Modern timber roofs are mostly framed with pairs of common rafters or prefabricated wooden trusses fastened together with truss connector plates. Timber framed and historic buildings may be framed with principal rafters or timber roof trusses. Roofs are also designated as warm or cold roof depending on how they are designed and built with regard to thermal building insulation and ventilation. The steepness or roof pitch of a sloped roof is determined primarily by the roof covering material and aesthetic design. Flat roofs actually slope up to approximately ten degrees to shed water. Flat roofs on houses are primarily found in arid regions.
In high wind areas, such as where a cyclone or hurricane may make landfall, the main engineering consideration is to hold the roof down during severe storms. Every component of the roof, as of course the rest of the structure, has to withstand the uplift forces of high wind speeds. This is accomplished by using metal ties fastened to each rafter or truss. This is not normally a problem in areas not prone to high wind or extreme weather conditions.
A simple rafter roof consists of rafters that the rafter foot rest on horizontal wall plates on top of each wall. The top ends of the rafters often meet at a ridge beam, but may butt directly to another rafter to form a pair of rafters called a couple. Depending on the roof covering material, either horizontal laths, battens, or purlins are fixed to the rafters; or boards, plywood, or oriented strand board form the roof deck (also called the sheeting or sheathing) to support the roof covering. Heavier under purlins or purlin plates are used to support longer rafter spans.
Roof framing must be designed to hold up a structural load including what is called dead load, its own weight and the weight of the roof covering, and additional loading called the environmental load such as snow and wind. Flat roofs may also need to be designed for live loads if people can walk on them.
