The roof structure is one of the main enclosing elements of the building, the quality characteristics of which are subject to rather stringent requirements.
One of the most common roof sheathing materials is metal shingles, which are made from thin sheets of steel, aluminum, or copper.
From above, the elements are equipped with a polymer coating, which protects metal from aggressive external influences.
Externally, the metal tile is similar to ceramic, but it is more durable. This material is used for covering pitched roofs, the slope of which must be at least 14 degrees.
This is the national team roof frame structure consisting of many wooden or metal parts. She rests on load-bearing walls, which are a reliable basis for all overlying elements. The rafter system serves as a kind of skeleton, on the basis of which the roofs are made, as well as the laying of the roofing finishing roofing layer.
Rafter system
The constituent elements of the rafter roof, and their main characteristics:
- Mauerlat. Softwood beams, which are the connecting element between the rafters and the underlying structures. It has a square cross-section with a side of 100 or 150 mm. Mauerlat is laid along the load-bearing wall along its entire length. With the help of the Mauerlat, the loads from the roof are evenly distributed throughout the building.
- Sill. A bar with a square cross-section similar to a Mauerlat. It fits transversely to the bearing walls, as it serves to redistribute the load from the roof racks.
- Rafter leg. From these elements, the main triangular roof structure is created, which experiences the full severity of external atmospheric influences (rain, wind, snow, hail, etc.).
- Rack. Vertical connecting elements that distribute the compressive loads from the ridge assembly over the entire area of the bearing walls. They are made of square beams, the length of the edge of which is determined by calculation.
- Tightening. It is the final horizontal element of the triangle of rafter legs, which does not allow them to creep under the pressure of external loads and the own weight of the roof. Used in hanging rafter systems.
- Braces. They perceive and redistribute bending loads from the ridge knot.
- Lathing. Consists of boards, bars or plywood sheets (in the case of subsequent laying of bituminous tiles), which are located at right angles to the rafter legs, being an additional stiffening element.
- . The junction of two roof slopes.
- Overhang. A roofing element that protrudes beyond the supporting wall structures at a distance of about 0.4 m. Its purpose is to limit the penetration of moisture to the walls.
- Filly. These elements are attached to the ends of the rafters if they are not long enough to organize the overhang.
Varieties of pitched roofs
Depending on the number of inclined planes, roofing structures can be divided into:
In private housing construction, the option is most often used gable roof since he has a number of advantages. These include:
- Practicality. The gable roof has a significant angle of inclination, due to which rainwater does not accumulate on its surface, and snow and wind loads are distributed in the most optimal way.
- Simplicity of the device and operation. The assembly and joining of two pitched elements is much easier than for complex roof structures. In addition, the repair of such a roof will also be easy.
- Aesthetics. A gable roof structure blends seamlessly with the surrounding infrastructure.
- Reliability(if done correctly).
- Democratic price constituent materials.
Types of pitched roofs
Gable roof - rafter system for metal tiles
Frame made of rafters for a gable roof made of metal tiles has no significant differences from structures with other covering roofing materials.
But, due to the fact that metal thin sheets have a low specific weight, the rafters will experience less constant stress.
This allows you to reduce the value of their cross-section, due to which will be able to save a lot on the purchase of wooden materials.
For roofing under metal tiles, the optimal the angle of inclination must be at least 14 degrees.
For roofs with two pitched elements, the following apply options for arranging the frame:
Roof rafters for metal tiles.
In this case, 2 supporting rafter legs are fastened together with the help of bed(horizontally) and racks(vertically). The bed is laid parallel to the Mauerlat element, while taking on part of the force. The rafter system under the metal tile is taken over bending loads only, which significantly affects the selection of the design cross-section. This system can be used for buildings with large and small spans.
Types of rafters
Hanging rafters.
Unlike layered systems, in this version, two rafter legs fastened together only in the ridge knot... In this case, significant expanding forces on the supporting elements arise, which limits the use of hanging rafters only for buildings with a span of no more than 6 m. In some cases, an additional connecting element is installed - a tightening that takes on part of the spacer loads.
They can be made of wood or metal, as well as installed at the bottom (act as a load-bearing beam) or at the top of a triangular structure. It is worth considering that the higher the puff is located, the more efforts it will perceive.
NOTE!
To ensure high-quality tightening work, care must be taken about the reliability of fastening with load-bearing rafter legs.
Combined option
Used to create an original roof structure. Includes elements of both hanging and layered systems.
How to calculate the angle of inclination of the rafters?
To implement a gable roof, you need to know a few geometric values of the building, namely:
- Half span width - L;
- Distance from the load-bearing wall to the ridge of the roof (or the height of the support post) - H.
Standard formula: α = arctan (L / H)
Where α is the required angle of inclination of the roof.
Knowing this value, you can calculate the length of the supporting rafter leg:
l = H / sinα.
Where l is the length of the rafter.
The angle of inclination of the rafters
How to calculate the load?
To carry out the correct selection of parts of the roof frame, it is necessary calculate the values of temporary and permanent load acting on its structural elements. 
The permanent load includes the weight of all elements, as well as the mass of the supporting elements themselves and the battens.
Temporary loading options include force effects from wind, snow cover, rain masses, as well as the weight of a person (to take into account options for subsequent repairs).
Constant load calculation
Roof cake weight.
It is determined by adding the masses of all its elements, namely steam, hydro and thermal insulation, as well as metal roofing. In this case, the weight of one running meter (can be found in the normative documentation) is multiplied by the value of its length.
The weight of the rafter system.
It is determined by adding the weight values of the lathing, rough flooring, as well as the supporting frame. The mass of each element is calculated by the formula:
M = V * p,
Where V is the volume of the element, calculated depending on the geometric characteristics of the cross-section and the length of the elements;
P - Density of the wood used (depends on the species).
Total permanent load = weight of the rafter system + weight of the roofing cake.
Calculation of temporary loading
Conducted in accordance with regulatory documents ( SNiP 2.01.07-85 "Loads and Impacts" or Eurocode “Actions on structures” part 1-4).
To determine the value of the wind effect, the roof structure is conventionally divided in height into several parts. For each of them, the value of the wind load is calculated. To obtain the total wind pressure, they must be summed up.
Formula for calculation:
Wm = Wo × k × c,
Where Wm is the value of the wind load;
Wo is the normative value of wind pressure, determined from zoning maps;
k - wind pressure coefficient (determined depending on the height according to the regulatory documentation);
c - aerodynamic coefficient (for a gable roof - 0.8).
Determined by the formula:
S = µ × So;
Where So is the normative value of the snow load, determined from the zoning map.
µ - coefficient, which is determined depending on the angle of inclination of the roof:
- For α≤30 deg. - µ = 1
- For α≥60 deg. - µ = 0
- For 30≤α≤60 deg. - µ = 0.033 × (60-α)
Snow load areas
How to choose a bar and calculate the pitch of the rafters for metal tiles?
Determination of the value of the cross-section of the rafter beam is carried out in several stages.
Calculation of the load distributed on each running meter of the structure:
Qр = L × Q;
L - Step of rafters.
The L value is calculated as follows:
The length of the roof slope is divided by the intended step of the structures (for convenience, it is most often taken equal to 1). Then, 1 is added to the resulting value. The resulting value reflects the number of rafters that need to be installed on one pitched roof surface. At the last stage, the value of the axial distance between the rafters is determined by dividing the length of the roof slope by the number of rafters.
The distance between the rafters under the metal tile - the standard pitch is 0.6-0.95 m.
Rafter step
Then we determine the maximum working area of the rafter leg (Lmax). Let's move on to calculating the cross section. To do this, we find its height using the formula:
H ≥ 8.6 * lmax * sqrt (Qp / (b * r)), with a roof slope α<30 град;
H ≥ 9.5 * lmax * sqrt (Qp / (b * r)), with a roof slope α≥30 degrees;
Where b is the width of the cross section,
r is the value of the normative resistance of wood to bending loads (determined according to the normative documentation, depending on the type of wood).
To simplify the calculations, you need to use the rafter standardization table (GOST 24454-80 “Softwood lumber. Dimensions ").
If the inequality is not observed, it is necessary to increase the value of the geometric characteristics of the section and repeat the calculation.
What is the difference between a rafter system for a cold and warm roof?
The main difference between these two roofs is the support system of the rafters. In the case of a warm attic, the main support element is the Mauerlat, as well as the support beam system. In a cold roof, rafters are installed directly onto load-bearing walls.
Installation of rafters for metal tiles
All installation work on the roofing is carried out at a sufficiently high height. To minimize the risk of falls and to make it much easier to work at heights, it is possible to assemble the frame of the supporting truss system on the ground.
To do this, you need to create a template from the boards, according to which further assembly will be carried out.
It is manufactured in several stages:
- The planks are lifted over the walls of the building, aligned, and then held together with a nail.
- Align the angle of inclination of the boards in accordance with the project, by lowering and raising them. The elements are fixed.
- The result should be a structure that resembles the shape of the future rafter system, made in accordance with the calculated geometric dimensions of the roof.
- The template is lowered to the ground, according to which the finishing elements are fixed to each other. More details in the video below.
Then you should take care of installing the supporting element - the Mauerlat. As mentioned earlier, it is laid on the load-bearing walls in the longitudinal direction. Fastening is carried out using pins (on an armored belt or masonry) or using a wire rod (for buildings with a low roof height).
CAREFULLY!
When using a hairpin connection, the connecting elements no need to walled up tightly into the wall. They should protrude from the wall by 30-40 mm, since the nut will be screwed onto the studs.
The next step is to create ridge run, which serves as a supporting part for the entire structure of a gable roof. It is made from a bar or hewn log. If the span of the building is not more than 6 m, it can be supported without additional supporting elements. Otherwise, construction trusses must be used for installation.
Mounting. Part 1
After installing these elements, you can lift and install the main rafter assembled according to the template. Bonding with a Mauerlat can be done in 2 ways:
Rigid connection. It is carried out using corners and beams. Less commonly, fastening is used by means of a saw on rafter legs, followed by fixation with nails or staples.
Features: in addition to the main connection, it is necessary to tie the rafters to the wall using anchors or a wire structure.
Sliding. It is based on the creation of a swivel joint. It is made by joining elements using cuts. The elements are connected with a metal embedded part with holes for bolts, or 2 nails, which must be driven in at an angle.
It is necessary to carry out the installation of wooden trusses in a certain sequence. First, the extreme trusses are installed, located at the ends of the building. Then a cord or rope is pulled between them, with the help of which the verticality of their installation is checked. Further, under the cord, further installation of truss structures is carried out in accordance with a given design step.
Mounting. Part 2
The creation of a roof from a metal tile is a rather laborious process that requires certain skills and a well-stocked hand. Therefore, to carry out the correct installation, you must at least work under the supervision of a competent specialist.
Useful video
Video instruction for self-installation of rafter legs:
A roof made of metal tiles is the coating that is most often found in the construction of not only country houses, but also city cottages. The metal tile is called a fake clay tile, it is very popular and has its own advantages over the original. The main thing is that the weight of the metal tile is much less than the weight of the clay tile. The difference in weight is very significant, it will be about 30 kg just for 1 sq. m. Due to the lightness of the roofing, the parameters of the rafter system can be reduced, for example, the pitch of the rafters under the metal tile.
Since the metal tile is a fairly light material compared to the original (clay tile), the pitch of the rafters can be reduced.
In addition, the installation of sheet metal tiles is much easier in terms of technology, which, of course, shortens the work period and saves not only time, but also money. In addition, this will simplify and facilitate the lifting of the tiles to the roof, which, of course, is important, since it requires some effort.
The basis of any well-done work is carefully calculated in advance.
In such an important matter as a roof covering system, you should not be mistaken, and you need to approach it responsibly.
An error, one wrong step or calculation threatens you with a violation of the entire coating, the collapse of the base or a change in the shape of the rafter legs. The more accurate your calculations are, the more reliable the frame will be, and the calmer and more confident you will feel in the house with your own roof.
Therefore, carefully study all the nuances that you should pay attention to when calculating.
In order to measure the step distance, you need to take into account all the loads that will act on the roof. For example, on the territory of Russia (its middle zone), the wind and snow load on the roof is approximately 200 kg per 1 sq. m.
In the north and in the areas closest to it, this number can almost triple. Naturally, from these calculations, the step of the rafter leg will change in order to ensure the reliability of the entire structure.
The material under consideration should be up to 950 mm long with a design load of 200 kg, this is with a normal section of 150 * 50 mm. When increasing the thickness of the insulation, do not forget to increase the cross-section of the rafters. At the same time, experts clarify that with a crate of 50 * 30 cm, you should not increase the step.
Rafter roof calculation system
- With the greatest accuracy, measure the length of the slope along the eaves of the roof.
- The length of the ramp is divided by the length of the selected step (the gap between the rafter legs).
- One is added to the result, and the resulting value is rounded up. thus, the number of rafters that will be needed for only one side of the roof slope under the metal tile is found out.
- Then the total length of the ramp is taken and divided by the number of rafters, which was found out using previous calculations. The number that comes out will be the value of the gap between the rafters. It is this figure, or rather the distance, that is called the step of the rafters.
rafter length 3.0 (m) 4.0 (m) 5.0 (m) 6.0 (m) 60 (cm) 40x150 50x150 50x175 50x200 90 (cm) 50x150 50x200 75x175 75x200 110 (cm) 75x125 75x175 75x200 100x200 140 (cm) 75x150 75x200 75x200 100x200 175 (cm) 75x150 75x200 100x200 100x250 215 (cm) 100x150 100x200 100x250 100x250
Note. The rafter sections are in millimeters.
The length of the roof slope is 25.5 m.
The length of the rafter step is 1 m.
28.5 / 1 = 28.5 + 1 = 29.5, now rounding: 29.5 = 30.
That is, you will need exactly 30 rafters for the roof slope.
28.5 / 30 = 0.95, that is, 95 cm is the step distance.
All calculations should be made taking into account the shape of the roof, the structural features of the frame, as well as the microclimate that is individual for each region. The calculation of the rafter roof is carried out at the design stage.
Also, experts advise, to improve ventilation, when installation is carried out, drill holes with a diameter of 10 mm in the rafters near the upper edge.
Mandatory precautions
To ensure durability and reliability, the rafter structure under the metal tile must be treated with a special solution, it is better to do this in advance in order to process all the places, since some of them will be inaccessible after fixing.
So that grinder beetles do not start up in the rafters, which over time can exude the entire wooden part of the roof, it is necessary to make a mandatory treatment with an antiseptic. Timeliness of processing will save you not only from insects, but also from mold and mildew, which can destroy rafters even faster than grinder beetles.
The need to treat the rafters with special means as protection against fire is beyond doubt. By choosing any impregnation suitable for its properties and having processed the rafters, you can be sure that you will greatly increase their resistance to fire.
Types of existing structures
For the structural rigidity of the rafter structure, it must necessarily consist of triangles, conjugated to each other and not having freedom. Roof structures are divided into two types:
- hanging;
- inclined.
Hanging rafter structure consists of rafter legs in the amount of two pieces, which are connected at the very bottom by a tightening. It is the tightening that does not give grounds to increase the distance, that is, to part.
If the spans exceed 8 m in size, then a rack (headstock) with struts is installed to reduce the deflection. For spans, the length of which is less than 8 m, an additional tightening is placed, which cuts between the top of the rafters and the lower tightening.
For it, you need to use a log or horizontal beam, which is attached to the base of the rafter legs, which reduces the bursting horizontal forces acting on the walls of the building.
"Filly" - this is the name of the continuation of the rafter leg, which is fastened with nails to the rafter leg. It can be made from a board that is half the size of the leg itself.
The rafter structure is supported by the middle part and the ends on the supports or walls of the building and is installed only in cases where the spans do not exceed 6 m.
The addition of a beam that cuts horizontally below the ridge between the two rafters allows the distance between the supports to be increased to 8 m.
If there is an additional support, then the width can be increased to 12 m, and in the presence of two supports - up to 16 m. In cobbled buildings or in chopped wooden buildings, the rafter legs rest on the upper crowns.
- In stone buildings - on a support beam, in timber frame - on the upper strapping. The dimensions of the building and the presence of internal supports (walls, columns) in it dictate the location of the rafters.
Some features
There are no fundamental differences between the roof structure for metal tiles and other structures. The main difference between them is that the upper support is installed on top of the ridge girder, and not on the ridge beam.
For a metal surface, air circulation is important, which is formed in the form of a free zone between docked wooden parts. This reduces the risk of condensation under the roof deck.
If the installation of a house involves replacing the Mauerlat on the upper log crown, then a calculation that determines the step of the rafters should be done as carefully as possible and double-checked.
Since it will be impossible to make even the slightest changes afterwards, since notches are made in the crown, which increase the reliability of the fastening.
Rafters for metal tiles (their installation) are carried out using special corners, which are used to fix structural elements.
The use of such anchoring angles will provide a high load-bearing capacity and, in addition, allow you to work without special tools. In this case, it will be possible to use ordinary nails and screws to fasten the corners.
Knowing the features of the calculations, the distance and the required step, you can easily understand the intricacies of this construction process. Good luck!
The roof frame must be reliable and durable. But without competently carried out calculations, this is difficult to achieve. In the process of carrying out the calculations, it is determined at what distance to put the rafters on the roof.
What can an incorrect or inaccurate calculation of the loads to which the rafter structure will undergo? To the most negative consequences, ranging from deformation of the rafter legs and damage to the roof covering and ending with the collapse of the base of the roof frame. Therefore, when designing buildings, the list of mandatory calculations contains data on what the distance between the roof rafters should be. There is a certain methodology that allows you to calculate this value.
Method for calculating the distance between the rafters
The distance between the rafters on the roof is called the rafter pitch. As a rule, the pitch of the rafter legs in the roof structure usually exceeds one meter, and the minimum gap ranges from 60 centimeters.
The calculation of the required number of rafters for a roof of a certain length and the pitch of the rafters is made as follows:
Roof structure for metal tiles
In the construction of suburban private real estate, metal roofing can be found most often. This roofing material is similar to clay tile decking, but has a number of advantages over it. Sheet metal shingles are easy to install, so the roof can be erected in a shorter time, the rafter system for metal shingles is also not difficult.
Metal tiles are lighter than ceramic products, the difference in weight sometimes reaches 35 kilograms per square meter, depending on the thickness of the products (read also: ""). Due to a significant reduction in the weight of the roofing, it becomes possible to reduce the thickness of the elements of the rafter structure and the dimensions of the cross-section of the sheathing bars, to increase the pitch of the rafters.
Under the metal roofing, the rafter legs are mounted with a distance of 600 to 950 millimeters, while the cross-section of the construction material is 150 by 50 millimeters. According to experts, in this case, if you place a 150 mm thick insulation between the rafters, then such thermal insulation will create comfortable conditions for staying in the attic room. At the same time, for greater reliability, it is advisable to choose a 200 mm insulation.
When installing the rafters, to ensure ventilation of the space that is filled with insulation, holes are drilled in the rafters near the upper roof with a diameter of 10-12 millimeters.
The technology of creating a rafter system for metal tiles does not differ significantly from structures for other types of roofing materials. The only peculiarity is that for the rafters, the upper support is mounted on the ridge girder from above, and not on the side of the ridge beam. The presence of a free zone between the rafters ensures air circulation under the roof deck, and this, due to the use of metal material, reduces the risk of condensation.
Shed roof rafter system, main advantages and characteristics ").
The distance between the rafters of the gable roof is made taking into account the size of the heat insulator, which is laid between them. The approximate step between the rafter legs is 1-1.2 meters (read: ""). Rafters regulate the size of the roof overhang.
The distance between the rafters under the metal tile necessarily takes into account the efforts resulting from the wind, snow load, the own weight of structures, and the roof. In addition, the following factors affect the step of the rafters under the metal tile:
- location of pipes - wooden elements of the roof should be 25-35 cm away from the chimney, should not interfere with the straight paths of ventilation ducts, fan pipes;
- roof configuration - a rafter leg is required at the junction of the ridge of the gable, hipped roof.
All wooden elements of the truss system are made of coniferous trees, the moisture content of which does not exceed 20%.
The step of the rafter legs is calculated at the design stage for budgeting. This will significantly reduce the amount of waste, cut sawn timber.
What do you need to know when choosing the optimal distance between the rafters?
Roof layout with hanging rafters.
Having collected the present loads during the strength calculation, the designer evenly distributes them to the load-bearing walls. The principle of calculation is the same for layered, hanging rafters, only the schemes for fastening the elements in the ridge, on the Mauerlat, differ.
The minimum, maximum pitch of wooden rafters for metal tiles is regulated by 0.7 m, 1.2 m, respectively.
When choosing a step of 60 - 100 cm, the length of the rafters cannot exceed 6 m, with its decrease, a run-up of up to 1.2 m is allowed. If you put your legs more than 60 cm, this will unnecessarily burden the roof and increase the construction budget. If the step is increased by more than 1.2 m, the bearing capacity and the resource of the structure will sharply decrease.
A solid lathing made of wood-containing slabs adds strength, rigidity to the rafter system. At the same time, it is allowed to increase the step by 0.3 - 0.2 m in comparison with the periodic lathing from a bar or edged board. However, to save the construction budget, a solid crate for metal tiles is rarely used. The material has sufficient strength, rigidity due to the additional transverse ribs of the profile.
A step of more than 1.2 m is not used even when using metal rafters, despite the sufficient margin of safety of the material. This is due to the possible deflection of roof sheets during heavy snowfalls and hurricane winds.
The cross-section of the timber from which the rafter legs are made also affects the step of the rafters, since the support area of the battens and the weight of the roof change. The best option is a bar of 150 x 50 mm with a discharged crate with a step of 4 - 7 cm, depending on the step of the shear wave.
Calculation example for a gable roof
When carrying out the project, the roofing material is known at the initial stage by specialists. In order to find out the recommended pitch of the rafters, you can use the SNiP tables, and then adjust the value in accordance with the operating conditions. An example table is shown below:
| Leg section (cm) | Rafters pitch (cm) depending on their length (m) | ||
| 5 | 4 | 3 | |
| board 20 x 2 | 70 | 120 | – |
| board 18 x 2 | – | 100 | – |
| board 16 x 2 | – | 70 | 130 |
| timber 22 x 6 | – | 120 | – |
| timber 20 x 5 | – | 110 | – |
| timber 18 x 5 | – | 90 | 150 |
| log 180 | 90 | 150 | – |
| log 150 | – | 90 | 150 |
| log 140 | – | 70 | 140 |
| log 130 | – | – | 110 |
The table values correspond to the rafters of simple single-pitched roofs. First, the section of the leg is selected, the length of the element, the distance between the centers of the log, the beam is obtained automatically. In the next step, the length of the slope in the ridge is divided by the rafter pitch, adding one. Thus, the number of legs is counted, rounded up. Then it remains to adjust the distance between the wooden rafters in fact. For example, with a ridge length of 7.5 m, a rafter leg with a section of 16 x 2 cm (board) 4 m long, you get:
7.5 / 0.7 = 10.7 + 1 = 11.7 pcs. round up to 12 rafters.
Specifying the size allows you to calculate the center-to-center distance for installation after installing the outer legs:
7.5 / 12 = 62.5 cm.
Dormer windows are placed between adjacent rafters, in places where pipes and chimneys pass, the legs are shifted to the distance specified in SNiP. All other elements of the system remain in place, the area of abutment to the pipes, if necessary, is enhanced:
- a bed is cut into two adjacent legs;
- a short rafter is cut into it with one end, the second adjoins in the ridge to the element of the opposite slope;
- the shifted legs in the upper part rest on the ridge girder, fixed to at least two trusses.
Thus, the system receives the required rigidity without loss of bearing capacity, the fire safety requirements of wooden roof parts are met.
Rafter leg material
The material of the rafters is often a bar of 25 x 10 cm - 15 x 4 cm, which allows you to reduce the construction budget.
When choosing lumber of natural moisture, the developer is guaranteed to receive a shrinkage of the structure in the first year and a half by 5-7 cm in height. By increasing the cost estimate for arranging the rafter system by 70%, you can purchase glued beams, significantly reducing the structural loads, and doubling the roof resource.
The distance between the rafters will remain unchanged, however, instead of the planed timber of 17.5 x 5 cm, recommended by building codes for five-meter legs, spaced 0.6 m apart, you can get by with glued laminated timber of a smaller section of 15 x 4 cm.Transportation costs will decrease, work at height will be easier , cut material.
Prefabricated rafters from boards are used with the same scheme for fastening hip roof trusses. The upper slopes are made of single boards, the lower ones are made of three boards sewn with self-tapping screws with an offset in the rows.
The choice of metal rafters is justified with a complex roof configuration, an abundance of ventilation pipes, chimneys, which cannot be bypassed in compliance with SNiP requirements, fire safety. In this case, the step between the rafters increases as much as possible, since the rolled metal is much stronger than lumber.
If the rafters are attached at the bottom to the Mauerlat, the step of the legs is not critical, the elements can, if necessary, be shifted to the desired distance in either direction. If a scheme is used to support the braces, which are floor beams, it is much more difficult to displace individual elements. In this case, the amount of cutting waste increases when trimming a rough ceiling, floor of an attic or attic room.
The rafter system for metal tiles is one of the most economical structures of modern roofs. This is due to the low weight of the steel profile sheets of the covering, the absence of solid substrates such as OSB or plywood. A lot of literature and instructions, videos are devoted to the process of proper installation of roofing elements. The process of choosing a rafter system for an individual developer is less clear. This article is an attempt to inform the average reader how to choose the right rafters for metal tiles.
The constructive solution of the truss system depends on the shape of the roof, the shape and size of the building. There are two main structural schemes of rafter systems: Rafter rafters are a simple and reliable non-spread rafter system in which the rafter beams rest (lean) at the top and bottom on walls or beam elements. This rafter system is most popular in individual construction. The rafter system consists of rafter legs resting on the Mauerlat at the bottom, and on the girder at the top, racks resting on the beds and pads, as well as struts, struts and trusses. 
Hanging rafters - a more complex system, used in the absence of internal walls, as a rule, with large distances between the supports. This design is a spacer, in which the spacer force is absorbed by puffs. The structural elements of the system are various types of roof trusses. The advantage of this system is that all elements can be manufactured and assembled both on site and in the factory. In the latter case, it remains only to carry out an enlarged assembly and installation. The use of such a system is justified for objects with a tight construction time. 
Another advantage is that the finished elements are usually exposed to fire-retardant impregnation in the factory. In addition, enterprises have a large selection of ready-made design solutions for different types of roofs. 
Dimensions and cross-sections of the elements of the rafter system
Ideally, all elements of the house, including the roof, are developed in detail by the design organization. If there is no project or we are talking about reconstruction, then the following information will come in handy.For the manufacture of load-bearing roof structures, wood with a moisture content of no more than 25% should be used. The constituent elements can be from logs, beams and edged boards.The overall massiveness of the rafter roof elements depends on the distance between the load-bearing walls of the building, the climatic region of construction and the slope of the roof. The larger the span, the amount of snow and the heavier the roof structure, the more massive the supporting frame of the roof.
Geometry of the layered rafters
Although the cross-section of almost all elements of the rafter system is a calculated value, in the construction literature the minimum cross-section of rafter legs is determined as 50 × 100 mm, the Mauerlat is 150 × 100 mm, and the minimum height of the bed and gaskets is 150 mm. It should be noted that the cross-section of the rafters of 50 × 100 mm looks like fireplace matches on the roof, and it is often increased for visual comfort. Reinforced rafters work like bending elements and are calculated depending on the span of the building and the actual load.
So, among the known values there are the length of the roof, the span of the building (the distance between the supports C), the height of the roof, and at least a general idea of the snow load. Among the unknowns there is a rafter pitch (B), a rafter section (a × b). Getting rid of the unknowns, we assign the step of the rafters under the metal tile. The rational distance varies between 0.6 and 1.2 m. If the roof has rafters made of logs, then the pitch of the rafters can be increased to 1.8 m. In a situation with metal tiles, this is not necessary, since you will have to perform a thickened lathing. 
The rafter pitch from 0.6 to 0.9 m is mainly used for roofs with a cold attic and single rafters. It should be noted that the minimum step is rarely used, and the installation is carried out with a step of at least 0.7 m. If the attic floor is not expected, any step of the rafters can be taken for a cold attic. The cross-sections of rafter legs for simple roofs under normal conditions in the climatic zone of central Russia can be preliminarily determined from the following table. Roof slope, hail | Step of rafter legs, m | Board section |
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Distance between supports, m |
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| 20 - 30 | 3,0 | 3,5 | 4,0 | 4,5 | |
| 0,7 | 50 × 120 | 50 × 150 | 50 × 175 | 50 × 175 | |
| 0,9 | 50 × 150 | 50 × 175 | 50 × 175 | 50 × 200 | |
| 1,2 | 50 × 175 | 50 × 200 | 2 × 50 × 150 | 2 × 50 × 200 | |
Roof roof
This is not the case with a mansard roof. Since the time of all-Union unification and typification (experienced builders will understand), the modular dimensions of the insulation boards have remained the same: a sheet of 600 × 1200 mm (TechnoNIKOL additionally produces insulation boards with a size of 500 × 1000 mm). If it is planned to place a dwelling in the attic space by completing an attic roof, the option with the maximum rafter pitch is often used. Moreover, with a thickness of insulation of 150 mm and the need to make an air gap of 50 mm for ventilation of the under-roof space, a rafter section height of 200 mm is required. And with a more massive layer of insulation (from 200 mm), which is not uncommon in our country, you have to fill in additional slats. Thus, in the presence of insulation between the rafters, a pitch of 1200 mm is taken from the condition of the convenience of installing the insulation without additional trimming, the possibility of installing various types of dormer windows and the manufacturability of the ventilation device for the under-roof space. With a distance between the rafters of 1.2 m and a snow load for 4 snow areas and above (everything north of Sergiev Posad, St. Petersburg, all the Volga region to Syzran, and further through the Ural ridge), a device of paired rafter legs is required. The sections of the elements can be preliminarily determined from the table below.table 2
Roof slope, hail | Step of rafter legs, m | Rafter leg section |
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Distance between supports, m |
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| 3,0 | 3,5 | 4,0 | 4,5 | 5,0 | ||
| 20 - 25 | 1,2 | 2 × 50 × 150 | 2 × 50 × 175 | 2 × 50 × 175 | 2 × 50 × 200 | 2 × 50 × 200 |
You need to connect two boards through spacers in increments of no more than 1 meter. A rafter system made of paired boards is more reliable than a single system... And in order to sleep under the new roof completely calmly, you need to turn to the designers for the calculation of the roof elements. I recommend watching the following video: