 2) or 85 kg/cm² (grade 3). Sqrt is the square root.
- Check whether the deflection value complies with the standards. It should not be greater than the figure obtained by dividing L by 200. L refers to the length of the working section. Correspondence of the deflection value to the ratio L/200 is feasible only if the inequality 3.125·Qr·(Lmax)³/(B·H³) ≤ 1 is true. Qr denotes 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 section width (cm), and H is the section height (cm).
- When the above inequality is violated, the indicators B and H increase.
Table: nominal dimensions of thickness and width of lumber (mm)
| Board thickness - section width (B) | Board width - section height (H) | ||||||||
| 16 | 75 | 100 | 125 | 150 | - | - | - | - | - |
| 19 | 75 | 100 | 125 | 150 | 175 | - | - | - | - |
| 22 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | - | - |
| 25 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 32 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 40 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 44 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 50 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 60 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 75 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 100 | - | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 125 | - | - | 125 | 150 | 175 | 200 | 225 | 250 | - |
| 150 | - | - | - | 150 | 175 | 200 | 225 | 250 | - |
| 175 | - | - | - | - | 175 | 200 | 225 | 250 | - |
| 200 | - | - | - | - | - | 200 | 225 | 250 | - |
| 250 | - | - | - | - | - | - | - | 250 | - |
Example of load-bearing structure calculation
Let us assume that α (roof inclination angle) = 36°, A (distance between rafters) = 0.8 m, and Lmax (working section of the rafter leg of maximum length) = 2.8 m. First grade pine material is used as beams , which means that Rben = 140 kg/cm².
Cement-sand tiles were chosen to cover the roof, 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 rafter system, beams 5 cm thick are used.
The following computational steps follow from this:
- Qr=A·Q= 0.8·303=242 kg/m - distributed load per linear meter of rafter beam.
- H ≥ 9.5·Lmax·sqrt(Qr/B·Rben).
- H ≥ 9.5 2.8 sqrt(242/5 140).
- 3.125·Qr·(Lmax)³/B·H³ ≤ 1.
- 3.125·242·(2.8)³ / 5·(17.5)³= 0.61.
- H ≥ (approximate height of the rafter section).
In the table standard sizes you need to find a cross-sectional height of the rafters that is 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 in regulatory documents, and this is proven by the inequality 3.125·Qr·(Lmax)³/B·H³ ≤ 1. Substituting the values (3.125·242·(2.8)³ / 5·(17.5)³) into it, we find that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.
Video: detailed calculation of the rafter system
Calculating the rafter system of a gable roof is a whole complex of calculations. In order for the beams 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 pitch between the rafters should be.
A gable roof is formed on the basis of a frame that combines the simplicity of the device and unsurpassed reliability. But the roof skeleton of two rectangular slopes can boast of these advantages only if the rafter legs are carefully selected.
Parameters of the gable roof rafter system
It’s worth starting the calculations if you understand that the rafter 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 rafter 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” indicates the height and is independently 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 assume that the width of half 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).
You should add 60–70 cm to the figure obtained from the Pythagorean formula. The extra centimeters will be needed to carry the rafter leg beyond the wall and make the necessary cuts.
The six-meter rafter is the longest, so it is suitable as a rafter leg
The maximum length of a beam used as a rafter leg is 6 m. If a durable board of greater length is required, then they resort to the fusion method - nailing a section 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 purlin and bracing;
- 10x10 or 10x15 cm - for a stand;
- 5x10 or 5x15 cm - for a bed;
- 2x10, 2.5x15 cm - for laths.
The thickness of each part of the supporting roof structure 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 cross-section of the rafter legs of a gable roof is affected by:
The most significant effect on the cross-section of the rafter legs is the pitch of the rafters. Increasing the distance between the beams entails increased pressure on the supporting structure of the roof, and this obliges the builder to use thick rafters.
Table: rafter cross-section depending on length and pitch
Variable impact on the rafter system
The pressure on the rafter legs can be constant or variable.
From time to time and with varying intensity, the supporting structure of the roof is affected by wind, snow and precipitation. In general, the roof slope is comparable to a sail, which can break under the pressure of natural phenomena.
The wind tends to overturn or lift 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 = 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 Russia, k is a correction factor determined by the height of the structure and the nature of the terrain, and c is the aerodynamic factor coefficient.
The aerodynamic coefficient can vary from -1.8 to +0.8. A negative value is typical for a rising roof, while a positive value is typical for a roof on which the wind presses. 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 determined from map 3 of Appendix 5 in SNiP 2.01.07–85 and a special table. The coefficient taking into account the change in wind pressure with height is also standardized.
Table: standard value of wind pressure
Table: k coefficient value
It's not just the terrain that affects wind loads. The location of the housing is of great importance. Behind a wall of tall buildings there is almost no threat to the house, but in an open space the wind can become a serious enemy for it.
The snow load on the rafter system is calculated using 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 roof slope.
The weight of the snow layer is indicated in SNiP “Rafter Systems” and is determined by the type of terrain where the building is built.
The snow load on the roof depends on where the house is located
The correction factor, if the roof slopes tilt 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 sloped 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.
Constant loads
Continuously acting loads are considered to be the weight of the roofing pie, including sheathing, insulation, films and finishing materials for the attic.
The roofing pie creates constant pressure on the rafters
The weight of the roof is the sum of the weight of all materials used in the construction of the roof. On average it is 40–45 kg/sq.m. According to the rules, per 1 m² of rafter system there should not be more than 50 kg of roofing material weight.
To ensure that there is no doubt about the strength of the rafter system, it is worth adding 10% to the calculation of the load on the rafter legs.
Table: weight of roofing materials per 1 m²
| Type of roofing finish | Weight in kg per 1 m² |
| Rolled bitumen-polymer sheet | 4–8 |
| Bitumen-polymer soft tiles | 7–8 |
| Ondulin | 3–4 |
| Metal tiles | 4–6 |
| Corrugated sheeting, seam roofing, galvanized metal sheets | 4–6 |
| Cement-sand tiles | 40–50 |
| Ceramic tiles | 35–40 |
| Slate | 10–14 |
| Slate roofing | 40–50 |
| Copper | 8 |
| Green roof | 80–150 |
| Rough flooring | 18–20 |
| Lathing | 8–10 |
| The rafter system itself | 15–20 |
Number of beams
How many rafters will be needed to arrange the frame of a gable roof is determined by dividing the width of the roof by the pitch between the beams and adding one to the resulting value. It indicates an additional rafter that will need to be placed on the edge of the roof.
Let's say it was 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 (including the additional timber).
The rafter system of a gable roof is a structure made from a certain number of rafters
The pitch of the beams of the supporting roof structure
To determine the distance between the beams of the supporting roof structure, 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 roof slope;
- level of wind and snow loads.
It is customary to place rafters at 90–100 cm intervals when choosing a lightweight roofing material
A normal step for rafter legs is 60–120 cm. The choice in favor of 60 or 80 cm is made in the case of constructing a roof inclined at 45˚. The same small step should be taken if you want to cover the wooden roof frame with heavy materials such as ceramic tiles, asbestos-cement slate and cement-sand tiles.
Table: rafter pitch depending on length and cross-section
Formulas for calculating the rafter system of a gable roof
Calculation of the rafter system comes down to establishing the pressure on each beam and determining the optimal cross-section.
When calculating the rafter system of a gable roof, proceed as follows:
- Using the formula Qr = AxQ, they find out what the load per linear meter of each rafter leg is. Qr is the distributed load per linear meter of a rafter leg, expressed in kg/m, A is the distance between the rafters in meters, and Q is the total load in kg/m².
- Proceed to determining the minimum cross-section of the rafter beam. To do this, study the data from the table included in GOST 24454–80 “Softwood lumber. Dimensions".
- Based on standard parameters, choose the section width. And the section height is calculated using the formula H ≥ 8.6 Lmax sqrt(Qr/(BRbend)), if the roof slope is 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 section width cm, Rbend is the bending resistance of wood, kg/cm². If the material is made from pine or spruce, then Ri can be equal to 140 kg/cm² (grade 1 wood), 130 kg/cm² (grade 2) or 85 kg/cm² (grade 3). Sqrt is the square root.
- Check whether the deflection value complies with the standards. It should not be greater than the figure obtained by dividing L by 200. L refers to the length of the working section. Correspondence of the deflection value to the ratio L/200 is feasible only if the inequality 3.125·Qr·(Lmax)³/(B·H³) ≤ 1 is true. Qr denotes 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 section width (cm), and H is the section height (cm).
- When the above inequality is violated, the indicators B and H increase.
Table: nominal dimensions of thickness and width of lumber (mm)
| Board thickness - section width (B) | Board width - section height (H) | ||||||||
| 16 | 75 | 100 | 125 | 150 | - | - | - | - | - |
| 19 | 75 | 100 | 125 | 150 | 175 | - | - | - | - |
| 22 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | - | - |
| 25 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 32 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 40 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 44 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 50 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 60 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 75 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 100 | - | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 125 | - | - | 125 | 150 | 175 | 200 | 225 | 250 | - |
| 150 | - | - | - | 150 | 175 | 200 | 225 | 250 | - |
| 175 | - | - | - | - | 175 | 200 | 225 | 250 | - |
| 200 | - | - | - | - | - | 200 | 225 | 250 | - |
| 250 | - | - | - | - | - | - | - | 250 | - |
Example of load-bearing structure calculation
Let us assume that α (roof inclination angle) = 36°, A (distance between rafters) = 0.8 m, and Lmax (working section of the rafter leg of maximum length) = 2.8 m. First grade pine material is used as beams , which means that Rben = 140 kg/cm².
Cement-sand tiles were chosen to cover the roof, 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 rafter system, beams 5 cm thick are used.
The following computational steps follow from this:
- Qr=A·Q= 0.8·303=242 kg/m - distributed load per linear meter of rafter beam.
- H ≥ 9.5·Lmax·sqrt(Qr/B·Rben).
- H ≥ 9.5 2.8 sqrt(242/5 140).
- 3.125·Qr·(Lmax)³/B·H³ ≤ 1.
- 3.125·242·(2.8)³ / 5·(17.5)³= 0.61.
- H ≥ (approximate height of the rafter section).
In the table of standard sizes, you need to find a section height of the rafters that is close to 15.6 cm. A suitable parameter is 17.5 cm (with a section width of 5 cm).
This value fully corresponds to the deflection indicator in regulatory documents, and this is proven 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), we will find that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.
Video: detailed calculation of the rafter system
Calculating the rafter system of a gable roof is a whole complex of calculations. In order for the beams 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 pitch between the rafters should be.
