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. 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 elements.

The types of structures that require roofs range from the size of a letter box to that of a cathedral or stadium, dwellings 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 legislation.