The service life and final reliability of the roof are in the correct preparatory work, to which the correctly chosen distance between the rafters can be attributed. It is from this element of the supporting frame that the distribution of the load on load-bearing frame, an error in the calculations of which can lead to deformation and collapse of the entire roof. So, calculating the total distance between truss elements is one of the most important tasks when planning and miscalculating a supporting house.
Basic calculation technology
Rafter pitch is the exact distance between two legs of one truss truss. Most often, in private construction, the distance between the legs is about one meter, but the exact figures should come from a detailed calculation of the bearing capacity of the selected type of roofing system. In order to independently perform such a calculation, you must adhere to the following scheme of actions:
- Having measured the eaves of the roof structure, it is necessary to determine overall length roof slope.
- The resulting value must be divided by the selected and the most optimal step rafters. In other words, if exactly one m is chosen in the rafters, then the length of the slope must be divided by this unit of measurement.
- One is added to the result, after which you can round the finished value. It is this method that allows you to determine the number of roof trusses on one roof slope.
- Now it’s worth dividing the length of the roof slope by the resulting number of trusses, which will allow you to determine the pitch of the rafters.
It is worth recalling that this method miscalculation of such a building element as the pitch of the rafters cannot be considered extremely accurate, because in such calculations it is necessary to take into account the features of the selected roofing material, which can be a problem in the absence of proper experience in this matter. It is for this reason that methods will be described below that will allow you to choose the right rafter pitch for popular roofing materials.
Ceramic tiles and rafter installation
The main feature of the manufacture and installation of a truss bearing system for ceramic tiles is the extremely large weight of the roofing material, which in its totality is ten times greater than the weight of any roofing material.
As a rule, this type and size of roofing material provides for up to 60 kg per square m of the truss bearing system. All this becomes the reason for the use of only dry lumber on the roof, which eliminates the possibility of further deformation during drying and pressure.
Using wooden beam on the roof, the cross section of which is 150x50 mm, the step between can vary from 70 to 120 cm.
In addition, the distance between the rafters may depend on the overall slope of the roof.
Do not forget about the length of the legs themselves rafter type. The longer this length is, the smaller the distance should be between the legs of the rafters, otherwise the supporting frame of the roof will be deprived of maximum strength and stability.
Experts recommend using no more than 80 centimeters of pitch between trusses if the roof slope is up to 45 degrees. Special attention should be paid to the lathing system when using ceramic roofing material on the roof.
To determine such a step, it is necessary to take into account the characteristics of each individual element of the roofing material, namely each tile. In order to perform the correct calculation of the crate step for such material, it is necessary to subtract the length of the lowest row and the distance between the last bar and the row of the crate from the pitched length.
As a rule, all varieties of such roofing material have a length of no more than 40 cm. When laying the tiles, they are overlapped, which takes up to 9 cm from the bottom row. In other words, the correct step in this calculation is from 31 to 35 cm.
Metal tiles and laying methods
As a rule, the use of metal tiles in private construction is considered the most common, for which there are a number of reasons and positive characteristics of this roofing material on the roof.
If we compare such material roof type with the one described above, several striking differences can be noted, each of which can be safely considered positive feature material. First, it is worth recalling the simplicity and speed of installation of such material, which is made in a sheet type, allowing you to cover a significant area of \u200b\u200bthe roof at a time.
The second feature of metal roof tiles can be considered the extremely low weight of the material, which has a positive effect on installation, work on the roof and the load on the supporting frame of the roofing system.
This feature can be considered the most important, because it is responsible for the lower costs of labor and time in organizing the truss carrier system. When using this roofing material, the pitch of the rafter bearing legs is from 60 to 95 cm, because the reduced weight and size of the roof does not require high load resistance parameters.
With such indicators, it is worth using thermal insulation material, whose thickness is not less than 15 cm, which fits between the rafters. When arranging ,thickness insulating material should or may be larger, because it is necessary for arranging a residential floor on attic. This also affects such an element as the thickness of the material that is used to make the rafter load-bearing legs.
All this points to the absence significant differences in the rafter system under the metal tiles on the roof. This roofing material provides standard knowledge about the construction of truss and roofing systems, which is simply necessary to install. The main difference of such a system can be the fastening of the top-type support to ridge run, and not through the ridge-type beam itself.
The free space at the top of the system, obtained through the installation of rafter legs, is ideal for ventilation of the entire under-roof space, which is often covered with condensate in metal roofing material.
In the event that changes are made to the roof of the old wooden house, it is necessary to completely remove the Mauerlat, replacing it with a common crown of the upper type. In such a crown, it is necessary to put marks and make selections with the desired step, into which the elements of the rafter bearing legs will be installed. This allows you to more accurately calculate the pitch of the rafter bearing legs and install them.
Decking and determining the distance between the racks
When using a roofing material such as corrugated board, the calculation the right step rafter load-bearing legs is 60-90 cm, which practically coincides with all the types of structures and roofing materials described.
If necessary, put more distance between the elements of the rafter legs, it is necessary to insure the transverse elements of the crate with a greater thickness and cross section. This will strengthen the entire supporting structure, which will extend the life of the entire roof. The step between is about 50 centimeters, when using a board that is at least 10 centimeters in size.
When it comes to such a moment as calculating the crate on the truss system, then do not forget that the last board should be 1.5 centimeters thicker than all the other elements of the system. Before installing the elements of the crate, it is worth remembering that there is a chimney and special system, which should be on every roof. This will save time on the calculation and alteration of the truss system, and the crate as a whole.
Ondulin and construction installation methods
As in the cases described above, ondulin provides for the calculation of the pitch of the rafter bearing legs from 60 to 100 cm, which makes such sizes standard when using the most popular roofing materials.
The truss trusses themselves are made of lumber with a section of 200x50 millimeters, which is quite enough to ensure that the strength and reliability of the truss system are at the maximum level, and it is easy to install them. It is immediately worth noting the fact that the described roofing material needs a continuous lathing system, which will allow the material to qualitatively resist the loads from snow thicknesses, as well as solar exposure.
In some cases, you can also use a thinned type of crate, the calculation of which is quite simple, if a wooden beam is used for this, the step between the elements of which is no more than 30 cm. As a rule, this option is more expensive than necessary for a continuous crate. In other words, it is irrelevant to use the thinned type of system in this case.
Slate and truss structure under it
Usually, slate coating considered the most common in our country. The reason for this indicator can be considered the extremely low cost of the material, the simplicity and speed of installation, as well as the possibility of replacing individual sheets in case of damage to individual sections of the roof.
The distance between the rafters for this type of roof should be 80 cm. It is this distance that is considered the most optimal solution when combining the load from the weight of slate sheets.
The crate in such a system needs a thinned type using a board or timber of any type, but of a certain section, which should not be less than 30 mm. Such a thickness of a board or timber is needed for high-quality and correct distribution of weight from the roofing material itself, as well as from winter precipitation.
Do not forget about the constant margin of safety when organizing the truss system, which may be required in case of unforeseen circumstances associated with mechanical loads and bad weather. All other features of the truss and crate system do not have any special differences from the options listed above. In other words, for all other types of coatings, we determine the distance in the same way as for the above roof.
An attic is an attic space that can be used as a living space. The mansard roof should provide normal functioning such a room. In the construction of the roof, a truss structure is usually used, and the pitch between the rafters of the mansard roof is an important indicator of its reliability.
Huge loads caused by the weight of the roof, wind, climatic factors are perceived by the rafter system. The distance between the rafters of a mansard type roof determines how much of the load falls on each element. Only the right choice of distance will ensure the stability of the entire roof.
Attic: system features
Rafters are called load-bearing beams, on which protective, additional and external coverings of the mansard roof are attached.
The elements are usually made in the form of a durable wooden beam or board with a thickness of at least 50 mm. Sometimes a log can be used. For particularly durable buildings, metal and reinforced concrete beams are used.
For the roof of the attic, one of two options for truss systems (layered or hanging) is used, as well as a combination of both options in one design. The layered type is characterized by the support of each of the rafters on the wall of the structure. The hanging type implies linking the elements into a common frame, which is fixed only on the extreme supports.
According to the type of truss system used, mansard roofs are conditionally divided into the following types: single-pitched, gable, broken, hip, hipped and vaulted. In private construction, the most wide application has a single-pitched, double-pitched or sloping roof. In a shed roof, the rafters rest on the walls different heights, which provides a slope (slope) of the roof in one direction. The gable roof has two inclined planes, with each of the rafters resting at one end on the wall, and at the other end connected to another beam. This design forms a triangle, and the angle between the elements determines the slope of the slope. The sloping roof also has two slopes, but each of them has a break line in which the laying angle changes.
Mounting Features
When installing the rafter system of the attic roof, fastening and installation of rafters in the form of simple geometric shapes. The greatest rigidity (strength) has a linkage into a triangle, which is used in the construction of a mansard roof. Yes, the most common gable roof includes a series of triangles of rafters, connected by longitudinal lags (girders). The linkage of the rafters into a triangle is provided by the lower transverse beam (Mauerlat). To facilitate the fastening of the outer roof covering and the redistribution of its weight on the rafters, a lattice is made in the form of transverse bars or boards.
The sloping roof combines two types rafter connections. The lower rafters with the help of a Mauerlat and racks are connected in right triangles, which, in turn, are fastened with a longitudinal run between themselves at the top. At the bottom, the attic rafters rest on the wall of the house. The upper ones are connected into a triangle by analogy with a gable design.
Figure 1. Table for selecting the section of a beam for rafters.
The lower end of the rafter is fixed on the transverse log, and the upper ends are connected together through the longitudinal upper run. The lower corners of the bonded triangle are interconnected using a longitudinal lower run. The system formed is fixed on the lower truss system. To strengthen the upper triangles, additional vertical racks are used. Thus, the mansard roof is a surface with a break on each side. From the wall, a slope with greater steepness begins, and then it takes on a more gentle appearance.
Mansards with longitudinal bars (including floor beams) are made by cutting rafters into a bar by a third of its height. Fastening to the cross beams is advisable to produce a screw connection. With such fastenings, the functions of two different truss systems are separated, and they are calculated as separate systems.
Parameters taken into account when choosing rafters
When choosing the design of the rafter system, the size of the beam and the number of elements, it is important to take into account all the loads acting on the rafters. These loads can be divided into permanent and temporary, periodic or short-term nature. Under constantly operating load should take the weight of all elements mansard roof: truss structure itself with grating, external roof deck, additional protective and insulating layers, elements of a hinged attic interior. The weight of the external roof can vary greatly depending on the type and material of the coating.
Natural factors should be taken into account as temporary or periodic loads. This is, first of all, the weight of snow in winter time. The wind has a significant impact, and the direction of such a load can be different. For some areas, this factor may be decisive. The possibility of storm water flows cannot be ignored either. In addition, it is necessary to take into account the weight of people and materials when carrying out repair work on the roof.
The geometry of the roof and rafter system has a significant impact on the distribution of loads. The main parameters include the length and width of the roof, as well as the steepness of the slope. The length of the roof greatly affects the distribution of the load, so for long lengths it is necessary to use reinforcing vertical posts. An increase in the width of the roof leads to an increase in the load on all attic rafters, as their length increases and total weight all elements. For wide roofs, a broken type is more suitable due to the presence of intermediate vertical racks in them and the redistribution of loads between different truss systems.
Changing the slope slope affects the parameters ambiguously. An increase in steepness, on the one hand, reduces the accumulation of snow cover and redistributes the load on the load-bearing walls of the house, on the other hand, the length of the rafters and the windage of the roof increase, which is dangerous in windy areas. The concentration of loads on the walls can also adversely affect the reliability of the house, because with a decrease in loads on the rafters, constant loads on the masonry walls increase.
Requirements for the material of the truss system
The calculation of the number of rafters and installation parameters is based on the fact that during construction it was used quality material. In this regard, the material for the truss system should be selected based on the following conditions.
Only high-quality timber with a section of at least 50x100 mm should be used as the main rafters.
All wooden elements during installation must be well dried (permissible moisture content - no more than 15%). The number of even small defects on the beam cannot exceed 3 pieces per 1 m. The tree is treated with an antiseptic before installation. Coniferous wood performed the best. Vertical racks are made of timber with a size of at least 100x100 mm with checking them vertical arrangement with the help of a plumb line.
Features of the calculation of rafters
After choosing the design of the mansard roof (based on the recommendations of experts and in accordance with the reference data), the main design parameters are the distance between the rafters () and their number. Usually the distance between the rafters is from 0.6 to 1.5 m. Calculations are based on the fact that the optimal load should be 40-60 kg per 1 m of rafter length, and the maximum allowable deflection beams - 1/250 of its length.
The number of rafters per slope is calculated after measuring the length of the slope and selecting. The length of the slope is divided by the step value, 1 (one) is added to the result. The result is rounded up to the nearest whole number in big side.
A specialist can calculate the distance between the rafters, taking into account all factors, but in practice they use reference recommendations. So, for example, for rafters from a board measuring 50x180 mm and a slope length of 3 m average step is 1.5 m; with a length of 3.5 m - 1.2 m; and with a length of 4 m - 0.9 m.
Distance between rafters for different roofs
The distance between the rafters varies significantly for roofs with different coating. Ceramic tiles are one of the heaviest roofing materials. For rafters from a bar measuring 50x150-60x180 mm, the recommended distance between them is 80-130 mm (depending) on the steepness of the slope. With a slope of 15 °, the pitch is chosen to be 80 cm. With an increase in the length of the rafter, the pitch is increased within the recommended range.
The distance between the rafters for roofs with metal roofing is set smaller than for natural tiles. The optimal step is 60-95 cm for a bar measuring 50x150 mm. When using a corrugated board coating, the step is in the range of 60-90 cm with a sufficient beam cross section from 50x100 mm to 50x150 mm.
The lightest coating is obtained when using ondulin. The optimal distance between rafters measuring 50x50 mm is 60-80 cm and decreases when a larger beam is installed. When covering the roof of the attic with slate, a beam measuring from 50x100 mm to 50x150 mm is used. The step is set in the range of 60-80 cm.
Required Tools
When installing rafters on a mansard roof, the following tools are used:
- Bulgarian;
- drill;
- hacksaw;
- saw;
- axe;
- chisel;
- a hammer;
- plane.
When installing a truss system on a mansard roof, it is important to determine the optimal distance between the rafters. Right choice this parameter will allow you to calculate the optimal amount of material and ensure the reliability of the entire roof.
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 rafter leg
The maximum length of the 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 truss system has its own 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.
Thickness of each piece load-bearing structure roofing 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 section of Russia, k is a correction factor determined by the height of the structure and the nature of the terrain, and c is the aerodynamic 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. Great importance has a housing area. 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
| Type of roof finish | Weight in kg per 1 m² |
| Rolled bitumen-polymer sheet | 4–8 |
| Bitumen-polymer soft tiles | 7–8 |
| Ondulin | 3–4 |
| metal tile | 4–6 |
| Decking, seam roofing, galvanized metal sheets | 4–6 |
| Cement-sand tiles | 40–50 |
| Ceramic tiles | 35–40 |
| Slate | 10–14 |
| slate roof | 40–50 |
| Copper | 8 |
| green roof | 80–150 |
| Draft flooring | 18–20 |
| crate | 8–10 |
| The truss system itself | 15–20 |
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:
- According to the formula Qr \u003d AxQ, they find out what is the load on running meter 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.
- 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".
- 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 maximum length in meters, Qr - distributed load per linear meter of the rafter leg in kg / m, B - section width cm, Rbend - wood bending resistance, 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.
- 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).
- When the above inequality is violated, the indicators B and H increase.
Table: nominal dimensions of thickness and width of lumber (mm)
| Board thickness - section width (B) | Board width - section height (H) | ||||||||
| 16 | 75 | 100 | 125 | 150 | - | - | - | - | - |
| 19 | 75 | 100 | 125 | 150 | 175 | - | - | - | - |
| 22 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | - | - |
| 25 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 32 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 40 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 44 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 50 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 60 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 75 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 100 | - | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 125 | - | - | 125 | 150 | 175 | 200 | 225 | 250 | - |
| 150 | - | - | - | 150 | 175 | 200 | 225 | 250 | - |
| 175 | - | - | - | - | 175 | 200 | 225 | 250 | - |
| 200 | - | - | - | - | - | 200 | 225 | 250 | - |
| 250 | - | - | - | - | - | - | - | 250 | - |
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 length) = 2.8 m. , which means that Rizg \u003d 140 kg / cm².
Chosen for roofing cement-sand tiles and therefore the weight of the roof is 50 kg/m². The total load (Q) experienced by each square meter, is equal to 303 kg / m². And for the construction of the truss system, bars 5 cm thick are used.
From this follow the following computational steps:
- Qr=A·Q= 0.8·303=242 kg/m - distributed load per linear meter of rafter beam.
- H ≥ 9.5 Lmax sqrt(Qr/B Rbend).
- H ≥ 9.5 2.8 sqrt(242/5 140).
- 3.125 Qr (Lmax)³/B H³ ≤ 1.
- 3.125 242 (2.8)³ / 5 (17.5)³ = 0.61.
- H ≥ (approximate height of the rafter section).
In the table of standard sizes, you need to find 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 in normative documents, and this is proved by the inequality 3.125 Qr (Lmax)³/B H³ ≤ 1. Substituting the values (3.125 242 (2.8)³ / 5 (17.5)³) into it, we find that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.
Video: detailed calculation of the 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.
The reliability of the roof and its service life mainly depend on how correctly the calculations are made.
One of the most important parameters The rafter system is how far apart the rafters are.
After all, the distribution of the load on the frame depends precisely on this desired parameter.
If the calculation is made incorrectly, then deformation may occur and the roof will collapse.
Therefore, it is extremely important to perform an accurate calculation of the interval between the rafter legs when drawing up a roof project.
Basic provisions of the calculation
The pitch of the rafters is the distance from one rafter leg to the other.
In the construction of roofs of private houses, this value is 1 meter.
But this figure is approximate.
To get the exact size of the interval, it is necessary to calculate the bearing capacity of the truss system of the selected type.
To perform the correct calculation, the following scheme should be used:
- first you need to determine the length of the roof slope;
- now the resulting figure should be divided by the value of the approximately selected rafter pitch. If you previously chose its value of 1 meter, then you need to divide by 1. If you chose 0.8, then you should divide by 0.8, etc.;
- after that, one is added to the result obtained and the resulting value is rounded up. This action is necessary in order to get the exact number of rafters that will need to be installed on one slope;
- the total length of the slope must be divided by the number of rafter legs obtained in the previous calculation. And we get the exact distance between them.
To better understand everything, let's look at an example.
By measuring the length of the roof slope, we got 27.5 meters.
We choose a step of 1 meter to make it more convenient to count.
Further calculations will look like this:
Now we add the unit 27.5 m + 1 = 28.5 m.
We round up to the nearest value and get the number 29.
That is, 29 rafter legs must be installed on one slope of our roof.
Now we divide the length of the roof by their number: 27.5 / 29 \u003d 0.95 m.
So, in our case, the step of the rafter legs should be 0.95 meters.
This is a general calculation.
In which the peculiarity of a particular roofing material is not taken into account.
And he can significantly change this parameter.
When you know what kind of material you will use on the roof, some changes will have to be made to the calculation procedure.
Optimal and minimum value
The step is the unit of account.
After all, it depends, first of all, on the load on the roof frame and the section of the rafter legs.
You can take a thick board for their construction and make the interval between them 120 cm.
At this distance, the crate may begin to sag.
Yes, and how to lay the heater?
Indeed, for most materials, the width is 1 meter.
So they take an approximate step of 1 meter when calculating.
The minimum distance between the rafter legs is 70 cm.
And to get optimal value for each material it is necessary to carry out the calculation.
Determining the pitch for a pitched roof
Shed roofing is the simplest.
Indeed, in the truss farm of such a roof there are only rafter legs.
There are no struts, racks, braces in its design.
Such a roof is usually arranged on garages, outbuildings, baths.
In order to determine the pitch of the rafters on a pitched roof, you can use the table:
As can be seen from the table, rafter step depends on their length and cross section.
Distance between rafters for a gable roof
A gable roof is much more common than a shed roof.
And to explain such popularity is simple: the design is very simple, but for all its simplicity, the gable roof is distinguished by its reliability.
Its design can be easily adjusted to any climatic conditions and any roofing material.
When calculating the pitch of the rafters of a gable roof, everything is performed according to common system calculation (see above).
If the slopes have the same length, then it is enough to perform calculations for one slope.
If the slopes are of different lengths, then the calculation is performed for each slope.
It should be remembered that the limit values for the pitch of the rafter legs are 70 and 120 cm.
Ondulin
If ondulin is used as a roofing material, then the rafter system is constructed from pine boards with a section of 50 x 200 mm, and the rafters should be located at a distance of at least 60 cm and no more than 90 cm from each other.
A crate of timber with a section of 40 x 50 mm is laid on top.
metal tile
Metal roofing during construction country houses used most often.
Because this material is much lighter than ceramic or cement tiles.
Although according to appearance very similar to her.
The light weight of the metal tile makes it possible to use boards of a smaller section when creating the rafter system, and to use thinner bars for the crate.
More about the counter-lattice under the metal tile.
About the hip roof with hanging rafters at the link. Also about the design of hanging rafters.
About the price of Viking metal tiles here. Specifications, photos, reviews.
Reducing the size of the elements of the rafter leg system, in turn, reduces the load on the walls of the building and its foundation.
During the construction of the rafter system under the metal tile, the rafters are mounted in increments of 60 - 95 cm.
The cross section of the material is 50 x 150 mm.
According to experts, if a heater having a thickness of 150 mm is placed between the rafters, then the most comfortable living conditions will be created in the attic.
To ensure ventilation of the space where the insulation is laid, when installing the rafter system, holes will be drilled near the upper roof with a diameter of 12 mm.
The technology for manufacturing a truss system for metal tiles does not differ significantly from the technology for manufacturing a frame for other roofing materials.
Its only difference is the installation at the top.
The upper support is mounted not on the side of the ridge beam, but on the ridge run.
The presence of a free zone between the rafters allows the air to fully circulate under the flooring, which helps to protect against the formation of condensate.
Ceramic tiles
System design under ceramic tiles has its own characteristics.
After all, clay is used to make such a roofing material.
And this is very heavy material.
If we compare metal and ceramic tiles, the latter weighs 10 times more.
Accordingly, the truss system is significantly different.
On 1 square meter of the roof surface there is a load of 40 - 60 kg, depending on the manufacturer and brand of the product.
Rafter for such frame system made from wood that has been dried for a long time.
Such wood should have a moisture content of no more than 15%.
A beam with a section of 50 x 150 or 60 x 180 mm is used.
So more reliable.
And the distance between the rafter legs can be 80 - 130 cm.
The exact value can be called if the angle of inclination of the slopes is known.
If the angle of inclination is 15 degrees, then the pitch of the rafters will be 80 cm.
And if the angle of inclination, for example, is 75 degrees, then the step can be larger - 130 cm.
More than 130 cm interval between the rafters do not.
Also, when calculating the pitch of the rafters, their length is taken into account.
The larger the dyne, the smaller the distance between them.
The shorter the rafter legs, the greater the distance can be.
If the angle of inclination is 45 degrees, then the roofer can safely move along the roof if the pitch of the rafters is 80 cm.
Decking
When creating a truss system for corrugated board minimum distance between the rafter legs is 60 cm.
The maximum value is 90 cm.
If for some reason the pitch of the rafters was more than 90 cm, then it becomes necessary to install large cross-section boards.
The rafter legs themselves can have a section of 50 x 100 or 50 x 150 mm.
Slate
Despite the emergence of a large number of new roofing materials, asbestos-cement slate remains one of the most popular.
If it is planned to lay slate on the roof, then the rafters should have a section of 50 x 100 or 50 x 150 mm.
The distance between them is made not less than 60 and not more than 80 cm.
The crate is made from a bar having a section of 50 x 50 mm or a board with a section of 25 x 100 mm.
When constructing any building structure, it should be remembered that there are still unforeseen situations.
And, therefore, when calculating the sections of the rafters and the distance between them, it is necessary to provide for a margin of safety.
about the installation of the truss system.
Sergey Novozhilov is an expert in roofing materials with 9 years of experience practical work in the field of engineering solutions in construction.
Source: http://ProRoofer.ru/montaz/stropilnye-sistemy/rasstoyanie-mezhdu-stropilami.html
The distance between the rafters of a gable roof and the pitch of the rafters
The quality and durability of the roof largely depend on the correct calculation of the pitch of the rafters. It affects the laying of insulation, the fastening of roofing material and the installation of additional parts.
Pay attention only to the requirements for the distance under the roofing sheets, then there may be problems with the insulation boards. And vice versa, adjusting the size of the insulation can make the frame too weak, and in winter there will be a threat of collapse.
The basics of determining the pitch of the rafters
As a rule, the pitch between the rafters depends on several factors. But usually the distance fluctuates between 0.6 and 1.2 meters. These figures are rounded, in practice the distance may differ by several centimeters downwards. For an accurate determination, the following calculations must be performed:
- Determine the length of the slope by measuring along the eaves of the house. Suppose that it turned out 17.8 meters.
- Divide the figure obtained by the planned step of the rafters. If it was decided that the distance between the beams would be 0.8 meters, then 17.8 / 0.8 = 22.25.
- Further, we round the result up and add one: 23 + 1 = 24. At this stage, the number of rafters required is determined.
- Now we calculate the distance between the axes of the beams. To do this, the length of the slope must be divided by the number of rafters: 17.8 / 24 \u003d 0.74 m.
So, you will determine the actual distance at which the rafters for a roof made of metal or some other material should stand.
Often, when planning a roof frame, specialized calculators are used. They enter basic data and get the finished result. Nevertheless, you should not completely trust the program, since only a person can take into account some conditions. When calculating, you can understand how the system works, what loads are acting. If necessary, adjustments are made to strengthen or facilitate the design. Self-calculation will help to more accurately determine the required step of the rafters of the gable and pitched roof.
Step calculation depending on the roofing material
For each coating, its own standards and dimensions are developed for the truss system. The main factors are the strength of the material, its weight and resistance to external loads. Consider the main types of coatings.
Decking
The step of the rafters under the corrugated board can vary from 0.6 to 0.9 m. The final result depends on additional conditions, which we will discuss below. If for some reason the step should be larger, a transverse board with a large section is added. In this case, usually the rafters themselves are chosen with dimensions of 50x100 or 50x150 mm.
In addition to the rafters, a crate of 30x100 millimeters is used. The gap between the boards should be about 0.5 meters. It can be more, it all depends on the type of corrugated board and the height of the wave. Sheathing boards protruding beyond the boundaries of the eaves should have a thickness of 1.5 cm more than usual. This is done based on the need to mount ventilation, a chimney or a drain.
Ceramic tiles
The main complexity of design roof frame under ceramic tiles is a large weight of the material for the roof. It is 10 times heavier than corrugated board and accounts for 40 to 60 kg per square meter.
Under such a coating, dry beams are used, with a moisture content of only 15%. Their cross section should be either 50x150 or 60x180 mm. With such indicators, the maximum allowable distance between the rafters can be 1.3 meters. The minimum allowable step through which rafters can be placed is 0.8 m. The roof slope affects the final decision: at 15 °, the step is 0.8 m, at 75 ° - 1.3 m.
The length of the rafter leg is also taken into account, the shorter it is, the larger the gap, and vice versa. In addition, if the slope is less than 45 °, and movement is planned on the surface, the beams are placed in increments of no more than 0.85 m.
The distance between the boards of the crate is selected so that each intersection of the tiles has its own base. The standard length of the material is 400 mm, and the amount of overlap during installation varies between 55 and 90 mm. It turns out that the distance between the central axes of the crate can be either 310 or 345 millimeters.
metal tile
AT last years the question is particularly acute: how to determine the distance between the rafters under the metal tile? It imitates more expensive ceramic material, but at the same time has positive properties corrugated board. Installation of such a coating is simple and does not require large financial costs. All this makes the material attractive for the roofs of private houses.
The mass of metal tiles is less than ceramic, so a simpler frame is required. The section of the beams is reduced to 50x150 mm and the step between the crate is increased. The step of the rafters under the metal tile varies between 0.6 and 0.95 m.
Ondulin
The allowable step for covering from ondulin is 0.6-0.9 meters. The standard section of the rafters is 50x200 millimeters. Such dimensions will allow to withstand the load of the roofing pie on the gable roof.
On top of the counter-lattice, a crate of bars measuring 40x50 millimeters is attached. The pitch between the central axis is 600 mm.
Additional factors
When calculating the pitch of the rafters, attention is paid not only to the type of roofing. Many other points are taken into account, data on which can be found in reference books and SNiPs. Here are some of the factors:
- snow and wind loads. The more snow falls in winter and the stronger the winds blow, the smaller the distance between the beams should be. But, if you increase the angle of the roof slope over 45 °, then it is allowed to increase the step.
- Choice of heater. To reduce the cost of insulation boards, it is recommended to take into account their standard sizes. They produce mats with a width of 600, 800 and 1200 mm. If this condition is neglected, then a large number of undercuts will appear, cold bridges and construction will be delayed.
- lumber quality. It takes into account the type of wood, grade and section. As mentioned above, strength is affected by how well the beam was dried. When buying wood, pay attention to evenness and the presence of defects in the form of knots and traces of pests.
- Floor beams and puffs. If under gable roof arrange attic floor, then the maximum distance between the rafters should be 0.75 meters.
The calculation of the distance between the beams for a gable and single-pitched roof is different. Even if he makes several slopes, for each it is necessary to make a calculation individually. This is especially true for buildings with different lengths of cornices.
Taking into account all these factors, it is possible to determine how far the rafters should be installed. Although all calculations can be done independently, since reference data is freely available, it is better to turn to professionals. They have experience in designing, they will quickly determine the required distance between the rafters. Yes, you will avoid possible errors and you will be safe.
Source: https://build-experts.ru/shag-stropil/
The distance between the rafters is one of the key parameters that affect the strength of the structure. Competent calculation of the installation step of the rafters allows you to build a roof that is resistant to high operational loads.
Roof loads and rafter system calculation
The development of a single-pitched or dual-pitched roof project begins with the choice of the type of truss system, the angle of inclination of the slopes (roof height) and materials for the construction of the structure. The calculation of the distance between the rafter legs is carried out taking into account the loads that the roof will experience during operation. Permanent loads include :
- the weight of the materials from which the truss system is made;
- roofing weight;
- weight of roofing cake materials (waterproofing, vapor barrier, insulation);
- the weight of the finishing elements of a residential attic or attic.
In addition to permanent loads, the roof also experiences temporary ones, which include :
A step is the distance between the rafters of one slope. When calculating a shed, gable or complex roof, the following scheme is usually followed :
- the length of the future roof slope is measured;
- the resulting value is divided by the optimal numerical value rafter steps;
- one is added to the obtained value, the result is rounded off;
- the length of the roof slope is divided by the rounded result.
The final result will determine how far the rafter legs should be placed.
The determination of the step cannot be extremely accurate, since it is necessary to take into account a number of additional factors, including the width of the insulation, the particular installation of the crate for various types of roofing material.
If a roof with a chimney is being designed, the pitch can be adjusted according to its location so that it is not necessary to remove part of the rafter in the future and install a supporting structure, such as a special rafter system.
The distance between the rafters under the slate
Slate is a traditional roofing material. Its advantages include such characteristics as resistance to external influences (excluding mechanical ones) and low cost. Slate allows you to create a roofing, the repair of which can be reduced to the replacement of individual elements. Slate is heavy and requires the installation of a sufficiently powerful truss system. The calculation of the distance at which it is necessary to put the rafters under the slate is carried out taking into account the cross section of the beam for the manufacture of rafter legs.
The optimal solution is to install the system under the slate, in which the gap between the rafters must be at least 800 mm. In order for the slate structure to withstand not only the weight of the material, but also increased external loads, the crate is made of timber or boards with a cross section of at least 30 mm. When calculating the truss system for slate, it should be read that this material has rather large restrictions on the choice of the angle of inclination of the slopes.
Rafters for metal roofing
Metal tiles are actively used as a practical and aesthetic roofing material when arranging a shed, gable, hip or complex roof. The frame for the metal tile is built according to standard principles.
To calculate at what distance it is better to place the rafters, it is necessary to take into account the loads and the angle of the roof. The metal tile is characterized by relatively low weight, so it can serve as a replacement for the old roofing of slate or ceramic tiles.
In this case, the rafters do not need to be strengthened or the pitch of their installation changed.
The standard step of rafters for metal tiles is 600-900 mm. The cross section of the elements can be 50-150 mm - this is enough to create a reliable frame for a metal tile.
But if it is intended to use insulation, the layer of which in areas with low winter temperatures should be 200 mm, it is recommended to use a 200x50 beam for rafters under metal tiles so as not to mount additional system holding the heater.
The gap between the rafters under the metal tile is best adjusted to the width of the sheet or roll heat insulator.
Decking: rafters and crate
The professional flooring belongs to easy and convenient roofing materials in application. Galvanized or covered with a decorative protective layer corrugated board can be used for installation pitched roof utility room or garage, and for the gable roof of a residential building. How to calculate the distance required to install the rafters under the corrugated board?
To ensure the necessary rigidity of the structure, it is enough to install the rafters under the corrugated board in increments of 600-900 mm. In this case, it is necessary to pay attention to the angle of the roof.
The calculation shows that with high external loads, it is better to lay the corrugated board on the system with a minimum step.
But if the distance between the rafters under the corrugated board is close to the maximum, and the angle of the roof slope is small, then the structure is strengthened by using a more frequent crate. In this case, the step of the crate under the corrugated board should be about 50 mm, the width of the elements should be at least 100 mm.
Rafter system for soft roofing
Soft roofs include bituminous and bitumen-polymer roll materials, roofing membranes, as well as soft tiles. A soft roof is characterized by a relatively low weight and no need to install a massive truss structure.
The minimum distance between the rafters is 600 mm, the maximum is 1500 mm. When installing the support under soft roof it is important to take into account the angle of inclination of the slopes: the smaller the angle, the smaller the distance between the supports for a continuous crate should be. The choice of step is also influenced by the thickness of the material for the crate - the thicker the sheet of plywood or OSB, the greater the installation step of the rafter legs can be.
Ondulin: calculation of rafters
Ondulin (bituminous slate) is laid on a flat continuous crate made of sheet material. This allows the roofing to successfully resist wind and snow loads. The crate under the ondulin rests on the rafters, which should be located in increments of 600 - 1000 mm, depending on the angle of inclination of the gable or shed roof.
Rafters for ondulin are made of timber with a section of 200 × 50 mm. When choosing at what distance to put the rafter legs for the structure under the ondulin, it is recommended to take into account the width of the insulation material in order to simplify its installation. This calculation allows you to reduce the financial costs of arranging the roof.
Sandwich panel roof
Sandwich roofing is most often erected on sip-panel houses or hangar-type buildings. A feature of the sandwich is its bending rigidity, which makes it possible to dispense with the installation of traditional rafters. Small spans from the ridge of a gable roof to the upper part of the wall (or the distance between the supporting structures of a shed roof) allow the installation of a sandwich without additional supports.
If the span exceeds 4 meters, additional girders must be installed. To install a sandwich roof on a residential building, a traditional truss system is often installed, but in this case the rafters can be positioned with a large step - they serve as a support for the girders. The distance between the rafter legs is selected based on the length of the available material for the runs and the length bearing walls. Technical specifications sandwich allow the roof to withstand high operating loads .
Polycarbonate: erection of the supporting structure
Polycarbonate in recent times actively used as a roofing material. First of all, polycarbonate is in demand in the construction of gazebos, sheds, winter gardens. The crate and truss system for polycarbonate can be made of wood or metal.
Polycarbonate varies in weight depending on the thickness of the sheet. The crate for polycarbonate is recommended to be done in increments of 600-800 mm. The crate (wooden or metal) is mounted on rafters, which can be straight or arched.
Usually the gap between the rafters for polycarbonate is from 1500 to 2300 mm.
Source: http://krovlya-mp.ru/shag-stropil-dvuskatnoj-kryshi.html
The distance between the rafters: what step should be between the rafters
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.
Here is a good video tutorial on how to independently calculate the distance between the rafters:
What determines the pitch of the rafters?
So, the distance between the rafters is determined by 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. BUT computer program can not always take into account everything that the human brain notices.
Decorative rafters: 0% load
First of all, decide on the most important point: the type of roof and its purpose. The fact is that 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.
Functional rafters: detailed calculation
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 sloping or simply heavy (for example, covered 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 even be increased by a distance of 1.2 m-1.4 m.
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.
understand, do you high roof or flat, this map of the average wind load will help:
The second point: in the Russian region on standard roof Houses are 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 set the pitch of the rafters under the standard dimensions of the insulation boards, which are 60, 80 cm and 120 cm.
Modern heaters are sold today standard width, usually on the same standard rafter pitch. If you then take them and adjust them to existing parameters, then there will be a lot of waste, cracks, cold bridges and other problems.
Point 4. 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 normative documents, 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 being built with truss trusses connected to each other, and their lower belt it is used simultaneously as floor beams, then the distance between the trusses must be made within 60-75 cm in order 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 along the edges you need to fasten directly into 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 presses on it, and not on common roof generally. 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 general load bearing capacity roofs 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 uniformly distributed load, which causes the bending of the rafters, but also the forces that are already acting along the axis of the rafters. 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!
The quality and durability of the roof largely depend on the correct calculation of the pitch of the rafters. It affects the laying of insulation, the fastening of roofing material and the installation of additional parts. Pay attention only to the requirements for the distance under the roofing sheets, then there may be problems with the insulation boards. And vice versa, adjusting the size of the insulation can make the frame too weak, and in winter there will be a threat of collapse. How to correctly calculate the distance between the roof rafters? We will talk about this in our article.
As a rule, the pitch between the rafters depends on several factors. But usually the distance fluctuates between 0.6 and 1.2 meters. These figures are rounded, in practice the distance may differ by several centimeters downwards. For an accurate determination, the following calculations must be performed:
- Determine the length of the slope by measuring along the eaves of the house. Suppose that it turned out 17.8 meters.
- Divide the figure obtained by the planned step of the rafters. If it was decided that the distance between the beams would be 0.8 meters, then 17.8 / 0.8 = 22.25.
- Further, we round the result up and add one: 23 + 1 = 24. At this stage, the number of rafters required is determined.
- Now we calculate the distance between the axes of the beams. To do this, the length of the slope must be divided by the number of rafters: 17.8 / 24 \u003d 0.74 m.
So, you will determine the actual distance at which the rafters for a roof made of metal or some other material should stand.
Often, when planning a roof frame, specialized calculators are used. They enter basic data and get the finished result. Nevertheless, you should not completely trust the program, since only a person can take into account some conditions. When calculating, you can understand how the system works, what loads are acting. If necessary, adjustments are made to strengthen or facilitate the design. Self-calculation will help to more accurately determine the required pitch of the rafters of a gable and single-pitched roof.
Step calculation depending on the roofing material
For each coating, its own standards and dimensions are developed for the truss system. The main factors are the strength of the material, its weight and resistance to external loads. Consider the main types of coatings.
Decking
The step of the rafters under the corrugated board can vary from 0.6 to 0.9 m. The final result depends on additional conditions, which we will discuss below. If for some reason the step should be larger, a transverse board with a large section is added. In this case, usually the rafters themselves are chosen with dimensions of 50x100 or 50x150 mm.
In addition to the rafters, a crate of 30x100 millimeters is used. The gap between the boards should be about 0.5 meters. It can be more, it all depends on the type of corrugated board and the height of the wave. Sheathing boards protruding beyond the boundaries of the eaves should have a thickness of 1.5 cm more than usual. This is done based on the need to mount ventilation, a chimney or a drain.
Ceramic tiles
The main difficulty in designing a roof frame for ceramic tiles is the large weight of the material for the roof. It is 10 times heavier than corrugated board and accounts for 40 to 60 kg per square meter.
Under such a coating, dry beams are used, with a moisture content of only 15%. Their cross section should be either 50x150 or 60x180 mm. With such indicators, the maximum allowable distance between the rafters can be 1.3 meters. The minimum allowable step through which rafters can be placed is 0.8 m. The roof slope affects the final decision: at 15 °, the step is 0.8 m, at 75 ° - 1.3 m.
The length of the rafter leg is also taken into account, the shorter it is, the larger the gap, and vice versa. In addition, if the slope is less than 45 °, and movement is planned on the surface, the beams are placed in increments of no more than 0.85 m.
The distance between the boards of the crate is selected so that each intersection of the tiles has its own base. The standard length of the material is 400 mm, and the amount of overlap during installation varies between 55 and 90 mm. It turns out that the distance between the central axes of the crate can be either 310 or 345 millimeters.
metal tile
In recent years, the question has been particularly acute: how to determine the distance between the rafters under the metal tile? It imitates more expensive ceramic material, but at the same time it has the positive properties of corrugated board. Installation of such a coating is simple and does not require large financial costs. All this makes the material attractive for the roofs of private houses.
The mass of metal tiles is less than ceramic, so a simpler frame is required. The section of the beams is reduced to 50x150 mm and the step between the crate is increased. The step of the rafters under the metal tile varies between 0.6 and 0.95 m.
Ondulin
The allowable step for covering from ondulin is 0.6-0.9 meters. The standard section of the rafters is 50x200 millimeters. Such dimensions will allow to withstand the load of the roofing pie on the gable roof.
On top of the counter-lattice, a crate of bars measuring 40x50 millimeters is attached. The pitch between the central axis is 600 mm.
Additional factors
When calculating the pitch of the rafters, attention is paid not only to the type of roofing. Many other points are taken into account, data on which can be found in reference books and SNiPs. Here are some of the factors:
- Snow and wind loads. The more snow falls in winter and the stronger the winds blow, the smaller the distance between the beams should be. But, if you increase the angle of the roof slope over 45 °, then it is allowed to increase the step.
- Choice of heater. To reduce the cost of insulation boards, it is recommended to take into account their standard sizes. They produce mats with a width of 600, 800 and 1200 mm. If this condition is neglected, then a large number of undercuts will appear, cold bridges and construction will be delayed.
- lumber quality. It takes into account the type of wood, grade and section. As mentioned above, strength is affected by how well the beam was dried. When buying wood, pay attention to evenness and the presence of defects in the form of knots and traces of pests.
- Floor beams and puffs. If you equip the attic floor under a gable roof, then the maximum distance between the rafters should be 0.75 meters.
The calculation of the distance between the beams for a gable and single-pitched roof is different. Even if he makes several slopes, for each it is necessary to make a calculation individually. This is especially true for buildings with different lengths of cornices.
Taking into account all these factors, it is possible to determine how far the rafters should be installed. Although all calculations can be done independently, since reference data is freely available, it is better to turn to professionals. They have experience in designing, they will quickly determine the required distance between the rafters. So, you will avoid possible mistakes and be sure of safety.
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