When building a roof, first of all choose the type truss system. This means that the rafters must be hanging or layered, or the entire rafter system will consist of both rafters, that is, combined. It is not difficult to calculate the distance between the rafters, knowing the magnitude of the loads that will act on the entire roof.
Loads are temporary and permanent. Permanent loads include:
- the weight of the materials of the truss system itself;
- weight of roofing material;
- the weight of the elements of the roofing cake, if the roof is insulated;
- weight interior decoration attic ceiling.
Live loads include: the weight of the snow cover, the weight of a person during roofing work and maintenance of the roof during operation, wind load and the weight of possible water flows in summer.
What are the calculations based on?
The distance between the roof rafters is also calculated based on the selected material and the section of the rafters. All calculations are carried out at the design stage of the house by specialists of design organizations. To do this, they use reference documents and building codes and regulations governing a particular step of the rafters, the step of the crate and other calculated values.
When building a private house, rarely does anyone order a project where, based on calculations, it is recommended what distance between the rafters must be maintained. Most often, developers rely on the experience of the masters and trust them to conduct all the calculations on the spot. However, there are those who want to make the calculation of the truss system themselves and build a roof on their own.
In this case, they must first of all decide on the type of roof, choose which attic will be - heated residential or cold, since the type of truss system and the calculation of the distance between the rafters directly depend on these parameters
The distance between the rafters of a pitched roof
Calculation for shed roof- simplest. The rafter system does not provide for racks and braces - only the rafters themselves. Such a roof is most often equipped in utility rooms, garages, bathhouses, etc.
The step of the rafters of a shed roof is presented in the table below and depends on the section of the beam:
If the developer wants to independently calculate the loads on the truss system, it will be useful for him to familiarize himself with such documents as SNiP 2.01.85 "Loads and impacts" and "Changes from SNiP 2.01.85", where maps are laid for the climatic regions of Russia.
As a material for the construction of the truss system, coniferous wood is chosen, the humidity of which is 20-22%. Boards, beams and other types of lumber should be smooth without pilosity, rot, knots and blue.
All wooden elements should be treated with antiseptics and flame retardants to protect the wood from decay and fire. Material used, length rafter legs and the cross section of the beam also affects the calculations (see table):
Approximate calculation
The step of the rafters is nothing but the distance between the rafter legs. As a rule, for residential private houses, this figure is 0.6-1.0 m.
The pitch of the rafters is influenced by a significant number of factors. Payment the right amount rafter legs are made in the following way:
- The length of the roof slope is measured. After that, the resulting value is divided by the selected step between the rafters.
- Then one is added to the resulting value and rounded up to get an integer. This is how the number of rafter legs required for a given length of one roof slope is obtained.
- After that, the entire length of the roof slope is divided by the resulting integer (number of rafter legs) and as a result, the distance between the axes of future rafters is obtained, which is called the rafter pitch.
- The axis should run in the center of the rafter. However, for different kind roofing pitch rafters can vary, it also varies depending on the angle of slope of the roof slope.
For example: the length of the roof slope is 16 meters, and the step between the rafters is 0.6 meters. Therefore, 16 / 0.6 + 1 \u003d 27.66 \u003d 28 meters (rafters are necessary for the slope device)
The greater the slope angle, i.e. the steeper the roof, the greater the pitch of the rafters. This is due to the fact that the load on steep roofs is not distributed over the entire roof plane, but is mostly transferred to bearing walls.
Rafters for ceramic tiles
Ceramic tile is a material that has a decent weight. Therefore, for a tiled roof, the cross section and length of the rafters should be calculated, given that one square meter ceramics weighs almost ten times the weight of metal tiles.
- If in the design of the rafter system it is supposed to tighten the rafters, then its cross section should be increased.
- The rafters themselves must be made of dry healthy wood, their cross section must be 50x150 or 60x180 mm.
- The pitch of the rafters is done in the range of 0.8 -1.3 m for ceramic tiles, and its exact determination depends on the slope angle of the slopes.
The higher the roof, the wider the pitch of the rafters. For example, with a slope angle of 15 degrees, the step will be 80 cm, and if the slope angle rises to 75 degrees, the distance between the rafter legs will increase to 1.3 m.
Another circumstance affects the pitch of the rafters - this is the length of the rafter legs. The longer they are, the shorter their step. This is due to the fact that with the maximum length of the rafters, increased deflection loads are created. They can be reduced if you arrange a system of support posts, struts and other elements of the truss system, which take on part of the load from the rafters.
Rafters for a metal roof
The truss system for a metal tile is not much different from other truss structures. But due to the fact that it has a weight several times less than ceramic tiles, rafters and all other elements of the roof structure can be made with a smaller margin of safety, i.e., a smaller section. The difference in the weight of clay and metal tiles is about 35 kg per 1 m2.
The pitch of the rafters under the metal tile, as a rule, is 0.6-0.9 m. Metal tile is widely used for covering country houses, in which attic rooms are often provided, so the roof is made insulated. The pitch of the rafters in such structures is adjusted to the dimensions of the insulation, which is inserted between the rafter legs.
When carrying out installation work, it is very important to correctly determine the pitch of the rafters under the metal tile. It is important to understand that it is quite difficult to make such calculations, and it is on how correctly everything will be done that the level of roof strength directly depends. Many experts argue that the operational life of not only the roof, but also the structure itself depends on the level of reliability of attaching the metal tile to the rafters. As a result of the fact that the metal tile has a rather large weight, the truss system must be strong. In addition, beginners will not be able to lay roofing material. In order to properly carry out installation work, it is recommended to responsibly approach the calculation of the step of the truss system.
The device of the truss system under the metal tile
Do-it-yourself rafter system for metal tiles is not as difficult to perform as it is to make the appropriate calculations. It is important to understand that the type of system used depends entirely on the type of roof. To date, the most popular option is the truss system for gable roof under the metal roof. Such truss systems, as practice shows, are great for installing metal tiles on the roof of a dwelling. To equip the rafters, wooden blocks of different lengths are used - the choice, as a rule, directly depends on the length of the legs. Many experts recommend paying attention to the fact that it is necessary to choose the right angle of inclination, which can vary from 15 to 30 degrees - this value is considered optimal.
Calculation of the distance between the rafters for metal tiles
The step of the rafter under the metal tile is the distance from one rafter leg to the other, to put it differently, then this distance is called the step of the rafter system by roofers. This distance directly depends on the type of roofing chosen. As practice shows, for all roofing materials, including for metal tiles, this size varies from 0.6 to 1.2 m. However, it is important to understand that it is best to carry out accurate distance calculations for each material separately - there must be an individual approach.
Before proceeding with the installation work, it is necessary not only to determine the distance between the rafters, but also to understand how much material is required for the legs. To find out the pitch of the rafters of a gable roof under a metal tile or a shed, it is recommended to adhere to the following recommendations:
- First of all, calculate the length of the roof slope from the metal tile. Measurements, as a rule, are carried out along the eaves of the roof.
- After that, it is possible to choose an arbitrary step. It is important to take into account the fact that the step limits for metal tiles can vary from 0.6 to 1 m. For example, initially it became necessary to adhere to a distance of 0.8 m. The length of the slope should be divided by the chosen value for the step of the truss system. Let's say that the length of the slope is 12 m, it turns out that 12 / 0.8 and it turns out 15. After that, you need to add 1 to the resulting value. In the case when the number turns out to be fractional, then it should definitely be rounded up. Thus, it turns out 15+1=16. This value shows exactly how many rafter legs should be installed on the roof. Then you need to take the length of the slope and divide it by the resulting number of legs. It turns out that 12/16 and it turns out 0.75 m.
Many experts recommend entrusting these works to professionals, which will allow you to get the desired result in the future. It is important to understand that there is a high probability that mistakes will be made, which will ultimately negatively affect finished construction roofs.
Attention! As practice shows, such calculations can be used for any roofing materials.
Installation of rafters under the metal tile
Since the installation work is carried out at a great height, if necessary, it is possible to assemble the frame for the supporting truss system on the ground, and then lift and fix it on the roof. For these purposes, it is best to create a special template or scheme of the truss system for the metal tile from the boards, according to which the installation will be carried out.
The whole process is carried out in stages:
- First of all, the boards are raised 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 design should be obtained, which, according to appearance and the shape resembles a truss system. 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 to install supporting elements, namely, Mauerlat. Laying is carried out on load-bearing walls in a longitudinal position. As fasteners, special studs or wire rods are used.
The next step is the installation of the ridge run, which is used as a supporting part for all roofs. For work, you will need to use a beam or a hewn log. In cases where the length of the slope does not exceed 6 m, then additional supporting elements can be dispensed with.
When the installation of all elements has come to an end, you can proceed with the installation and further installation work on the arrangement of the truss system according to the attached template.
Mauerlat can be fixed in several ways:
- rigid connection- in this case, a beam or corners should be used, nails and staples are used much less often, a feature is the fact that it will be necessary to additionally tie the rafters to the wall, using anchors for this purpose;
- sliding way- in such cases, an articulated connection must be created, it can be done by joining the elements with cuts.
During operation, the step distance of the truss system must be observed.
When choosing a rafter pitch for a metal tile, you should pay attention that the distance should not be less than the minimum allowable value and more than the maximum allowable value, that is, it can vary from 0.6 m to 1.2 m.
If it was decided to choose a distance from 0.6 to 1 m, then the length of the rafters in this case should not be more than 6 m. In the event that it is reduced, it is allowed to make a run up to 1.2 m.
If the legs of the truss system are installed more often than after 0.6 m, then this will only contribute to a significant weighting of the roof and, as a rule, an increase in costs. If necessary, the distance can be 1.2 m, but it is important to understand that in this case the level of bearing capacity will be significantly reduced, which will negatively affect the structure of the metal tile.
If necessary, you can increase the level of strength, then for this you will need to perform a continuous crate of wood boards. In such situations, the step can vary from 0.2 to 0.3 m. This option is used extremely rarely, since its implementation requires much more investment.
Spacing of 1.2m almost never during exercise installation work not used. This is due to the fact that despite the rather high level of strength, the metal tile will begin to sag under the weight of snow in winter period time or strong wind gusts.
An important value is the cross section of the timber used for arranging the truss system. It depends on him what distance will be chosen. The best option the choice of wooden bars with a cross section of 150 * 150 mm is considered, while the crate can be installed at intervals of 4 to 7 cm. In this case, it is recommended to take into account the step of the transverse wave.
Conclusion
The pitch of the rafters under the metal tile can only be determined after the appropriate calculations have been made. It is important to understand that if the distance is quite small, this can significantly increase the material consumption and project budget, and if the distance is made too large, then the roofing material will begin to sag during operation, which will lead to deformation of the metal tile.
The 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 a roof depend on many factors: building materials, climatic conditions, crate reliability.
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 pitch
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 " 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 Exact size step, 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 finishing materials attic
- 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 a 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 step.
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, 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: for a gentle one- or gable roof 63 - 67 cm is enough. 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 mansard roof under the metal tile, you will have to lay a heater with a thickness of 150 - 200 mm.
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 peculiarities 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
Distinctive feature 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).
In building a house, it is important that the roof frame and its rafter structure are reliable. To do this, you need to know the optimal size of the gap between the rafters. How to correctly carry out calculations for the gable roof truss system, taking into account various roofing materials, our experts will tell in this article.
How to correctly calculate the distance between the rafters (for a gable roof)
Distance for various roofing materials
Experts advise: when determining the step between the rafter supports, consider the roofing material from which the roof of the gable roof will be created. Consider the most common roofing options suitable for private residential buildings.
- Decking - for this roofing material, the distance we need for a digital indicator is from 60 to 90 centimeters (if you increase the step to at least one meter, you will have to install additional transverse boards with a large section - 5 × 10 cm). At the same time, under the corrugated board, it will be necessary to install the crate, which will provide the possibility of attaching the mandatory vertical components - the chimney, the ventilation system.
- Ceramic tiles - a roof made of such material has a rather heavy weight, therefore, the load on the truss system will be higher than when the roof is covered with corrugated board. Accordingly, the distance that we calculate will be different: from 80 centimeters to 1 meter 30 centimeters. But in this option, the steepness of the roof slope is also taken into account (if the angle of inclination is 15 degrees, then the gap will be minimal - 80 cm, at 75 degrees - 130 cm).
- Metal tile - this roofing material is lighter than the previous one and is sold immediately in sheet form, therefore the thickness of the components of the truss system is reduced, and, consequently, the distance between the elements in it is also reduced. For roofing material with a cross section of 15 × 5 centimeters, the gap should be selected from 60 centimeters to 95 centimeters. Such a distance will make it possible to qualitatively insulate the attic space (for this, it is necessary to place the insulation material directly between the rafters). Installation feature roof structure under the metal tile lies in the fact that the supports are installed on top of the ridge run (usually they are mounted on the side of the ridge beam). But in this case, do not forget about ventilation holes around the top roof.
- Ondulin - this material is called "Euroslate", and it is perfect for the roof of private residential buildings. For the truss structure under the ondulin, rafters from boards of 5 × 20 centimeters are used, which provides a certain margin of safety (this also takes into account the rafter runs). The distance between such rafter boards is standard - from 60 to 90 centimeters, and the wooden beam for the crate should be 4 × 5 cm in section, with an interaxal step of 60 cm.
- Slate is one of the most common roofing materials in our country; under it, rafters should be selected with a section of 5 × 10 / 5 × 150 centimeters. The distance between them will be in a slightly smaller range - 60-80 cm. wooden beam has its own gap between the elements, which directly depends on the angle of the slope of the roof slope: with a small slope - from 45 cm, with a larger one - from 60 cm.
Thus, the general digital indicators for determining the distance between the rafters (depending on the roofing material) will fluctuate within the following limits: the minimum is 60 cm, and the maximum is 130 cm. If you yourself cannot decide on the choice of such a digital indicator, then you should contact to qualified professionals for help.
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 to the rafter system small gazebo, hidden under the crowns of trees, have completely different requirements.
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:
Second point: Russian region the standard roof of the house is constantly affected by such an atmospheric phenomenon as snow. 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 put the step of the rafters under standard sizes insulation plates, and these 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 can certain moment affect 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 technical specifications materials and SNiPam.
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. talking plain language, 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!
