A roof is the covering on the uppermost part of a building.A roof protects the building and its contents from the results of weather. Structures that need roofs range all the way from a letter box to a cathedral or stadium, dwellings being the most common.In most states a roof protects essentially against rain. Relying on the character of the building, the roof might also defend against heat, daylight, cold and wind.

Other kinds of structure, for instance, a garden conservatory, might use roofing that protects against cold, wind and rain but admits light. A terrace might be roofed with material that gives protection against daylight but admits the other elements. The features of a roof are dependent on the object of the building that it covers, the available roofing materials and the local practices of construction and broader ideas of architectural design and practice and can be ruled by local or countrywide legislation.

The supporting structure of a roof customarily comprises beams that are long and of robust, reasonably firm material such as timber, and since the mid 19th century, solid iron or steel. In states that use bamboo at length, the suppleness of the material causes a specific curving line to the roof, characteristic of Oriental design. Timber adapts well to a large range of roof shapes. The timber structure can fulfil a classy as well as practical function, when left exposed to view.

Stone lintels have been employed to support roofs since stone age times, but can't bridge huge distances. The stone arch came into in depth use in the traditional Roman period and in variation forms could be used to span spaces up to 140 feet across.The stone arch or vault, without or with ribs, ruled the roof structures of major architectural works for roughly two thousand years, only giving way to iron beams with the Business Revolution and the creation of such buildings as Paxton's Crystal Palace, finished 1851.With repeated enhancements in steel girders, these became the important structural support for massive roofs, and at last for standard homes also. Another sort of girder is the fortified concrete beam, in which metal rods are encased in concrete, giving it bigger strength under tension.