There are a number of ways to design and construct pitched roofs using timber. Traditionally, builders constructed them on site, cutting and fitting each piece to its neighbours. Following the Second World War, various forms of prefabricated truss were developed, with the intermediate rafters and purlins cut and fitted on site.
Currently, the highly Design > Engineer;ed and slender trussed rafter, where the rafters and ceiling joists are triangulated to form a lightweight prefabricated truss, is the most widely used form of domestic roof construction.
This Wood Information Sheet (WIS) covers the general principles of pitched roof design only. It does not cover the design of hatches, chimney openings or the additional support that is required for water tanks or other added loads.
Structurally, the most efficient solution is to use ceiling joists to tie the rafter feet together, thereby relieving the walls of horizontal thrust.
By adding a collar to the roof construction, it is possible to relieve the walls of thrust.
An alternative to the ventilated cold roof is the warm roof, where all of the insulating material is placed above the roof structure.