Wood is often used to construct two-part ventilated flat roofs. Wooden roofs, both those with external drainage and those with internal drainage, they are most often constructed with the use of purlin-claw trusses or lattice trusses.
Claw-type purlin trusses
In flat roofs 8-12 m, a purlin-tick-type structure with two stud walls is used (this is the standard solution, also used in steep roofs). The purlins are arranged along the roof slopes, approximately in the middle of the span and supported by columns. The columns stand on foundations arranged perpendicular to the direction of the structural ceiling span. Full trusses, placed every third or fourth pair of rafters, have tongs covering the poles and rafters. In flat roofs with a smaller span – 8 – 9 m it is enough to support the rafters in the ridge with one ridge purlin supported by columns. Purlin, the pole and rafters are clamped with tongs.
In flat roofs with a very low slope, for design reasons, forceps only connect the poles. Swords are used to stiffen the roof truss in the longitudinal direction – diagonal elements approx 1 m, fixed to columns and purlins at an angle of 45 °.
Purlin construction – the forceps transfer most of the loads through the stud walls to the attic ceiling, and only part of the loads on external walls. This has a positive effect on the strength and stability of the roof.
Lattice trusses
It is the simplest and cheapest way of making the supporting structure of the ceiling and at the same time the supporting layer of the upper part of the flat roof. Lattice trusses (trusses, girders) they look like connected triangles. They consist of upper and lower belts, posts and diagonals. Structural elements are made of slender ones – thin and long – boards, the most common cross-section 38 x 89 mm i 38 x 140 mm. They are connected with two-sided nailed overlays made of waterproof plywood 12 mm or on perforated steel plates 2 or 2,5 mm – usually they are nail plates. You can also use a minimum thickness of OSB 10 mm. For top and bottom straps (elements determining the durability of the structure) it is good to use wood with greater bending strength (33 MPa) class C35 and on diagonals and posts (secondary elements) – grade C33.
Lattice trusses are usually produced in factories and assembled on the construction site. You can also buy them ready-made, and also prepare according to an individual project. They are light, assembly is very simple, the execution takes from several to several hours.
The trusses are based only on the outer walls. This solution allows for any arrangement of the rooms, and, if necessary, to change their functions.