Flat roofs are not actually flat but are built on a slight incline to prevent ponding (accumulation of water). Materials used in flat roofing need to be water resistant and watertight. Flat roofs require regular inspection because damage is not visible until there is leak. The five main types of flat roofs are roll asphalt, single-ply, multiple ply, modified bitumen and flat-seamed metal.
Roll asphalt has been used in flat roofing for more than 100 years. This type of roof consists of layers of asphalt-coated fibreglass felt, tar and water resistant materials, which are kept in place by nails or asphalt cement. The topmost layer is covered with gravel or crushed rock, which serves as a counterweight and provides protection against ultraviolet rays from the sun. It is the cheapest kind of flat roof, but it generally doesn't last more than 10 years.
Single-ply flat roofs are made from a single layer of materials that have properties similar to rubber or plastic. These materials offer a less expensive, but sturdier and more energy efficient, alternative to roll asphalt flat roofs. The most commonly used modern materials in single-ply flat roofs include ethylene propylene diene monomer (EPDM), polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO). These materials are preferred because they are mould resistant and generally maintenance-free. On average, single-ply roofs last 10 to 12 years, although TPO roofs may last as long as 30 years.
Multiple-ply roofs, also called built-up roofs, are made of a number of layers of fibreglass felt packed between layers of water-resistant asphalt. In comparison to single-ply roofs, multiple-ply roofs are thoroughly reinforced, which means that they last longer and are less prone to damage. The average lifespan of a multiple-ply roof is between 10 and 30 years.
Some multiple-ply roofs make use of a modified bitumen membrane, which is made from a mixture of polymers and asphalt. The modified bitumen membrane has the flexibility of polymers and the water-resistant characteristics of asphalt. The topmost layer is covered with a reflective material, gravel or mineral blends. Modified bitumen flat roofs last between 10 and 20 years.
Flat-seamed metal roofs are more expensive than other types of flat roofs because they are made from metals, such as copper and stainless steel. Metal roofs are lighter than other types of flat roofs, which make them an excellent choice for buildings in areas with high seismic activity. Depending on weather conditions, metal roofs last between 20 and 50 years.