There is no point in arguing about the importance of a roof for any building. It is not for nothing that over the entire history of mankind, more than a dozen different types of roofs have been invented, from simple to quite complex in design and construction. An important element When planning the construction of a roof, there is a step between the rafters - strong bars that are the basis of the structure. This will be discussed in this article.
The distance between the base of the roof slopes is not a constant value and depends on the following components:
- type of roof;
- slope angle;
- type intended for installation roofing material;
- rafter section sizes.
Before starting the construction process top structure At home, you should perform a calculation, determining the optimal distance between the rafters.
Gable roof rafter spacing
Gable roofs are most widespread in our country. They are a design that has two parallel to the plane, with an inclination angle relative to the horizon from 20 to 50 degrees.
If the roof slope of a gable roof is insufficient in snowy areas, there is a risk of accumulation of large snow masses, which can lead to the destruction of the structure. An increase in the angle of the slopes in regions with a predominance of strong winds is also fraught with high loads and the danger of breaking not only the roof, but the entire structure as a whole.
Mansard roof rafter system
Most private houses have a usable under-roof space called an attic. This design is characterized by an increased height of the slope, which is caused by the need to create a living space comfortable height. As a rule, stingrays mansard roof broken lines with varying slope angles. For their installation, a double rafter system is used.
The steepness of the lower slopes of the attic roof significantly exceeds the slope of their upper extensions. The plane load perceived by them is not large. Thanks to this, the rafters in the lower part can be installed with maximum spacing. It is recommended to install the upper ridge slopes with a reduced gap from each other.
Rafters in a pitched roof
For outbuildings and some private houses, roofs with one slope are used. Due to the limitation of the angle of inclination, they are exposed to high pressure. Experts recommend for rafters pitched roof use lumber with an increased cross-section, setting a minimum step from each other.
When calculating the distances at which roof beams are installed, Special attention should be given to the magnitude of the snow load in a particular area. With a small slope, this characteristic is of great importance. It is better to choose roofing material for such roofs with a minimum dead weight, which will reduce the bending load.
Hip roof rafter system
The hip roof rafter system is considered the most complex in construction. This type is called hipped, since the roof is formed not only by side, but also by additional end slopes, where the rafters are installed not on the ridge, but on the corner strings. This places special demands on the organization of the roof frame.
Under hip roof th attic is not often arranged. It's connected with small angle the slope of the rafters and the roof as a whole. If the angle of the slopes to the horizon increases, the distance between the rafters increases; if it decreases, vice versa. An additional aspect of the calculation is the roofing material used.
Dependence of rafter pitch on roofing material
In addition to snow and wind loads, which are variable, the roof is also subject to constant (static) loads, the force of which depends on the roofing material used. It's no secret that different kinds roofs have their own mass, which can differ by 10 or more times.
The correct choice of material affects not only the upper, but also all other parts of the structure of a residential building and other buildings. It is not without reason that when designing the foundation it is necessary to decide in advance on the choice of roof.
Corrugated sheet roofing
Currently, one of the most common roofing materials is profiled sheet, produced galvanized or with subsequent polymer coating. TO distinctive features profiled sheet will include the following parameters:
- High corrosion resistance;
- As a result, a long (more than 15 years) service life;
- Easy installation even without the necessary qualifications;
- Low leaf mass (weight of 1 m2 is 4-5 kg).
Since this roofing material does not place a high load on the rafter system, the distance between the elements is selected as much as possible for a particular angle of inclination. In addition, the profiled sheet does not require high strength characteristics from the roof sheathing. All this together allows us to minimize the overall load on the foundation and walls.
Metal roofing
The second common type of steel roofing materials is metal tiles. This type of corrugated sheet successfully imitates natural clay material, but with less weight (10 or more). A special feature of rafters for metal tiles is their smaller cross-sectional size.
When choosing at what distance to install the rafters, you should first of all be guided by dynamic load. Like corrugated sheets, metal tiles are not demanding on the size of the rafter legs and can be easily mounted on a sheathing made of inch boards coniferous species. All this does metal roofing low-cost.
Rafter system for ondulin
In the 21st century, corrugated sheet materials were replaced by a more durable and lightweight analogue - ondulin. Among others, it is the lightest material. The weight of the sheet does not exceed 6 kg.
The small thickness of ondulin sheets with slope angles of less than 15° requires the construction of a continuous sheathing made of plywood sheets, for example, which will require an appropriate spacing of the rafters. This should be taken into account when making calculations.
Slate roofing
Not so long ago, a wavy material made from an asbestos-cement mixture, called slate, was widespread. High mass and fragility are the main disadvantages, however, even today it finds its fans in the construction of various outbuildings.
High mass comparable to weight clay tiles will not allow using the same rafter system as for metal tiles. Building codes determine the minimum slope angle of a slate roof to be 22 degrees or more. Otherwise, the load from the material itself and rafter system with sheathing exceeds permissible parameters. The pitch of the inclined beams, as well as their cross-section, are selected individually in each specific case.
Polycarbonate on the roof
IN last years more and more often artificial artificial wood began to be used on the roofs of verandas and gazebos. polymer material– polycarbonate. Available in two versions - monolithic and cellular. The first one is similar in its properties to the ordinary one. quartz glass, but significantly exceeds it in strength. The second has lower mechanical properties, but high thermal insulation and light transmittance.
Cellular polycarbonate is usually much lighter than its monolithic counterpart. It is used as a roof without the use of lathing, provided that the pitch does not exceed ½ the width of the sheet of material. The high strength of the monolithic analogue also allows you to avoid elements transverse to the rafters. Sufficient flexibility allows you to cover semicircular roofs on metal frame, the step of which does not exceed 0.9 meters.
Thematic material:
Rafters for soft roofing
An original pattern can be obtained through the use of soft roofing materials, spread with an adhesive layer. Which are installed on continuous sheathing from plywood or OSB. The pitch of the rafters should allow the sheets to be secured, so it is chosen as a multiple of ½ the width. Provided the standard plywood dimensions are 1520x1520 mm, the center distance between the rafters will be equal to: 1520:3 = 506 mm.
Rafter spacing for insulation
The installation of residential under-roof spaces is often combined with the laying of insulation sheets in the rafter gap. The most common slabs with dimensions are 600x1000mm. We use these parameters as starting points.
Scheme for calculating rafter pitch
By building regulations The pitch of the roof rafters is in the range of 0.6 - 1 meter. Its final calculation is performed using a simple formula depending on the total length of the roof. To calculate, you need to perform the following list of actions:
- determine what distance should be between the rafters for your specific construction conditions. The reference book determines the magnitude of wind and snow loads in the area.
- The length of the roof is divided by the desired distance, adding one. The result obtained will be equal to the number of rafter legs that are installed on one roof slope. If the value is not a whole number, it is rounded.
- The length of the roof is divided by the number of rafters calculated above, we get the final pitch in meters.
For example, with a slope slope of 30 degrees, the maximum distance between the rafters of a gable roof under metal tiles is 0.6 measures. The length is assumed to be 16 meters. Hence:
- 16:0,6+1=27,66;
- rounding the result, we get 28 rafters per slope;
- 16:28 = 0.57 meters - the center distance of the rafter legs for these specific conditions.
As you can see, the calculation technology is not complicated, but it is only approximate diagram. Taking into account many of the other parameters mentioned above can make certain adjustments.
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 spacing 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 perform a calculation bearing capacity 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 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 roof truss Such a roof has 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 70 and 120 cm.
What is the pitch depending on the roofing material
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.
A small one makes it possible to use boards of a smaller cross-section when creating a rafter system, and to use thinner bars for sheathing.
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 you place insulation with a thickness of 150 mm in the space between the rafters, then attic will be created the most 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.
Mounting of the upper support is not carried out ridge beam on the side, and on the ridge run.
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.
On 1 square meter the roof surface bears 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 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 can be greater - 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 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 appearance large quantity 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, at and the distance between them it is necessary to provide a safety margin.
Video about installation of the rafter system.
When carrying out installation work, it is very important to correctly determine the pitch of the rafters for the metal tiles. It is important to understand that making such calculations is quite difficult and the level of strength of the roof directly depends on how correctly everything is done. Many experts argue that the service life of not only the roof, but also the structure itself depends on the level of reliability of fastening the metal tiles to the rafters. As a result of the fact that metal tiles have quite heavy weight, the rafter system must be strong. In addition, beginners will not be able to lay roofing material. In order to carry out installation work correctly, it is recommended to take a responsible approach to calculating the pitch of the rafter system.
Construction of a rafter system for metal tiles
Making a rafter system for metal tiles with your own hands is not as difficult as making the appropriate calculations. It is important to understand that the type of system used depends entirely on the type of roof. Today, the most popular option is the rafter system for a gable roof under metal tiles. Such rafter systems, as practice shows, are excellent for installing metal tiles on the roof of a residential building. For arranging rafters they use wooden blocks different lengths - 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 rafters for metal tiles
The pitch of the rafters for metal tiles is the distance from one rafter leg to the other; to put it differently, roofers call this distance the pitch of the rafter system. 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 calculations of the distance for each material separately - there must be individual approach.
Before starting installation work, it is necessary not only to determine the distance between the rafters, but also to understand how much material will be required for the legs. To find out the pitch of the rafters of a gable roof under metal tiles or a pitched roof, it is recommended to adhere to the following recommendations:
- The first step is to calculate the length of the metal roof slope. Measurements are usually taken along the roof eaves.
- After this, it is possible to select an arbitrary step. It is important to take into account the fact that the pitch limits for metal tiles can vary from 0.6 to 1 m. For example, initially it became necessary to maintain a distance of 0.8 m. The length of the slope should be divided by the selected value for the pitch of the rafter system. Let's assume that the length of the slope is 12 m, it turns out that 12/0.8 and the result is 15. After this, you will 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 will 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 to be 0.75 m.
Many experts recommend entrusting this work 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 impact the finished roof structure.
Attention! As practice shows, such calculations can be used for any roofing materials.
Installation of rafters under metal tiles
Since installation work is carried out on high altitude, then, if necessary, you can assemble the frame for the load-bearing rafter system on the ground, then lift it and fix it on the roof. For these purposes, it is best to create a special template or diagram of the rafter system for metal tiles made from boards, according to which installation will be carried out.
The whole process is carried out in stages:
- First of all, the boards are lifted above the wall of the building, leveled and fixed with nails.
- According to the project, the boards must be aligned at a certain angle of inclination, after which all elements are securely fixed.
- Ultimately, a structure should be obtained that resembles a rafter system in appearance and shape. Manufacturing must be carried out taking into account the calculations made.
- Then you need to lower the template to the ground and install all the finishing elements.
Don't forget about installation supporting elements, namely, the Mauerlat. Laying is carried out on load-bearing walls in a longitudinal position. Special pins or wire rods are used as fastening elements.
The next step is the installation of the ridge girder, which is used as a supporting part for all roofing. For the work you will need to use timber or a hewn log. In cases where the length of the slope does not exceed 6 m, you can do without additional supporting elements.
When the installation of all elements has come to an end, you can begin installation and further installation work on arranging the rafter system according to the attached template.
The Mauerlat can be secured in several ways:
- rigid connection– in this case, beams or corners should be used, nails and staples are used much less often, the peculiarity is the fact that it will be necessary to additionally tie the rafters to the wall, using anchors for these purposes;
- sliding method– in such cases, a hinged connection must be created; it can be achieved by joining the elements with cuts.
During operation, the pitch distance of the rafter system must be observed.
When choosing a rafter pitch for metal tiles, it is worth paying attention that the distance should not be less than the minimum permissible value and greater than the maximum permissible 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 spread of up to 1.2 m.
If the legs of the rafter system are installed more often than every 0.6 m, 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 load-bearing capacity will be significantly reduced, which will negatively affect the metal tile structure.
If necessary, you can increase the level of strength, but for this you will need to make a continuous sheathing 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 exercise. installation work not used. This is due to the fact that despite quite high level strength, the metal tiles will begin to sag under the weight of snow in winter period time or during strong gusts of wind.
The cross-section of the timber used for arranging the rafter system is also important. It depends on him what distance will be chosen. The best option The choice of wooden blocks with a cross-section of 150 * 150 mm is considered, while the sheathing 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 for metal tiles 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 consumption of material and the project budget, and if the distance is made too large, then during operation the roofing material will begin to sag, which will lead to deformation of the metal tile.
The distance between the rafters is one of 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;
- weight roofing;
- weight of roofing materials (waterproofing, vapor barrier, insulation);
- weight of finishing elements 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 step cannot be extremely accurate, since it is necessary to take into account whole line additional factors, including the width of the insulation, features of installation of sheathing for various types of 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 a roofing covering, 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 sheets can be used for installation pitched roof utility room or a garage, or 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 sheets 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, as well as 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 supporting structures pitched 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. In order to correctly calculate at what distance it is better to lower the rafters, it is necessary to take into account the glazing area, the size and thickness of the sheets, and take into account that polycarbonate is attached with gaps for thermal expansion.
The distance between the rafters of a gable and pitched roof
Find out what distance between the rafters needs to be made for a gable and shed roof. Maximum distance between the rafters under metal tiles, ondulin and soft roofing.
What is the distance between the rafters of a gable roof?
Erection of the roof - The final stage any construction. This structural element is load-bearing structure, which takes on all the loads and then evenly distributes them over the surface of the load-bearing walls of the house.
Gable roof
A gable roof is rightfully considered the most common option for arranging suburban housing. And there is a completely logical explanation for this. It's no secret that what simpler design, the easier it is to build and the higher its reliability and performance characteristics.
The gable roof has a triangular shape and consists of two pitched planes connected at the highest point by a ridge girder, and the same number and shape of gables.
In principle, the construction of a gable roof is not considered such a complex operation. But, despite all its simplicity, it requires the most responsible and thoughtful approach.
Structurally, a gable roof has two slopes located in relation to the wall elements of the house at a certain angle, ensuring natural removal of precipitation from its surface. Also gable roofing structures can be attic or mansard type.
As a rule, gable roofs are installed on one-story, architecturally simple structures. A huge advantage is that the rafters in this case will not be particularly difficult to install.
The angle of inclination of the roof slopes of gable roofs
The angle of inclination of a gable roof cannot be less than 5 degrees, and in some cases it can even reach 90 degrees. Only at this angle of inclination of the roof slopes will the rafters be able to adequately cope with the function assigned to them.
To choose the angle of inclination of the slopes for gable roof influenced by a huge number of factors, such as climatic conditions, the type of roofing material chosen and architectural style building.
For example, flat roofs are ideal for construction in areas where strong hurricane winds constantly blow, but structures with steep slopes are an excellent option for those areas where significant amounts of precipitation fall.
Also, the angle of the roof affects its cost, this is due to the fact that a roof with a small angle of roof slopes will use much more roofing material than one with a large one.
However, this is not the only important parameter when building a gable roof. No less important when forming a gable roof is correct installation and fastening of the rafter system, as well as the step size between its elements.
The quality of the structure is also affected by the attachment of the rafters to the mauerlat and the ridge girder. Especially reliable fastening can be formed using cutting and additional fixation of connections with bolts.
The distance between the rafter legs (otherwise known as the pitch of the rafters) is considered one of the most important parameters when installing a roof, which affects its strength characteristics.
A correctly calculated pitch between the rafters of a gable roof allows you to create a roof that is resistant to the high loads to which it will be subjected during operation.
Let's take a closer look at what the distance between the rafter legs should be.
Rafter spacing for a gable roof
The distance between the rafters of a gable roof is calculated during design. Correcting any mistakes made at this stage is quite problematic, and therefore if you are not sure own strength, then it is better to entrust its implementation to professional architects.
Reliable rafters are the basis of any house roof.
The rafter pitch is calculated taking into account the loads to which the structure will be subjected during operation and the angle of inclination of the roof slopes.
When making a proper calculation, the presence or absence of an insulating layer and the cross-section must be taken into account. structural elements rafter system, type of sheathing and roofing. It is necessary to correctly calculate the loads, as this will help you avoid making mistakes when choosing the rafter section.
The rafter pitch is the distance between two adjacent trusses.
Step-by-step instructions for calculating rafter pitch for a gable roof:
- We calculate the estimated length of the roof slopes.
- Divide the resulting number by optimal sizes step.
- Add one to the resulting number and round the result.
- We divide the length of the slopes by the rounded result.
As a result of these simple calculations, you will get the distance at which the rafters should be installed.
The pitch between the rafters cannot be calculated accurately, since to obtain more accurate data it is necessary to take into account a huge number of parameters, ranging from the width of the insulating layer, the type of roofing material and even the distance between the gable roof sheathing elements. Rafters installed correctly will serve you faithfully for decades.
Rafter spacing under asbestos-cement slate
An important parameter of asbestos-cement slate is its significant weight, due to which it requires the installation of powerful rafters that can withstand heavy loads. The distance between the rafters is selected based on the cross-section of the timber used to make the rafters.
It is equally important that the rafters of a gable roof are fastened correctly, as this will provide it with a high degree of reliability and a long service life.
The optimal pitch between the rafters when using asbestos-cement slate should not be less than 800 mm. Thanks to this, the rafters will be able to easily cope with the task assigned to them.
Rafters for metal tiles
We will not talk about all the advantages and disadvantages of this material; a lot has already been said about this. Let's take a better look at what the step should be between load-bearing elements designs. Rafters for metal tiles are installed in accordance with existing standards and requirements.
Thanks to low weight this coating can be used to cover roofs on which ceramic tiles or asbestos-cement slate were previously laid.
The size between the rafters in this case ranges from 600 to 900 mm, and the cross-section of the lumber is 50 by 150 mm. This step between the rafters will create the most reliable load-bearing frame for the roof.
However, in areas characterized by snowy winters, it is preferable to arrange a distance between rafters of 200 mm, and to use timber with a section of 50 by 200 mm for the manufacture of structural elements.
In order not to install a special system to maintain the insulation, you can adjust the distance between the rafters to the width of the thermal insulation material.
Rafters for ondulin
Ondulin or flexible tiles must be laid on a continuous, perfectly flat sheathing, and it is better if it is made of some kind of sheet material, which will allow the roof to withstand all loads.
The sheathing is attached to the rafter legs, located in increments of 600 to 1000 mm, depending on the angle of inclination of the roof slopes of the gable roof.
The correctly selected distance between the elements of the rafter system guarantees everything supporting frame high degree of strength and durability of the entire structure.
What is the distance between the rafters of a gable roof, YouSpec Construction Encyclopedia
The correctly selected distance between the elements of the rafter system guarantees the entire supporting frame a high degree of strength and durability of the entire
Distance between rafters: correct calculation
The reliability of the roof and its service life mainly depend on how correctly the calculations are made.
One of the most important parameters of the 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 the other.
When constructing roofs of private houses, this value is 1 meter.
But this figure is approximate.
To obtain the exact size of the interval, it is necessary to calculate the load-bearing capacity of the rafter system of the selected type.
To perform the correct calculation, use the following scheme:
- 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 obtain the exact number of 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.
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 obtain the optimal value for each material, it is necessary to perform a calculation.
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.
As can be seen from the table, the rafter pitch 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 of a gable roof, everything is performed according to the general calculation system (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 the maximum pitch values for the rafter legs are 70 and 120 cm.
What is the pitch depending on the roofing material
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 is most often used in the construction of country houses.
Because this material is much lighter than ceramic or cement tiles.
Although in appearance he is 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 the hip roof with hanging rafters, follow the link. Also about the design of hanging rafters.
About the price of Viking metal tiles here. Specifications, photos, 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 gap between the rafters, then the most comfortable living conditions will be created in the attic.
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.
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.
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 under corrugated sheeting, the 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.
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 constructing 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.
Distance between rafters: single-pitched, gable roof
One of the most important parameters when calculating a rafter system is the distance between the rafters. After all, how strong and stable it will be depends on it -> Calculation of the rafter system
The main element of the roof, which absorbs and resists all types of loads, is rafter system. Therefore, in order for your roof to reliably withstand all impacts environment, it is very important to do correct calculation rafter system.
To independently calculate the characteristics of the materials required for installing the rafter system, I provide simplified calculation formulas. Simplifications have been made to increase the strength of the structure. This will cause a slight increase in lumber consumption, but on small roofs of individual buildings it will be insignificant. These formulas can be used when calculating gable attic and mansard roofs, as well as single-pitch roofs.
Based on the calculation methodology given below, programmer Andrey Mutovkin (Andrey’s business card - mutovkin.rf) for his own needs developed a rafter system calculation program. At my request, he generously allowed me to post it on the site. You can download the program.
The calculation methodology is based on SNiP 2.01.07-85 “Loads and Impacts”, taking into account “Changes...” from 2008, as well as on the basis of formulas given in other sources. I developed this technique many years ago, and time has confirmed its correctness.
To calculate the rafter system, first of all, it is necessary to calculate all the loads acting on the roof.
I. Loads acting on the roof.
1. Snow loads.
2. Wind loads.
In addition to the above, the rafter system is also subject to loads from roof elements:
3. Roof weight.
4. Weight of rough flooring and sheathing.
5. Weight of insulation (in the case of an insulated attic).
6. The weight of the rafter system itself.
Let's consider all these loads in more detail.
1. Snow loads.
To calculate the snow load we use the formula:
Where,
S - desired value of snow load, kg/m²
µ - coefficient depending on the roof slope.
Sg - normative snow load, kg/m².
µ - coefficient depending on the roof slope α. Dimensionless quantity.
The roof slope angle α can be approximately determined by dividing the height H by half the span - L.
The results are summarized in the table:
Then, if α is less than or equal to 30°, µ = 1 ;
if α is greater than or equal to 60°, µ = 0;
If 30° is calculated using the formula:
µ = 0.033·(60-α);
Sg - standard snow load, kg/m².
For Russia it is accepted according to map 1 of mandatory appendix 5 of SNiP 2.01.07-85 “Loads and impacts”
For Belarus, the standard snow load Sg is determined
Technical code of PRACTICE Eurocode 1. EFFECTS ON STRUCTURES Part 1-3. General impacts. Snow loads. TKP EN1991-1-3-2009 (02250).
For example,
Brest (I) - 120 kg/m²,
Grodno (II) - 140 kg/m²,
Minsk (III) - 160 kg/m²,
Vitebsk (IV) - 180 kg/m².
Find the maximum possible snow load on a roof with a height of 2.5 m and a span of 7 m.
The building is located in the village. Babenki Ivanovo region. RF.
Using Map 1 of Mandatory Appendix 5 of SNiP 2.01.07-85 “Loads and Impacts” we determine Sg - the standard snow load for the city of Ivanovo (IV district):
Sg=240 kg/m²
Determine the roof slope angle α.
To do this, divide the roof height (H) by half the span (L): 2.5/3.5=0.714
and from the table we find the slope angle α=36°.
Since 30°, the calculation µ will be produced using the formula µ = 0.033·(60-α) .
Substituting the value α=36°, we find: µ = 0.033·(60-36)= 0.79
Then S=Sg·µ =240·0.79=189kg/m²;
the maximum possible snow load on our roof will be 189 kg/m².
2. Wind loads.
If the roof is steep (α > 30°), then due to its windage, the wind puts pressure on one of the slopes and tends to overturn it.
If the roof is flat (α, then the lifting aerodynamic force that arises when the wind bends around it, as well as turbulence under the overhangs, tend to lift this roof.
According to SNiP 2.01.07-85 “Loads and impacts” (in Belarus - Eurocode 1 IMPACTS ON STRUCTURES Part 1-4. General impacts. Wind impacts), the standard value of the average component of the wind load Wm at a height Z above the ground surface should be determined by the formula :
Where,
Wo is the standard value of wind pressure.
K is a coefficient that takes into account the change in wind pressure with height.
C - aerodynamic coefficient.
K is a coefficient that takes into account the change in wind pressure with height. Its values, depending on the height of the building and the nature of the terrain, are summarized in Table 3.
C - aerodynamic coefficient,
which, depending on the configuration of the building and the roof, can take values from minus 1.8 (the roof rises) to plus 0.8 (the wind presses on the roof). Since our calculation is simplified in the direction of increasing strength, we take the value of C equal to 0.8.
When building a roof, it must be remembered that wind forces tending to lift or tear off the roof can reach significant values, and therefore, the bottom of each rafter leg must be properly attached to the walls or mats.
This can be done by any means, for example, using annealed (for softness) steel wire with a diameter of 5 - 6 mm. With this wire, each rafter leg is screwed to the matrices or to the ears of the floor slabs. It's obvious that The heavier the roof, the better!
Determine the average wind load on the roof one-story house with the height of the ridge from the ground - 6 m. , slope angle α=36° in the village of Babenki, Ivanovo region. RF.
According to map 3 of Appendix 5 in “SNiP 2.01.07-85” we find that the Ivanovo region belongs to the second wind region Wo= 30 kg/m²
Since all buildings in the village are below 10m, coefficient K= 1.0
The value of the aerodynamic coefficient C is taken equal to 0.8
standard value of the average component of the wind load Wm = 30 1.0 0.8 = 24 kg/m².
For information: if the wind blows at the end of a given roof, then a lifting (tearing) force of up to 33.6 kg/m² acts on its edge
3. Roof weight.
Different types of roofing have the following weight:
1. Slate 10 - 15 kg/m²;
2. Ondulin (bitumen slate) 4 - 6 kg/m²;
3. Ceramic tiles 35 - 50kg/m²;
4. Cement-sand tiles 40 - 50 kg/m²;
5. Bituminous shingles 8 - 12 kg/m²;
6. Metal tiles 4 - 5 kg/m²;
7. Corrugated sheeting 4 - 5 kg/m²;
4. Weight of rough flooring, sheathing and rafter system.
The weight of the rough flooring is 18 - 20 kg/m²;
Sheathing weight 8 - 10 kg/m²;
The weight of the rafter system itself is 15 - 20 kg/m²;
When calculating the final load on the rafter system, all of the above loads are summed up.
And now I'll tell you little secret. Sellers of certain types of roofing materials as one of the positive properties note their lightness, which, according to them, will lead to significant savings in lumber in the manufacture of the rafter system.
To refute this statement, I will give the following example.
Calculation of the load on the rafter system when using various roofing materials.
Let's calculate the load on the rafter system when using the heaviest one (Cement-sand tiles
50 kg/m²) and the lightest (Metal tile 5 kg/m²) roofing material for our house in the village of Babenki, Ivanovo region. RF.
Cement-sand tiles:
Wind loads - 24kg/m²
Roof weight - 50 kg/m²
Sheathing weight - 20 kg/m²
Total - 303 kg/m²
Metal tiles:
Snow load - 189kg/m²
Wind loads - 24kg/m²
Roof weight - 5 kg/m²
Sheathing weight - 20 kg/m²
The weight of the rafter system itself is 20 kg/m²
Total - 258 kg/m²
Obviously, the existing difference in design loads (only about 15%) cannot lead to any significant savings in lumber.
So, we figured out the calculation of the total load Q acting per square meter of roof!
I especially draw your attention: when making calculations, pay close attention to the dimensions!!!
II. Calculation of the rafter system.
Rafter system consists of separate rafters (rafter legs), so the calculation comes down to determining the load on each rafter leg separately and calculating the cross-section of an individual rafter leg.
1. Find the distributed load per linear meter of each rafter leg.
Where
Qr - distributed load per linear meter of rafter leg - kg/m,
A - distance between rafters (rafter pitch) - m,
Q is the total load acting on a square meter of roof - kg/m².
2. Determine the working area in the rafter leg maximum length Lmax.
3. We calculate the minimum cross-section of the rafter leg material.
When choosing material for rafters, we are guided by the table of standard sizes of lumber (GOST 24454-80 Softwood lumber. Dimensions), which are summarized in Table 4.
| 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 |
A. We calculate the cross-section of the rafter leg.
We arbitrarily set the width of the section in accordance with standard dimensions, and determine the height of the section using the formula:
H ≥ 8.6 Lmax sqrt(Qr/(BRben)), if the roof slope α
H ≥ 9.5 Lmax sqrt(Qr/(BRben)), if the roof slope α > 30°.
H - section height cm,
B - section width cm,
Rbend - bending resistance of wood, kg/cm².
For pine and spruce Rben is equal to:
1st grade - 140 kg/cm²;
2nd grade - 130 kg/cm²;
3rd grade - 85 kg/cm²;
sqrt - square root
B. We check whether the deflection value is within the standard.
The normalized deflection of the material under load for all roof elements should not exceed L/200. Where, L is the length of the working section.
This condition is satisfied if the following inequality is true:
3.125 Qr (Lmax)³/(B H³) ≤ 1
Where,
Qr - distributed load per linear meter of rafter leg - kg/m,
Lmax - working section of the rafter leg with maximum length m,
B - section width cm,
H - section height cm,
If the inequality is not met, then increase B or H.
Condition:
Roof pitch angle α = 36°;
Rafter pitch A= 0.8 m;
The working section of the rafter leg of maximum length Lmax = 2.8 m;
Material - 1st grade pine (Rbending = 140 kg/cm²);
Roofing - cement-sand tiles (Roofing weight - 50 kg/m²).
As it was calculated, the total load acting on a square meter of roof is Q = 303 kg/m².
1. Find the distributed load per linear meter of each rafter leg Qr=A·Q;
Qr=0.8·303=242 kg/m;
2. Choose the thickness of the board for the rafters - 5 cm.
Let's calculate the cross-section of the rafter leg with a section width of 5 cm.
Then, H ≥ 9.5 Lmax sqrt(Qr/BRben), since the roof slope α > 30°:
H ≥ 9.5 2.8 sqrt(242/5 140)
H ≥15.6 cm;
From the table of standard sizes of lumber, select a board with the closest cross-section:
width - 5 cm, height - 17.5 cm.
3. We check whether the deflection value is within the standard. To do this, the following inequality must be observed:
3.125 Qr (Lmax)³/B H³ ≤ 1
Substituting the values, we have: 3.125·242·(2.8)³ / 5·(17.5)³= 0.61
Meaning 0.61, which means the cross-section of the rafter material is chosen correctly.
The cross-section of the rafters, installed in increments of 0.8 m, for the roof of our house will be: width - 5 cm, height - 17.5 cm.
