Distance between runs. All about gable roofs: the optimal distance between the rafters
The gable roof is the most popular design in private housing construction. The truss system of a gable roof should provide a support platform for mounting the roof. Properly calculated pitch of the rafters of a gable roof allows you to ensure the strength and stability of the structure to all external loads, create a durable and reliable roofing. This article discusses the design features of the truss system, the calculation and installation procedure. This information will allow you to correctly understand the methodology for creating a gable roof, so that during construction you do not ask unnecessary questions to workers.
The distance between the rafters for a gable roof
Among the people involved in the construction of a private or country house, there are constant discussions about the most successful roof design and the number of supporting elements. In order to understand these opinions and take one side or another, it is necessary to consider the general structure of the roof.
There are two types of rafter construction:
hanging.
Layered.
They are used on relatively small houses with a length of supporting elements of not more than 6 m. The structure consists of a number of truss trusses having the shape of an isosceles triangle. The trusses are installed on a strapping made of timber (Mauerlat), interconnected by batten strips. The bearing capacity of hanging rafters is relatively small, but the simplicity of design, economy and high speed of installation are their advantages. There are quite a few options for implementing hanging rafters, which is explained by the prevalence of small buildings that do not need a complex and massive roof structure.
Design layered truss system somewhat more difficult. Along the perimeter of the ceiling of the upper floor, a strong timber is laid - Mauerlat. Two (or more) vertical racks are installed along the longitudinal central axis, the height of which determines the angle of inclination of the slopes. A ridge run is installed between the posts, which runs along the entire length of the roof and serves as a reference line for the rafter legs. Each of them has two points of support - at the bottom it is a Mauerlat, and at the top - a ridge run.
For the formation of additional support, which excludes sagging of the supports, struts are used - inclined strips attached to the rafter legs at an angle close to a straight line and abutting in the lower part against the central lower bar - lying.
The step lag for the roof is the distance between two adjacent rafters. It is determined by the number of lags evenly distributed along the length of the longitudinal axis of the roof. The main supporting structures for the roof are rafters and lathing, which form inclined surfaces with a given geometry and area. The angle of inclination determines the wind and snow loads, and, with an increase in the angle, the wind load increases, and with a decrease, the snow load on the roof.
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What determines the pitch of the rafters
Consider the factors that determine the step between the rafters of a gable roof, in more detail. It must be immediately taken into account that the number of rafters determines the bearing capacity. Reducing their number complicates the installation of insulation, contributes to the sagging of the laths of the crate and the roof installed on them.
At the same time, too frequent placement of rafters also creates difficulties in the installation of insulation. It has to be adjusted in width, which creates waste, i.e. waste of material. Therefore, the size of the heat insulator is often used as a criterion for determining the pitch of the rafters. For example, many types of mineral wool are 60 cm wide, which is quite suitable for most gable roof designs.
In addition, the pitch of the rafters directly depends on their number. This is a calculated value, which is determined based on the bearing capacity of the supporting elements. The loads acting on the roofing, as well as the weight of the roofing itself, are quite large, so a stable and durable structure is needed to accept them. It should be borne in mind that the type of rafter system also matters, since layered rafters are much heavier than hanging rafters, they form a noticeable load on the walls and foundation of the house.
Wind and snow loads are external factors that affect the roof structure. The weight of snow in winter can crush or even break a roof if its design is not properly designed. The SNiP applications contain special data on the average annual amount of snow in different regions. Some of them have up to half a ton of snow per square meter.
In addition to snow, wind creates serious loads. At the same time, if the snow load is static, then the wind effects are sudden and uneven. The wind can rise at any time of the year, both in winter and in summer, which makes us take it seriously. When choosing the angle of inclination, you should find out the strength and direction of the prevailing winds in the region, find out about the possibility of hurricane gusts. Available in SNiP applications.
All these factors determine the parameters of the truss system and the distance between the individual supports.
The dependence of the pitch of the rafters on the material of the roof
There are many types of roofing. Most of them are noticeably outdated and are little used in modern construction. The most popular today are:
profiled sheet.
metal tile.
It should be noted that profiled sheet and corrugated board are one group of profiled coating. Some sources consider them to be different names for the same material, others separate them by wave height. The profiled sheet has a higher wave height, and the corrugated board is somewhat flatter. The first type can be used not only as a roof covering, but also as a material for the construction of fences, fences, metal garages, etc.
The corrugated board has a smaller wave and is intended for laying as a roof. Both materials have a common specific feature - profiling is performed only in the longitudinal direction, which expands their capabilities and allows you to make roofs for round roofs and canopies.
The step of the rafters of the gable roof under the corrugated board is selected taking into account this feature of the material. If the value is too high, it will be possible for the roof to sag between the rafters. In addition, there will be an excessive load on the battens, which may not be ready for it. An important parameter also becomes the area of the slope. The pitch of the rafters of a shed roof under corrugated board is much less than that of a gable roof, since the length of the supporting elements and the magnitude of the load in it increase significantly and require an increased number of supports.
The metal tile looks like a canvas made of natural ceramic tiles. In all its operational qualities, it is very similar to corrugated board or profiled sheet, but physically it has an important feature - the presence of not only longitudinal, but also transverse waves. This allows you to slightly increase the step between the rafters of a gable roof under a metal tile, or be guided when choosing the size of a heat insulator.
When using slate or ondulin, the pitch of the rafters is taken according to the degree of rigidity of these materials. Alternatively, increase the density of the lathing, or install a solid version of thick plywood.
Method for calculating the distance between the rafters
There are two calculation options:
Engineering calculation using appropriate formulas and techniques.
Using online calculators, giving an answer after entering their own initial data.
The first method is only suitable for a professional roof system designer. For an unprepared person, it is extremely difficult to perform such calculations, since various specific data, coefficients, tabular values are needed, which are difficult to find and must be used correctly. Calculations obtained using online calculators are also not recommended for use in real construction, but as an option to refine or verify the results of calculations performed by specialized organizations.
Video description
In the video you can see what the step of the rafters should be:
Conclusion
In conclusion, it should be recalled that all design and calculation activities must be carried out by trained and specially trained specialists. The pitch of the rafters is an important and crucial value that determines the bearing capacity of the roof. The damage caused by the inability to take on existing loads is quite serious and can call into question the possibility of living in a house, so it is necessary to take this issue as seriously as possible.
Before the construction of the truss system and the organization of the roof, a number of simple measures must be carried out, which will be the key to the integrity of the structure. This includes choosing the type of roof.
It can be hanging or layered, or maybe a combined version. This is when two types are combined in one building.
Live loads are snow cover on the roof in winter, water flows from rain in summer, wind, the load that a person exerts while carrying out maintenance work during the operation of the roof, etc.
Permanent loads include the weight of the rafter system itself, the weight of the roofing material and the roofing pie, the mass of interior decoration, if it is planned to organize an attic in the attic of the building.
When creating a project for a single-pitched or dual-pitched roof, first of all, they choose the type of truss structure, the angle of inclination of the roof slopes, as well as materials for the construction of the structure. When calculating the distance between the rafters, one should take into account the loads that will affect the roof of the building during operation.
- The weight of the roofing material;
- The weight of the building materials from which the roof truss frame is built;
- Weight of insulation, vapor barrier, waterproofing;
- The weight of the finishing material of the attic or residential attic space.
The following temporary loads also affect the roof of the structure:
- Snow weight;
- The weight of a worker performing maintenance and repair of the roof.
For the correct calculation of the pitch of the rafters, it is necessary to take into account the cross section of the elements of the structure under construction, the presence of thermal insulation of the roof, the type of lathing and roofing. Calculations should be performed on the basis of SNiP 2.01.85 "Loads and impacts".
Video on how to calculate the truss system:
Scheme for calculating the distance between the rafters
All calculations are carried out at the design stage of the building. They are carried out by design engineers of the organization where the house project was ordered.
If the project has not been drawn up and the person relies only on the experience of the masters who are entrusted with the construction of the roof, then in this case you need to familiarize yourself with the relevant documentation. These are SNiP 2.01.85 "Loads and Impacts" and "Changes from SNiP 2.01.85".
There is a detailed calculation scheme and a map of the country's climatic zones.
Further, they are determined with the type of roofing system and the purpose of the attic space. Indeed, depending on whether it is an unheated attic or a living room, the loads on the rafters will be different, and, consequently, the course of calculations also changes.
The calculations must necessarily take into account the climatic zone where the work is carried out, the structural material of the rafters and its cross section.
So, the pitch of the rafters is the distance between the rafter legs. It can vary within 0.6 m - 1.0 m.
Calculation progress:
- First, the length of the roof slope is measured. Further, this value is divided by the step length of the rafters of the selected material. This indicator is taken from the SNIP and is unique for each building material and also depends on the beam section used;
- one is added to the result of previous calculations and rounded up. Thus, an integer value is obtained and it indicates the required number of beams;
- the length of the roof slope is divided by the resulting integer. The result will show how many beams are needed to build a high-quality and durable roof.
Experts point to the following feature of roofs with a steep slope. When erecting them, you can reduce the distance between the rafters. This is due to the transfer of the load from the beams to the wall of the house.
The distance between the rafters for different types of roofs
However, the calculation of the distance between the rafters cannot be carried out so unambiguously. After all, this indicator also depends on the type of structural material used for the roof.
The distance between the beams for a roof made of metal
The average weight exerted on the roof is 35 kg/m². To withstand it, the roof must have a step of 0.6-0.9 m. In this case, a beam with a section of 50x150mm is used. 
However, metal tiles are often used in the construction of country houses and cottages. And in such structures, the attic space is often equipped as a living room.
This leads to the installation of a roofing pie and all kinds of heaters, which, in turn, put an additional load on the rafters. Therefore, it is recommended to increase the cross section of the beam for beams. This is especially true for regions with low air temperatures in winter. The recommended dimensions of the rafters are 50x200mm.
In addition, the distance between the rafters can be adjusted to the width of the insulation. This will not only increase the bearing capacity of the roof, but also simplify the arrangement of the heat-insulating layer, and reduce the consumption of insulation.
The distance between the beams for the roof of corrugated board
The recommended rafter pitch for a roof is 0.6 m - 0.9 m. However, here it is necessary to take into account the operational characteristics of the corrugated board itself. If the step is large, then the corrugated board will “sag” under its own weight, thereby changing the geometry of the roof and its technical characteristics.
To prevent this, you need to mount additional boards with a large section as a crate. They will act in some way as rafters.
The cross section of the rafters for roofing from corrugated board is 50x100 mm or 50x150 mm. The crate is made of a board with a section of 30x100 mm.
The distance between the beams for a roof made of ceramic tiles
Ceramic tiles are a heavy material for roofing. It exerts a load on the rafters in the range of 40-60 kg per m². That is why the distance between the rafters in this case will be minimal - 80-130 cm. The greater the weight, the smaller the distance. However, the latter indicator may decrease, depending on the angle of the roof. The larger it is, the less often you can mount beams.
Installation of the structure for ondulin
The step of the rafter legs under the ondulin should be 60-100 centimeters. For the manufacture of rafters, lumber with a section of 200 × 50 mm is used. This is enough to create the most durable and reliable truss frame.
It should be noted that under this roofing material it is necessary to make a continuous crate. Due to this, the material will better resist snow loads and solar exposure.
Sometimes a thinned type of crate is used. For its manufacture, a wooden beam is used. The distance between adjacent elements should be less than 30 centimeters. This option, as a rule, is more expensive compared to a continuous crate.
Rafter system under slate
Slate roofing is considered the most common in Russia. The main reason is the low cost of this roofing material, the ease and speed of installation work. Also an important advantage is the possibility of replacing individual damaged slate sheets with new ones.
The distance between the rafter legs for a slate roof should be 80 centimeters. This distance is considered the most optimal.
The crate under the slate should be thinned out. For its manufacture, a board or timber with a cross section of at least 30 millimeters is used. It is this thickness of the timber or board that is required for the qualitative distribution of the weight of the roofing and winter precipitation.
When erecting a rafter system, one should remember about a constant margin of safety. It may be needed in case of bad weather and under mechanical stress.
Rafter frame for a soft roof
Soft roofing includes soft tiles, bitumen-polymer and bituminous roll materials, as well as roofing membranes. The main advantages of this type of roof include low weight, as well as the absence of the need to build a massive rafter system.
The minimum pitch of the rafters is 60 centimeters, and the maximum is 150 centimeters. When erecting a rafter frame under a soft roof, the angle of inclination of the slopes should be taken into account. In other words, the smaller the slope of the slope of the roof slopes, the smaller the distance between the rafter legs for a continuous crate.
The distance between the rafters also depends on the material from which the crate will be made. The greater the thickness of the plywood or OSB sheet, the greater the pitch of the rafters can be.
Roof made of sandwich panels
Roofing of this type, as a rule, is erected on hangar-type buildings or houses made of sip panels. Sandwich panels have bending rigidity, so their installation does not require the installation of traditional rafter legs.
If the spans from the top of the wall to the ridge of the gable roof are small, then the sandwich panels are mounted without additional supports.
With a span length of more than 400 centimeters, it is necessary to install additional runs. When building a roof from sandwich panels on a residential building, a traditional truss frame is often erected. However, in this case, the distance between the rafter legs can be made large, since they serve as a support for the runs.
Choose the distance between the rafters based on the length of the bearing walls and the length of the material for the runs. Roofing made of sandwich panels is able to withstand high operational loads.
Construction of a truss frame for polycarbonate
Recently, polycarbonate is often used in the construction of roofing. It is used in the construction of sheds, winter gardens and arbors. The rafter frame and crate are made of metal or wood.
Polycarbonate differ in weight, which depends on the thickness of the sheet. The step of the crate for polycarbonate should be from 60 to 80 cm. The crate made of metal or wood is attached to the rafters (straight or arched).
The distance between the rafter legs for polycarbonate, as a rule, is 150-230 cm. To correctly calculate the distance between the rafters, it is necessary to take into account the glazing area, the thickness and dimensions of the sheets. It should also be remembered that polycarbonate sheets are mounted with small gaps.
The roof is one of the main elements of the house, so you should pay special attention to the correct calculation of the roof frame. Which will serve as a skeleton in your home. Incorrect calculation of all loads can lead to disastrous results in the form of deformation of the roof with its subsequent collapse.
Before proceeding with the installation of the roofing, it is necessary to calculate the truss system, which will depend on the material used, construction, and climatic conditions.
What you should pay attention to before making a calculation
Before starting the construction of the roof, it is necessary to choose the type of truss system. Then calculate all the loads that will fall on the roof. The main loads include the weight of the frame, roofing material, insulation, ceiling, and temporary loads are taken into account, among which are the weight of the snow cover, possible gusts of wind, the weight of a person during the installation and operation of the roof.
The distance between the rafters is calculated based on the selected type of rafters and the material with which the roof will be covered.
Types of rafters
In the construction of roofs of various designs, hanging or layered rafters are used. Sloped - used when installing one- or two-slope roofs. They have two points of support - load-bearing walls or a load-bearing wall and a ridge beam. Hanging rafters are used when it is necessary to cover large spans or create a sloping roof. In this case, the rafters rest with one end on the wall, and the other on the opposite rafter. The quality of the material from which the frame is made will directly affect the reliability of the entire roof.
Material for rafters
Currently, wood and metal are used. A wooden beam or log is used in the construction of residential buildings, garages and other buildings. Reinforced concrete or metal rafters are used in the construction of industrial facilities, shopping centers, where wide spans are needed.
Method of calculation
The distance between the rafter legs is called the pitch of the rafters. It should not exceed one meter, and the minimum value - 60 cm. Before making a calculation, it is necessary to measure the size of the roof slope. Then we divide the obtained value by the approximate value of the step between the rafters. One is added to the result and rounded up to the next whole number. So we find out the number of rafters that we need. In order to find out the exact distance between the rafters, for this we need to divide the length of the roof bevel by the number of rafter legs.
For greater clarity, consider an example calculation:
roof bevel length - 28.5 m
the step between the rafters is chosen equal to 80 cm
add one to the resulting number: 35.625 + 1 \u003d 36.625
as a result, we get that we need 37 rafter legs
the exact step of the rafters will be: 28.5 / 37 = 0.77 m
This is a general calculation method that requires adjustment depending on the roofing material.
The dependence of the pitch of the rafters on the material of the roof
The most common roofing materials are slate, metal tiles, corrugated board, ondulin, soft roofing.
Roof truss structure for slate roofs
Slate is widely used as a roofing material, due to its low cost, resistance to temperature extremes. Due to the severity of the material, it is necessary to install a powerful truss system. The distance between the rafters in this case will depend on the cross section of the beam. The optimal value is a distance of 80 cm, which will allow not only to withstand increased weight, but also significant snow and wind loads. In this case, the crate is made with a board at least 3 cm wide. Depending on the type of slate, the crate is made continuous for smooth sheets or sparse for wave ones.
Important: the slate must have at least three support points along the edges and in the center.
Step between rafters for metal tiles
Metal tiles are becoming more and more common among roofing materials. Which is characterized by a simple installation process, and will also help to create your own unique look for your home. Sheets of metal tiles are lightweight, which allow you to increase the distance between the rafters from 600 to 950 mm, with a beam section of 150x50 mm. The crate in this case will depend on the pitch of the sheet wave. So for a wave of 350 mm, a distance between the boards of 30-40 cm is possible.
Important: on the overhangs of the cornices, the edges of the bevels, the step of the lathing is made minimal for greater reliability of the roof.
Rafter pitch under corrugated board
Decking is a galvanized steel sheet coated with a special coating that protects it from the harmful effects of the environment. A large selection of colors and corrugation widths will allow you to create a reliable and aesthetically attractive roof. The step of the rafters under the corrugated board will directly depend on the characteristics of the type of profile used, the angle of the roof, its configuration. You should also take into account the fact that the crate under the profiled sheet is attached perpendicularly. The rafter system is made of beams with a section of 50-75 mm or from a board 20-50 mm thick and no more than 15 cm wide. sparse - the distance between the boards is 50-75 cm. The step of the rafters under the corrugated board is calculated according to the general scheme and is 60-90 cm.
Step of roof rafters from ondulin
Ondulin is a wavy sheet that has high strength and durability. With such a roof, the truss system is made of pine boards with a section of 50x200 mm, in increments of 60-90 cm. A sparse crate of 40x50 cm is made from above.
Important: if the roof has an angle of inclination greater than 50 degrees, then the crate must be solid.
Determining the pitch of the rafters for a pitched roof
A shed roof is simple and does not require any special installation skills from you. Most often it is installed on garages, baths, extensions. The roof frame consists of beams that rest on the walls of the building. The distance between the rafters of a shed roof will depend on the length of the rafters. Here you should correctly approach the choice of the section of the beam, since the greater the distance, the greater the load on the rafters. For the correct selection of the pitch of a shed roof, use the data from the table:
The distance between the rafters for a gable roof
The gable roof is the most common, not only because of the simplicity of the device, but also because of its high reliability. The pitch of the rafters of a gable roof is calculated according to the general scheme. If the bevels are the same on both sides, then one part can be calculated. To do this, we determine the angle of inclination of the bevel, here you should take into account the material with which you will cover the roof. So at an angle of 45 degrees or more, you can use all types of coatings. For slate, tiles, the minimum angle of inclination is 22 degrees, for corrugated board and soft tiles - 12 degrees, for metal tiles - 14 degrees, for ondulin - 6 degrees. The length of the rafters for a gable roof is calculated using the Pythagorean theorem, in which the length is the hypotenuse, and the height of the roof and half of its width are the legs. If the spans are more than 6 meters, then the rafter system is additionally reinforced with struts and headstocks, which will not allow the rafter legs to deform under the weight of the roofing material.
The distance between the rafters is a fundamental parameter on which the strength and reliability of the roof structure, its service life, and the possibility of using certain roofing materials depend.
The strength and service life of the roof depend on many factors: the quality of building materials, climatic conditions, the reliability of the crate.
But the supporting structure of the roof is the foundation on which the entire structure rests.
The rafter system must be accurately calculated, correctly mounted and reliably protected from destructive external influences.
General scheme for calculating the rafter step
The truss system is the supporting structure of the entire roof. It consists of rafter legs, vertical struts and inclined struts.
Each rafter is located at a certain distance from the next - this distance is called the "rafter step".
The strength of the roof structure, the maximum allowable load per square meter and the materials that can be used for roofing depend on it.
According to GOSTs, the minimum allowable value of the rafter pitch is 60 cm, the average is more than 1 m.
To determine the approximate pitch, you can use the following formula: D / (D / m + 1), where D is the length of the roof from ridge to ridge, m is the approximate rafter pitch.
All results must be rounded up to the nearest higher integer. It is obvious that such a formula serves only for approximate calculations.
To determine the exact step size, the following factors must be taken into account:
- own weight of the truss system, i.e. the materials from which it is made;
- the weight of the material with which you plan to cover the roof;
- weight of additional heaters, seals, hydro and vapor barrier systems;
- crate weight;
- weight of attic finishing materials;
- climatic loads (wind, snow accumulation).
In addition to the loads listed above, the roof must be capable of supporting the weight of at least one adult person, so that in the event of a repair or installation of an antenna, the installer can safely climb onto the roof.
If you plan to install a chimney, then its location must be included in the calculations initially so that in the future it will not be necessary to remove part of the roof and install additional support points.
Shed and gable roof: the difference in truss systems
For a shed roof, building a truss system is quite simple. Most often, the rafters are laid directly on the crown, without the use of additional supports and supporting structures.
That is why the maximum angle of inclination of a shed roof is limited to 30 degrees: the absence of additional load-bearing structures and supporting beams means that the entire load falls on the walls of the building and the foundation.
The optimal angle of inclination is 15 - 25 degrees. The maximum allowable span of rafters does not reach 6 m.
When building a shed roof, be sure to take into account the direction of the wind and the possible additional load from the weight of snow accumulated on the roof.
For houses located in regions with strong winds and low rainfall, you can guess the slope angle at which the roof is cleared of snow due to gusts of wind.
A gable roof is a system of two inclined slopes connected by a ridge. One of the main advantages of this design is the ability to more evenly distribute the load between the truss system and the load-bearing walls of the building.
In addition, the gable frame allows you to lean the rafters on each other, which gives it additional strength.
The overall strength of the roof structure increases as the slope angle approaches 45 degrees. It is this slope that is considered optimal for regions with heavy rainfall.
With an increase in the angle of inclination, on the one hand, stability increases significantly, so you can take a larger rafter pitch.
On the other hand, the windage of the roof increases, so for windy regions the optimal angle of inclination does not exceed 20 degrees.
Rafter system under slate
Despite the emergence of a large selection of modern roofing materials, the classic version - slate - is still very popular, mainly because of the cheapness and ease of installation.
The distance between the roof rafters under the slate is calculated taking into account the characteristics of the material: the slate is quite fragile, but at the same time it is able to withstand heavy weight loads.
The permissible range of the rafter step for slate is from 80 cm to 1.5 m. The average step length, 120 cm, is most often used.
Since the slate itself weighs quite a lot, the material for the supporting structure must be chosen durable, for example, bars with a cross section of at least 75 mm by 150 mm.
The length of the rafter pitch and the thickness of the crate are interrelated: the more durable the crate you plan to install, the smaller the pitch, and vice versa.
The slate sheet has a standard length of 175 mm, the lathing pitch is selected so that each slate sheet has at least three support points (one in the center of the sheet and two closer to the edges).
The step of the lathing depends on the degree of the slope of the roof: 63 - 67 cm is enough for a flat one- or two-slope roof. The minimum gap for a steep roof is 45 cm.
You can set the exact length of the rafter pitch for slate only by making accurate measurements and calculating the total weight of all roofing materials.
Do not forget to take into account the weather conditions (possibility of snow accumulation, strong wind gusts) and the load of additional equipment (antenna or chimney). If an attic is equipped in the attic, then consider the weight of the heaters.
The nuances of the system for metal tiles
Metal roofing is one of the most popular roofing materials on the market. It is unpretentious in handling, durable, looks beautiful.
In addition, metal tile is one of the lightest roofing materials (only 35 kg per square meter), it can be laid on a fairly light support, thereby reducing the load on the walls of the building and the foundation.
The average distance between the rafters under the metal tile is 60 - 95 cm for a gable roof with a slope of 20 - 45 degrees.
The size of the bars is chosen taking into account insulating and waterproofing materials. For a simple roof made of metal, a section of 50 - 150 mm is sufficient.
But in most cases, to create a mansard roof, it will be necessary to lay a heater with a thickness of 150 - 200 mm under the metal tile.
Taking into account the weight of the insulation, the truss system should be more durable, the recommended size of the bars increases to 200 mm by 50 mm.
When calculating the distance between the rafters, consider not only the length of the metal sheets, but also the insulation.
With the correct location of the rafters, you can significantly save on timber.
Do not forget about the features of mounting a roof made of metal: this material does not pass air well, as a result of which condensation often accumulates under the roof.
Fasten the top support of the truss system to the ridge run instead of the side part. This will create a small air gap, increase ventilation and help protect the roof from destructive moisture.
Rafter system for corrugated board and ondulin
A distinctive feature of corrugated board is lightness and rigidity, therefore, as in the case of metal tiles, the requirements for the rafter system are not so high.
The distance between the rafters under the corrugated board should be in the range from 60 cm to 120 cm. The optimal section of the rafter leg depends on the span between the supports.
So, for a span of 3 m, a beam with a size of 40 mm by 150 mm is chosen, for a span of 5 m - a size of 50 mm by 180 mm.
What distance is permissible between the rafter legs depends directly on the cross-section of the bars: the larger the rafter step, the more durable the material must be used for the rafters. When choosing a step of more than 80 cm, increase the thickness of the bars by 20 - 25%.
Do not forget to take into account the weight of the crate under the corrugated board. The distance between the rafters of a shed roof of 60 cm will require a crate with a minimum cross section of beams of 25 mm by 100 mm.
With a rafter pitch of 80 cm, it is 30 mm by 100 mm, etc. The angle of inclination of the roof also plays an important role: with a slope of less than 15 degrees, it is recommended to lay a continuous crate under the corrugated board, which is much heavier than a sparse one.
Because both are relatively light weight, the supporting structure can be light enough to reduce stress on the load-bearing walls and foundation of the building.
The higher the degree of inclination, the greater the distance between the rafters is allowed.
On a gable roof with a slope of less than 10 degrees, it is recommended to install a continuous crate, which increases the load on the roof structure.
In this case, it is better to use a thicker beam measuring 40 mm by 50 mm, and minimize the rafter step (60 cm).
The task of correctly calculating the distance between the rafters is very responsible. Not only the reliability and durability of the roof, but also all subsequent work on it will depend on how seriously you begin to solve it: laying insulation, installing roofing, installing additional elements. If you manipulate the step of the rafters under the roof sheets, as many do, then it is not a fact that a heater will then enter between the rafters.
If you focus only on insulation, the very first winter with its abundant Russian snow will crush the truss system. That is why the whole point is to choose the optimal rafter pitch for all slopes, and now we will teach you this skill.
What determines the pitch of the rafters?
So, the distance between the rafters is determined by such important factors:
- The shape of the roof (gable, single-pitched or multi-pitched).
- Roof angle.
- The parameters of the timber that is used for the manufacture of rafters (width, thickness).
- The design of the truss system (layered, hanging or sliding).
- The totality of all loads on the roof (covering weight, 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 must be taken into account, which is exactly what this article is about.
Online calculators vs. notebook and pencil
Today, there are many complex formulas for the correct calculation of the cross section of the rafters and the step of their installation. But remember that such formulas were at one time developed more in order to be able not only to ideally calculate the roof constructs, but to study the work of such elements.
For example, simple online programs that calculate the parameters of rafters are popular today. But ideally, if you can independently set specific tasks and calculate everything you need. It is important to understand to the smallest detail what exactly happens in the truss system during operation, what kind of forces act on it and what loads. And a computer program cannot always take into account everything that the human brain notices. Therefore, we advise you to make all the calculations manually.
Decorative rafters: 0% load
First of all, decide on the most important point: the type of roof and its purpose. The fact is that the roof of a residential building in winter can withstand a large cap of snow, a constant wind at a height and it is often insulated from the inside, but completely different requirements are imposed on the rafter system of a small gazebo hidden under the crowns of trees.
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 already a purely aesthetic factor:
In the above illustration, it can be seen that even in such a building there is a step of rafters. After all, here it provides both an aesthetic factor and the rigidity of the structure itself. But the step is chosen arbitrarily.
We approach the main question: what distance should be between the roof rafters of a residential building? Here, be patient and carefully study all the nuances.
Item 1. Wall length and rafter spacing
First of all, the step of installing rafters on the roof of a residential building is usually chosen according to the structural size of the building, although taking into account many other factors.
For example, it is easiest to install rafters in increments of 1 meter, so for a wall 6 meters long, 7 rafters are installed as standard. At the same time, you can save money by placing them with a distance of 1 and 2 meters, and you get exactly 5 rafters. It can also be placed with a distance of 2 and 3 meters, but reinforced with a crate. But it is highly undesirable to make a step of rafters more than 2 meters.
Item 2. 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 an ordinary roof is 1 meter. But, if there is a significant snow or wind load in the area, 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 zoom to a distance of 1.2m-1.4m.
Why is it so important? Let's figure it out. The fact is that the air flow collides on its way with the wall under the roof of the building, and turbulence occurs there, after which the wind hits the eaves of the roof. It turns out that the wind flow, as it were, goes around the slope of the roof, but at the same time trying to raise it. And in the roof at this moment there are forces that are ready to rip or overturn it - these are two windward sides and one lifting.
There is another force that arises from wind pressure and acts perpendicular to the slope, trying to push the roof slope inwards. And the greater the angle of inclination of the roof slope, the more important are the safe wind forces and the less tangential. And the larger the angle of the slope, the less often you need to put rafters.
To understand whether you should make a high roof or a flat one, this map of the average wind load will help:
The second point: in the Russian region, such an atmospheric phenomenon as snow constantly affects the standard roof of a house. And here, too, you need to consider that the snow bag usually accumulates more on one side of the roof than on the other.
That is why in such places where such a bag is possible, it is necessary to insert paired rafter legs or make a continuous crate. The easiest way to determine such places is by the wind rose: single rafters are placed on the windward side, and paired rafters 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 the standard width of mats
If you will insulate the roof, then it is advisable to set the pitch of the rafters under the standard dimensions of the insulation boards, which are 60, 80 cm and 120 cm.
Modern heaters today are sold in standard widths, usually at the same standard rafter spacing. 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. The quality and strength of the lumber used
Of great importance is also what kind of material you use to build the truss system. So, for each type of wood there is its own regulatory documentation, which concerns its bearing capacity:
Because for the manufacture of the roof truss system 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, then you can display the correction factor.
In addition, if there are sections, cuts or bolt holes on the rafter, in this place it is necessary to calculate the bearing capacity of the bar with a factor of 0.80.
Item 5. Distance between puffs and floor beams
One more thing: if the roof is built with interconnected roof trusses, and their lower belt is used simultaneously as floor beams, then the distance between the trusses must be made within 60-75 cm to take into account the design of the future floor.
Item 6. Loads on truss nodes
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, 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 able at a certain moment to act on the roof at the same time, and therefore there is such a thing as a critical value. This is exactly the value of the loads at which the roof does not withstand and deforms.
Therefore, if the building is being built with significant spans, then steel truss trusses are necessarily used. The fact is that there is no 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 farms is calculated depending on the type of roof and the design of the roof itself.
Usually, a unified truss is placed with a span that is a multiple of six, and therefore a distance is made between the truss nodes that is a multiple of one and a half meters.
Item 7. Weight of the rafter system and roofing cake
Do not forget that the main purpose of the rafters is to hold the entire roof on itself, and its weight is crucial:
Point 8. Ease of installation of roofing
The distance between the rafters is also influenced by such a factor as the selected roofing. The higher the slope of the roof, 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 a full slab? 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 fixed directly to the rafter or crate, and it is important that they match. Otherwise, roofing work will easily turn into a living hell at a height, believe me.
That is why, even before starting installation, it is necessary to make a layout, check everything several times. And to know some important subtleties for each type of coating.
Determining 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 truss system: the weight of the truss system, a snow cap, wind pressure. After you add all the loads together, be sure to multiply them by a factor of 1.1. So you all count on unexpected favorable conditions, that is, lay an additional 10% percent strength.
And now you just have to divide the total load by the planned number of rafters and see if each of them will cope with its task. If it seems that the design will be frail - feel free to add 1-2 rafters to the total, and you will be calm for your home.
You need to make a calculation for destruction, i.e. to the full load that acts on the roof. All these loads are determined by the technical characteristics of materials and SNiPs.
The standard roof structure is rafters, lattice runs, and each of these elements works only on the load that puts pressure on it, and not on the overall roof as a whole. Those. each individual rafter has its own load, total, but divided by the number of rafter legs, and by changing the step of their location, you change the load collection area on the rafters - reducing it or increasing it. And, if it is inconvenient for you to change the pitch of the rafters, then work with the parameters of the section of the rafter legs, and the total bearing capacity of the roof will increase significantly:
In 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 a slope of up to 30 degrees, the rafters are the so-called "bendable elements". Those. they work specifically for bending, and there are certain requirements for them. And the possibility of deflection of the rafters is calculated according to 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 the roof with a warehouse slope of more than 30 degrees, which rafters are already considered “bendable-compressed” elements. That is, they are affected not only by a uniformly distributed load, which causes the rafters to bend, but also by forces that are already acting along the axis of the rafter. In simple terms, here the rafters not only bend a little under the weight of the roof, but are also squeezed from the ridge to the Mauerlat. In addition, the crossbar, which usually holds back two rafter legs, must also be checked for tension.
As you can see, even a person who is far from construction can cope with 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 later all the work goes like clockwork!