Any sheet of material used to cover a flat or low-pitched roof is tyically known as a membrane and the primary purpose of these membranes is to waterproof the roof area. Materials that cover flat roofs typically allow the water to run off from a slight inclination or camber into a gutter system. Water from some flat roofs such as on garden sheds sometimes flows freely off the edge of a roof, although gutter systems are of advantage in keeping both walls and foundations dry. Occasionally, flat roofs are designed to collect water in a pool, usually for aesthetic purposes, or for rainwater buffering.
Flat roofs exist all over the world and each area has its own tradition or preference for materials used. In warmer climates, where rainfall is less and freezing is unlikely to occur, many flat roofs are simply built of masonry or concrete. In the right climate, this can be effective at insulating the sun’s heat, and this technique is cheap and easy to build where timber is not readily available. In areas where the roof could become saturated by rain and possibly leak, or where water soaked into the brickwork could freeze to ice and thus lead to ‘blowing’ (breaking up of the mortar/brickwork/concrete by the expansion of ice as it forms) these roofs are not suitable.
Gutters on smaller roofs often lead water directly onto the ground, or into a “soak away.” Gutters on larger roofs usually lead water into the rainwater drainage system of any built up area.