Large rafter pitch. How the rafter system of a shed roof is arranged and the correct fastening of the rafters

One of the important stages of any construction is the construction of the roof of the building, the basis of which is the installation of a truss system. The uniform distribution of the weight of the roof on the frame, the reliability of the roof and the duration of its operation as a whole will completely depend on how correctly the pitch of the rafters is chosen, that is, the distance between them. With any slight calculation error, damage and collapse of the entire roof can occur.

Collection of the required calculated values ​​\u200b\u200bto establish the pitch of the rafters

The rafter system includes the following components:

• farms;
• ridge and side runs;
• Mauerlat;
• diagonal elements.

Farms consist of a crossbar, base, rafter legs, binding and fastening elements. The rafter legs are beams located along the slopes, the lower ends of which are connected to the Mauerlat, and the upper ends to the girders.

Rafter pitch is the exact distance between two rafter legs.

To get the exact indicated value, you must first find out the values ​​​​of some quantities important for this.

All calculations for the roof covering system should be correctly made at the design stage of the house. Such a responsible business can be carried out by yourself or entrusted to its specialists. The more accurate the calculations are, the stronger the base for the roof will be.

In order to correctly make all the required calculations, including the rafter pitch, you need to know:

• permanent and temporary loads;
• their size;
• slope angle;
• type of roofing;
• material for the manufacture of rafters;
• roof type.

One of the indicators that must be taken into account when calculating the pitch of the rafters are permanent and temporary loads. These include:

• the weight of the roofing and truss structure;
• weight of the interior roof finish;
• snow cover weight;
• the weight of the person carrying out the maintenance of the roof.

It should be noted that these data are taken in special tables developed by scientists, depending on climatic zones, for various territorial regions.

The angle of the slope can be determined by knowing the length of the wall of the house and the height of the roof. The steeper the roof, the more installed, since the load on such roofs will be transferred mainly to the load-bearing walls. After making further calculations, the length of the rafters is easily calculated. Most often they have standard sizes: 4 and 6 m.

Coniferous wood is well suited for the manufacture of a roofing system. Lumber must be of high quality, without knots, rot. Before work, all wooden elements must be treated with antiseptic special compounds.

The step of the rafters will also depend on the type of roof, that is, on the functional features and its shape. The size of a simple and complex roof will be different.

How the type of roofing will affect the pitch of the rafters should be considered in more detail.

General technology for calculating the distance between the rafters

Having made all the necessary measurements, it is possible to make a constructive calculation of the pitch of the rafters. Based on practice, this indicator is in the range from 0.6 to 1.0 m.

In order to correctly perform this calculation on your own, you must adhere to the following technology:

1. The total length of the roof slope is taken.
2. The optimal step of the truss truss is selected.
3. The length of the slope must be divided by the pitch of the rafters.
4. One is added to the resulting value.
5. To get a whole number, the result is rounded up.
6. This calculation is the number of trusses (rafter legs) located on one roof slope.
7. Now the entire length of the roof slope is divided by the resulting value, showing the number of trusses. This determines the pitch of the rafters.

It should be noted that this calculation is made in the center of future rafters. However, the calculations cannot be considered accurate and constant. In each case of the construction of the roof of a house, the angle of the roof slope, the features of the selected roofing material, and other indicators are taken into account. In this regard, it is necessary to consider the features of choosing a rafter pitch for the most common roofing materials: metal and ceramic tiles, corrugated board, slate and ondulin.

Features when choosing the distance between the rafters for metal and ceramic tiles

Metal tiles as roofing are extremely common in both urban and suburban construction. The very design of the roof truss system when using metal tiles will differ little from the general scheme. However, due to the fact that this building material is light in weight, rafters and other elements of the system can be taken with a smaller section, that is, with a smaller margin of safety. The distance between the rafters when using metal tiles is 60-95 cm. All work on the installation of roofing from metal tiles is much easier than from other roofing materials. A feature of the lathing device of this material is that the board near the eaves should be about 1.5 cm thicker than the others. Usually, the cross section of metal rafters is 50x150 mm.

Ceramic tile is a roofing material that has a significant weight, which is 10 times the weight of a metal tile. In this regard, it is necessary to more thoroughly approach the calculation of the roof truss system. should be slightly increased - up to 60x180 mm. The average distance between the rafters for covering ceramic tiles is set in the range of 0.8-1.3 m. The angle of the slopes of the slopes is necessarily taken into account: the distance between the rafters is made larger, the higher the roof.

The length of the rafter legs in this case will have a strong influence on the result. The distance between the rafters decreases with increasing their length.. With their long length, increased deflection loads may occur, which can be significantly reduced if an additional system of struts, supporting racks and other elements is installed.

Features when choosing a rafter pitch for slate, corrugated board and ondulin

The most common roofing material is slate. This is due to its low cost, ease of installation, and the possibility of replacement if individual elements of the roof are damaged.

The features of the crate for slate is its thinned type, consisting of any type of board or timber, but with a mandatory cross section of at least 30 mm. This is necessary for the correct and high-quality distribution of the weight of the slate on the crate.

The optimal distance between the rafters, when combined with the load from the weight of the slate sheets, should be 80 cm. It is worth constantly taking care of the margin of safety of the truss system, which may be due to various unforeseen circumstances. Otherwise, its installation under a slate roof does not differ from other options.

Now you should pay attention to some features when installing a truss system for roofing materials such as corrugated board and ondulin.

The weight of corrugated board and ondulin is much lighter than that of ceramic tiles or slate, so the technology for installing rafters for these materials is practically no different from installing metal tiles. The distance between the rafters for a corrugated roof is 60-90 cm, and from ondulin - 60-100 cm.

If it is necessary to increase the distance between the rafter legs, it is necessary to strengthen the structure of the system with transverse elements of the crate.

The sheathing device for corrugated board is very similar to the sheathing device made of metal tiles. It is important to remember that in this case, the last board should also be slightly thicker than all the others.

A feature of the installation of the crate for ondulin should be considered the need for its continuous type for high-quality resistance to various loads. In some cases, a thinned type crate can also be used, but then the step between the rafters should be no more than 30 cm.

Thus, being able to make all the necessary measurements, knowing the basics of installing a truss system for basic roofing materials (tiles, corrugated board, slate and ondulin), you can correctly calculate the distance between the rafters.

It is pointless to argue about the importance of the roof for any building. It is not for nothing that over a dozen different types of roofs have been invented in the history of mankind, from simple to quite complex in design and construction. An important element in planning the construction of a roof is the 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;
• the type of roofing material to be installed;
• rafter section dimensions.

Before proceeding with the process of erecting the upper structure of the house, a calculation should be performed by determining the optimal distance between the rafters.

Gable roof rafter pitch

The most widespread in our country are gable roofs. They are a structure that has two parallel planes, with an angle of inclination relative to the horizon from 20 to 50 degrees.

With an insufficient slope of the gable roof in snowy areas, there is a danger of accumulation of large snow masses, which can lead to the destruction of the structure. An increase in the slope angle in regions with a predominance of strong winds is also fraught with a high load and the risk of breaking not only the roof, but the entire structure as a whole.

Mansard roof truss system

Most private houses have an exploited 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 of a comfortable height. As a rule, the slopes of the mansard roof are broken, having a varying angle of slope. For their installation, a double truss system is used.

The steepness of the lower slopes of the mansard roof significantly exceeds the slope of their upper extensions. The plane load perceived by them is not great. Thanks to this, the rafters in the lower part can be installed with a maximum pitch. The upper ridge slopes are recommended to be mounted with a reduced gap from each other.

Rafters in a shed roof

For outbuildings and some private houses, roofs with one slope are used. Due to the limited angle of inclination, high pressure is exerted on them. Experts recommend using lumber of increased cross-section for the rafters of a single-pitched roof, with the installation of a minimum step from each other.

When calculating the distances at which roof beams are installed, special attention should be paid to the amount of snow load in a particular area. With a small slope, this characteristic is of great importance. Roofing material for such roofs is best chosen with a minimum dead weight, which will reduce the bending load.

Hip roof truss system

The hip roof truss system is considered the most difficult in construction. This type is called four-pitched, since the roof is formed not only by side, but also by additional end slopes, where the installation of rafters is carried out not on the ridge, but on the corner bowstrings. This places special demands on the organization of the roof frame.

Under the hip roof, the attic is not often arranged. This is due to the small angle of inclination of the rafters and the roof as a whole. In the case of an increase in the angle of the slopes to the horizon, the distance between the rafters is increased, with a decrease, vice versa. An additional aspect of the calculation is the roofing material used.

The dependence of the pitch of the rafters on the roofing material

In addition to variable snow and wind loads, a constant (static) one also acts on the roof, the force of which depends on the roofing material used. It is no secret that different types of roofs have their own weight, which can differ by 10 or more times.

The right choice of material affects not only the top, but also all other parts of the structure of a residential building and other buildings. Not without reason, when designing the foundation, it is necessary to decide in advance on the choice of the roof.

Roofing from profiled sheet

Currently, one of the most common roofing materials is a profiled sheet, which is produced galvanized or followed by a polymer coating. The distinguishing features of the profiled sheet include the following parameters:

1. High corrosion resistance;
2. As a result, a long (more than 15 years) service life;
3. Easy installation even without the necessary qualification;
4. Small sheet weight (weight 1 m 2 is 4-5 kg).

Since this roofing material does not exert a high load on the truss system, the distance between the elements is chosen as much as possible for a particular angle of inclination. In addition, the profiled sheet does not require high strength characteristics from the roof lathing. All this together allows minimizing the total load on the foundation and walls.

Roofing from a metal tile

The second common type of steel roofing materials is metal tiles. This type of profiled sheet, which successfully imitates natural clay material, but with a lower mass (10 or more). A feature of the rafters under the metal tile is the smaller section size.

When choosing at what distance to install the rafters, first of all, you should be guided by the dynamic load. Like a profiled sheet, a metal tile is not demanding on the size of the rafter legs and is well mounted on a crate made of an inch coniferous board. All this makes a metal roof 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 - the most lightweight material. The weight of the sheet does not exceed 6 kg.

The small thickness of ondulin sheets at slope angles of less than 15 ° requires the construction of a continuous crate of plywood sheets, for example, which will require an appropriate rafter pitch. This should be taken into account in the calculations.

slate roof

Not so long ago, a wavy material from an asbestos-cement mixture, called slate, was widespread. High mass and fragility are the main disadvantages, however, even today he finds his fans in the construction of various outbuildings.

A high mass, comparable to the weight of a clay tile, will not allow the use of the same truss system as under a metal tile. Building codes define a minimum angle of slope for a slate roof of 22 degrees or more. Otherwise, the load from the material itself and the truss system with the crate exceeds the permissible parameters. The step of the inclined bars, as well as their cross section, are selected individually in each case.

Polycarbonate roof

In recent years, more and more often on the roofs of verandas and gazebos, an artificial polymer material, polycarbonate, has been used. Produced in two versions - monolithic and honeycomb. The first in its properties is similar to ordinary quartz glass, but significantly exceeds it in strength. The second one has less mechanical properties, but high thermal insulation and light transmission.

Cellular polycarbonate is standardly much lighter than a monolithic counterpart. It is used as a roof without the use of lathing, provided that the step does not exceed ½ of the width of the sheet of material. The high strength of the monolithic analogue also makes it possible to avoid elements transverse to the rafters. Sufficient flexibility allows you to cover semi-circular roofs on a metal frame, the pitch of which does not exceed 0.9 meters.

Thematic material:

Rafters under a soft roof

The original pattern can be obtained through the use of soft roofing materials, which are spread with an adhesive layer. They are installed on a continuous crate made of plywood or OSB. The step of the rafters should allow you to fix the sheets, so it is chosen as a multiple of ½ of the width. Given the standard dimensions of plywood 1520x1520 mm, the center distance between the rafters will be: 1520: 3 = 506 mm.

Rafter step 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 plates with dimensions of 600x1000mm. These parameters are used as starting points.

Rafter pitch calculation scheme

According to building codes, the pitch of the roof rafters is in the range of 0.6 - 1 meter. Its final calculation is performed according to a simple formula, depending on the total length of the roof. For the calculation, you must perform the following list of actions:

1. determine how much distance should be between the rafters for your specific building conditions. According to the reference book, the magnitude of wind and snow loads in the area is determined.
2. the length of the roof is divided by the desired distance by adding one. The result will be equal to the number of rafter legs that are installed on one roof slope. If the value is not an integer, it is rounded off.
3. the length of the roof is divided by the number of rafters calculated above, we get the final step in meters.

For example, with a slope of 30 degrees, the maximum distance between the rafters of a gable roof under a metal tile is 0.6 measure. The length is assumed to be 16 meters. Hence:

1. 16:0,6+1=27,66;
2. rounding the result, we get 28 rafters per slope;
3. 16:28 \u003d 0.57 meters - the center gap of the rafter legs for these specific conditions.

As you can see, the calculation technology is not complicated, but this is just an approximate scheme. Taking into account many other parameters mentioned above can make certain adjustments.

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

Parameters of the gable roof truss system

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

Rafter length

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

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

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

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

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

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

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

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

Section of rafter legs

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

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

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

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

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

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

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

Variable impact on the truss system

The pressure on the rafter legs is constant and variable.

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

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

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

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

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

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

Table: standard value of wind pressure

Table: value of coefficient k

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

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

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

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

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

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

Permanent loads

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

Roofing cake creates constant pressure on the rafters

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

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

Table: weight of roofing materials per 1 m²

Number of bars

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

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

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

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

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

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

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

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

Table: rafter pitch depending on length and section

Formulas for calculating the truss system of a gable roof

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

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

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

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

An example of the calculation of the supporting structure

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

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

From this follow the following computational steps:

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

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

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

Video: detailed calculation of the truss system

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

It is pointless to argue about the importance of the roof for any building. It is not for nothing that over a dozen different types of roofs have been invented in the history of mankind, from simple to quite complex in design and construction. An important element in planning the construction of a roof is the 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;
• the type of roofing material to be installed;
• rafter section dimensions.

Before proceeding with the process of erecting the upper structure of the house, a calculation should be performed by determining the optimal distance between the rafters.

Gable roof rafter pitch

The most widespread in our country are gable roofs. They are a structure that has two parallel planes, with an angle of inclination relative to the horizon from 20 to 50 degrees.

With an insufficient slope of the gable roof in snowy areas, there is a danger of accumulation of large snow masses, which can lead to the destruction of the structure. An increase in the slope angle in regions with a predominance of strong winds is also fraught with a high load and the risk of breaking not only the roof, but the entire structure as a whole.

Mansard roof truss system

Most private houses have an exploited 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 of a comfortable height. As a rule, the slopes of the mansard roof are broken, having a varying angle of slope. For their installation, a double truss system is used.

The steepness of the lower slopes of the mansard roof significantly exceeds the slope of their upper extensions. The plane load perceived by them is not great. Thanks to this, the rafters in the lower part can be installed with a maximum pitch. The upper ridge slopes are recommended to be mounted with a reduced gap from each other.

Rafters in a shed roof

For outbuildings and some private houses, roofs with one slope are used. Due to the limited angle of inclination, high pressure is exerted on them. Experts recommend using lumber of increased cross-section for the rafters of a single-pitched roof, with the installation of a minimum step from each other.

When calculating the distances at which roof beams are installed, special attention should be paid to the amount of snow load in a particular area. With a small slope, this characteristic is of great importance. Roofing material for such roofs is best chosen with a minimum dead weight, which will reduce the bending load.

Hip roof truss system

The hip roof truss system is considered the most difficult in construction. This type is called four-pitched, since the roof is formed not only by side, but also by additional end slopes, where the installation of rafters is carried out not on the ridge, but on the corner bowstrings. This places special demands on the organization of the roof frame.

Under the hip roof, the attic is not often arranged. This is due to the small angle of inclination of the rafters and the roof as a whole. In the case of an increase in the angle of the slopes to the horizon, the distance between the rafters is increased, with a decrease, vice versa. An additional aspect of the calculation is the roofing material used.

The dependence of the pitch of the rafters on the roofing material

In addition to variable snow and wind loads, a constant (static) one also acts on the roof, the force of which depends on the roofing material used. It is no secret that different types of roofs have their own weight, which can differ by 10 or more times.

The right choice of material affects not only the top, but also all other parts of the structure of a residential building and other buildings. Not without reason, when designing the foundation, it is necessary to decide in advance on the choice of the roof.

Roofing from profiled sheet

Currently, one of the most common roofing materials is a profiled sheet, which is produced galvanized or followed by a polymer coating. The distinguishing features of the profiled sheet include the following parameters:

1. High corrosion resistance;
2. As a result, a long (more than 15 years) service life;
3. Easy installation even without the necessary qualification;
4. Small sheet weight (weight 1 m 2 is 4-5 kg).

Since this roofing material does not exert a high load on the truss system, the distance between the elements is chosen as much as possible for a particular angle of inclination. In addition, the profiled sheet does not require high strength characteristics from the roof lathing. All this together allows minimizing the total load on the foundation and walls.

Roofing from a metal tile

The second common type of steel roofing materials is metal tiles. This type of profiled sheet, which successfully imitates natural clay material, but with a lower mass (10 or more). A feature of the rafters under the metal tile is the smaller section size.

When choosing at what distance to install the rafters, first of all, you should be guided by the dynamic load. Like a profiled sheet, a metal tile is not demanding on the size of the rafter legs and is well mounted on a crate made of an inch coniferous board. All this makes a metal roof 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 - the most lightweight material. The weight of the sheet does not exceed 6 kg.

The small thickness of ondulin sheets at slope angles of less than 15 ° requires the construction of a continuous crate of plywood sheets, for example, which will require an appropriate rafter pitch. This should be taken into account in the calculations.

slate roof

Not so long ago, a wavy material from an asbestos-cement mixture, called slate, was widespread. High mass and fragility are the main disadvantages, however, even today he finds his fans in the construction of various outbuildings.

A high mass, comparable to the weight of a clay tile, will not allow the use of the same truss system as under a metal tile. Building codes define a minimum angle of slope for a slate roof of 22 degrees or more. Otherwise, the load from the material itself and the truss system with the crate exceeds the permissible parameters. The step of the inclined bars, as well as their cross section, are selected individually in each case.

Polycarbonate roof

In recent years, more and more often on the roofs of verandas and gazebos, an artificial polymer material, polycarbonate, has been used. Produced in two versions - monolithic and honeycomb. The first in its properties is similar to ordinary quartz glass, but significantly exceeds it in strength. The second one has less mechanical properties, but high thermal insulation and light transmission.

Cellular polycarbonate is standardly much lighter than a monolithic counterpart. It is used as a roof without the use of lathing, provided that the step does not exceed ½ of the width of the sheet of material. The high strength of the monolithic analogue also makes it possible to avoid elements transverse to the rafters. Sufficient flexibility allows you to cover semi-circular roofs on a metal frame, the pitch of which does not exceed 0.9 meters.

Thematic material:

Rafters under a soft roof

The original pattern can be obtained through the use of soft roofing materials, which are spread with an adhesive layer. They are installed on a continuous crate made of plywood or OSB. The step of the rafters should allow you to fix the sheets, so it is chosen as a multiple of ½ of the width. Given the standard dimensions of plywood 1520x1520 mm, the center distance between the rafters will be: 1520: 3 = 506 mm.

Rafter step 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 plates with dimensions of 600x1000mm. These parameters are used as starting points.

Rafter pitch calculation scheme

According to building codes, the pitch of the roof rafters is in the range of 0.6 - 1 meter. Its final calculation is performed according to a simple formula, depending on the total length of the roof. For the calculation, you must perform the following list of actions:

1. determine how much distance should be between the rafters for your specific building conditions. According to the reference book, the magnitude of wind and snow loads in the area is determined.
2. the length of the roof is divided by the desired distance by adding one. The result will be equal to the number of rafter legs that are installed on one roof slope. If the value is not an integer, it is rounded off.
3. the length of the roof is divided by the number of rafters calculated above, we get the final step in meters.

For example, with a slope of 30 degrees, the maximum distance between the rafters of a gable roof under a metal tile is 0.6 measure. The length is assumed to be 16 meters. Hence:

1. 16:0,6+1=27,66;
2. rounding the result, we get 28 rafters per slope;
3. 16:28 \u003d 0.57 meters - the center gap of the rafter legs for these specific conditions.

As you can see, the calculation technology is not complicated, but this is just an approximate scheme. Taking into account many other parameters mentioned above can make certain adjustments.

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 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 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 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, 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 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

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).