A roof is the uppermost, covering, part of a building. The purpose of the roof is to protect both the building itself and its living or material contents from the effects of weather. The types of structures that require roofs range from the size of a letter box to that of a cathedral or stadium, being the most numerous and the most socially significant.The characteristics of a roof are dependent upon the purpose of the building that it covers, the available roofing materials and the local traditions of construction and wider concepts of architectural design and practice and may also be governed by local or national .In most countries a roof protects primarily against rain. Depending upon the nature of the building, the roof may also protect against heat, against sunlight, against cold and against wind. If the roof is the covering for a house, then all these protective functions are called into play. Other types of structure, for example, a garden conservatory, might utilise roofing that protects against cold, wind and rain but admits light. A verandah may be roofed with material that protects against sunlight but admits the other elementsThis section briefly summarises the elements of roof design. See following sections for greater detail.The elements in the design of a roof are :-its materialits constructionits functionThe material of a roof may range from banana leaves, wheaten straw or seagrass to lamininated glass, aluminium sheeting and precast concrete. In many parts of the world ceramic tiles have been the predominant roofing material for centuries.The construction of a roof is determined by its method of support and how the underneath space is bridged and whether or not the roof is pitched. The pitch is the angle at which the roof rises from its lowest to highest point. Most domestic architecture, except in very dry regions, has roofs which are sloped, or pitched. The pitch is partly dependent upon stylistic factors, but has more to do with practicalities. Some types of roofing, for example thatch, require a steep pitch in order to be waterproof and durable. Other types of roofing, for example pantiles, are unstable on a steeply pitched roof but provide excellent weather protection at a relatively low angle. In regions where there is little rain, an almost flat roof with a slight run-off provides adequate protection against an occasional downpour.The function of a roof includes its durability and how well it serves of its required purposes. The durability of a roof is a matter of concern in both material and construction, because in general the roof is the least accessible part of a building for purposes of repair and renewal, while its damge or destruction can have serious effects. In terms of serving the required purpose, different roofing materials and constructions have very different properties, and if possible, these should be taken into account in designing and building a roof to suit prevalent local weather conditions.In general, there are two essential parts to a roof, its supporting structure and its outer skin, or uppermost weatherproof layer. In a very small 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 , 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. In countries that use extensively, the flexibility of the material causes a distinctive curving line to the roof, characteristic of Oriental architecture. 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 fulfil an aesthetic as well as practical function, when left exposed to view.Stone lintels have been used to support roofs since prehistoric times, but cannot bridge large distances. The stone arch came into extensive use in the Ancient Roman period and in variant forms could be used to span spaces up to 140 feet across. The stone arch or vault, with or without ribs, dominated the roof structures of major architectural works for about 2,000 years, only giving way to iron beams with the and the designing of such buildings as Paxton's Crystal Palace, completed 1851.With continual improvements in steel girders, these became the major structural support for large roofs, and eventually for ordinary houses as well. Another form of girder is the reinforced concrete beam, in which metal rods are encased in concrete, giving it greater strength under compression.