Any pitched roof requires a reliable support system. Her role perform rafters. Lightweight and durable frame that can withstand the weight of the roof and the layer of snow on it, must be correctly calculated, carefully extinct, well-assembled and securely mounted on the house.
Structure of rafters
Rafters are more often made of coniferous wood - it is lightweight and contains natural resin, which contributes to its durability. There are special requirements for the material for the roof system:
- type of wood - for puffs in hanging rafters - not lower than the first, for rafter legs - the first or second, for racks and struts - the third;
- wood moisture should be no more than 2%;
- strength - when calculating the load, the weight of the roof and snow carpet is taken into account (200 kg / m² for most of the territory of Russia) and is divided by the planned number of rafters;
- resistance to biological effects and environmental influences.
The rafter for the roof is a triangular design of one section of the supporting system (it is also called a truss) and includes the following elements:
- rafter legs - inclined beams connecting the wall with the ridge;
- inclined struts - struts that rest against the rack or supporting structure of the building;
- crossbar - horizontal beams between the rafters;
- headstock - vertical racks for stretching, used with hanging rafters;
- screeds - horizontal elements that prevent the walls from spreading with truss feet.
Wall plate is laid over the walls of brick and stone houses along the perimeter of the building - subrafter timber 140–160 mm thick (to save, you can put trimming of the same section under each leg instead of a solid bar). The rafters are based on it. In wooden log cabins, the role of the Wall plate is performed by the upper crown, in frame houses - the upper trim.
Types of trusses
The angle of inclination of the roof and the design of the truss system depend on the type of roof (single-slope, double-slope, multi-slope), provided for by the project of roofing, norm of precipitation, wind loads typical for a particular area.
There are two types of truss system:
- mounted on one side of each truss foot on the external load-bearing walls and with the uprights on the internal load-bearing structures (walls or columns). If the span is less than 4.5 m, it is not necessary to install racks. In the center of the construction, the opposite truss legs converge on the ridge bar (usually with a cross section of 100 × 100 mm). This roof system facilitates the weight of the roof, working only on bending;
- trailing - they rest only with the ends of the rafter legs on the external walls of the building. In the hanging system, the truss legs are connected by tightening. Therefore, the only force transmitted to the wall is the vertical pressure of the roof. Such a system distributes the weight of the roof on the bearing walls, working on bending and compression. If the span width is less than 6 m, the ceiling beam can act as a tightening.
For large spans, truss trusses are performed, which are based on the same types of construction (on-board, hanging), but there are more connecting elements that evenly distribute the load on the support and prevent deformation of truss legs.
Layout of truss structures
Roof rafters are marked on the ground for safety and ease of use. All the necessary elements are customized to one size.
To mark the rafter legs, take a bar or round timber of small diameter and length equal to the distance between the walls of the house. In the center they fill a bar with a length equal to the distance from the level of the rafter support on the wall of the house to the ridge.
Each truss leg set between the edge of the longitudinal beam and the upper end of the perpendicular to it slat. At the top point with a pencil mark the place of cutting. Similarly, measure the tightening in the system with hanging rafters. When marking a complex truss structure with heads, struts, supports perform temporary fixation of rafters, grabbing elements with small studs.
Building and assembling rafters
Rafters are hewn on the ground. The assembly is also preferred downstairs - in any case it is more convenient. But not always assembled design can withstand the subsequent rise to the top.
Installation of truss system
First of all, extreme rafters are installed at opposite ends of the roof. Before fixing them completely, you need to check and adjust the symmetry. Therefore, when roofing is trussed, temporary rafters are attached to the roof rails or walls. The tops of the outer rafters are connected with a twine - a ridge bar will pass along the line indicated in this way. The rest of the rafters are exposed on it, more often - with a pitch of 600-1200 mm, with the use of temporary fastening. When installing roof rafters, it is necessary to leave a safe distance from them to the chimney - at least 130 mm.
The slopes of the equipped roof are measured with a twine: the top of each rafter is connected to the lower corner of the opposite, first on one side, then on the other. On each slope of the roof, the twines should cross in the middle of the roof, slightly touching each other. If the geometry is broken, the location of the rafters or their design is adjusted.
When all the rafters are measured and exposed, they are fixed to the wall or the walls of the house completely. The rafters tie the ridge beam, passing it under their peaks.
Rafters should be securely fastened to the wall plates and between themselves with the help of brackets, plates, steel corners. Power connections are performed with bolts and studs. In the manufacture of rafters with the observance of technology can be guaranteed good quality and durability of the roof.