Roof Systems
*photo used courtesy of GAF
There are two parts to a roof, its supporting structure and its outer skin, or uppermost weatherproof layer. In a minority of buildings, the outer layer is also a self-supporting structure.
The roof structure is generally supported upon walls, although some building styles, for example, geodesic and A-frame, blur the distinction between wall and roof. The supporting structure of a roof usually comprises beams that are long and of strong, fairly rigid material such as timber, and since the mid 19th century, cast iron or steel. Timber lends itself to a great variety of roof shapes. Moreover, because timber can be worked in a variety of ways, the timber structure can fulfill an aesthetic as well as practical function, when left exposed to view.