The slope of the roof rafters depends on the type of covering material used. Coatings made of less airtight materials must have a greater slope. According to PN-89B-0226 the following slopes of the roof are recommended:
- for a tile covering 22-51 °,
- for a sheet covering 3-51 °,
- for a roofing felt 3-27 °.
The types of roof trusses can be divided into four groups:
a) rafter-beam and rafter-collar beam trusses,
b) purlin trusses,
c) suspension trusses,
d) strut-strut trusses.
Rafter-beam roof truss: a) axonometric view, b) detail of the connection between the rafters and the ceiling beam, c) detail of the rafters connection in the ridge; 1 – ceiling beam, 2 – krokiew, 3 – pin.
For a small wall spacing - a small span of the covering, ok. 6 m - beam-rafter systems are used. The load-bearing structure consists of triangular trusses composed of two rafters and a beam. For larger wall spacings, do ok. 9 0 m, rafter-beam trusses with unsupported collar beam are used. The hose is placed slightly above half the length of the rafters.
Rafter-collar beam truss: a) view of the roof truss, b) cross-section, c) longitudinal section; 1 — krokiew, 2 - mayfly, 3 - windchest, 4 - bracing board, 5 - ceiling beam.
The figure below shows a rafter-beam truss with a collar beam supported by stools (poles). The moat rests on a cap placed on stools of stools placed in the main trusses. Additionally, the cap is connected to the stools by means of swords, thanks to which the roof truss is stiffened in the longitudinal direction. The overhang of the collar beam beyond the stool should not exceed approx. 80 cm.
A gable roof: a) roof diagram, b) cross-section, c) longitudinal section, d) detail of the rafter support on the lower purlin, if the rafter spacing is different from the spacing. belek, e) detail of the shoulder groove on the upper purlin; 1 — krokiew, 2 - mayfly, 3 - cap, 4 - pole (stool), 5 - ceiling beam, 6 - bottom purlin, 7 - sword.