The distance between the rafters under the metal tile necessarily takes into account the efforts of the wind, snow load, its own weight of structures, roof. In addition, the following factors influence the rafters pitch under the metal tile:
All wooden elements of the truss system are made of coniferous trees, the humidity of which does not exceed 20%.
The rafter pitch is calculated at the design stage for budgeting. This will significantly reduce the amount of waste, trimming cut lumber.
Scheme of the roof with hanging rafters.
Having collected the present loads in the strength analysis, the designer evenly distributes them onto the load-bearing walls. The principle of the calculation is the same for nasal, hanging rafters; only the mounting patterns of the elements in the ridge, on the mauerlate, differ.
The minimum, maximum step of wooden rafters for metal tile are 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 reduction it is allowed to run up to 1.2 m. If you put your feet more often 60 cm, this will make the roof unnecessarily heavy and increase the construction budget. If you increase the pitch more than 1.2 m, the bearing capacity, the life of the structure, will decrease dramatically.
The continuous lathing of wood-containing plates adds strength and rigidity to the truss system. In this case, an increment of 0.3–0.2 m is allowed as compared to a periodic crate from a bar or a cut board. However, to save the construction budget, solid crate for metal tiles is used extremely rarely. The material has sufficient strength, rigidity due to the additional transverse edges of the profile.
Device for sheathing slate roofs around pipes.
A pitch greater than 1.2 m is not used even when using metal rafters, despite an adequate margin of safety of the material. This is due to the possible deflection of the roof sheets during heavy snowfall, hurricane wind.
The cross-section of the timber from which the rafter legs are made also affects the rafter spacing, since the sheathing area and the weight of the roof change. The best option is a timber 150 x 50 mm with a discharged crate with a step of 4 - 7 cm, depending on the shear wave pitch.
When the project is carried out, specialists at the initial stage know the roofing material. In order to find out the recommended step of rafters, you can use the SNiP tables, and then adjust the value in accordance with the operating conditions. An example of the table is shown below:
|Leg cross-section (cm)||Graft spacing (cm) depending on their length (m)|
|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|
Tabular Values correspond to the rafters simple one-gable to LUNs. 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. At the next stage, the length of the slope in the skate is divided by the rafter step with the addition of one. Thus, the number of legs is calculated with rounding up. Then it remains to adjust the distance between the wooden rafters in fact. For example, with a skate length of 7.5 m, a rafter leg with a cross section of 16 x 2 cm (board) with a length of 4 m, the result is:
7.5 / 0.7 = 10.7 + 1 = 11.7 pcs. round up to 12 rafters.
Table of distances between the rafters.
Specifying the size allows you to calculate the center-to-center distance for installation after installing the extreme legs:
7.5 / 12 = 62.5 cm.
Dormer windows are located between adjacent rafters, in the places of passage of pipes, chimneys, feet are shifted by a distance specified in the SNiP.All other elements of the system remain in place, the area of junction to the pipes is reinforced if necessary:
Thus, the system obtains the necessary rigidity without loss of bearing capacity, and the fire safety requirements of the wooden parts of the roof are complied with.
The material of rafters is often a bar 25 x 10 cm - 15 x 4 cm, which allows to reduce the construction budget.
Table of choice of section of rafter legs.
When choosing lumber of natural moisture, the developer is guaranteed to receive a construction shrinkage in the first one and a half year by 5-7 cm in height. By increasing the cost estimate for arranging the truss system by 70%, it is possible to purchase glued laminated timber, significantly reducing structural loads, and doubling the roof resource.
The distance between the rafters will remain unchanged, but instead of the 17.5 x 5 cm planed bar recommended by the building code for five-meter legs spaced 0.6 m apart, you can do with a 15 x 4 cm glued bar. Transport will decrease costs, easier work at height, cut material.
Prefabricated rafters from planks are used with the eponymous mounting scheme for hip-roof trusses. The upper slopes are made of single boards, the lower of the three boards, stitched with screws with offset in rows.
The choice of metal rafters is justified with a complex roof configuration, an abundance of ventilation pipes, chimneys, which cannot be circumvented in compliance with the requirements of SNiP and fire safety. In this case, the step between the rafters is maximized, as the metal is much stronger than lumber.
If the rafters are attached at the bottom to the mauerlat, the pitch of the legs is not critical, the elements can, if necessary, be displaced to the desired distance in either direction. If the scheme is used to support puffs that are beams of overlap, it is much more difficult to displace individual elements. In this case, the amount of cut waste increases during the lining of the draft ceiling, attic floor or attic room.
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