A flat roof is a roof which is almost level in contrast to the many types of sloped roofs. The slope of a roof is properly known as its pitch and flat roofs have up to approximately 10°. Flat roofs are an ancient form mostly used in arid climates and allow the roof space to be used as a living space or a living roof. Flat roofs, or "low-slope" roofs, are also commonly found on commercial buildings throughout the world. The U.S.-based National Roofing Contractors Association defines a low-slope roof as having a slope of 3 in 12 (1:4) or less. Flat roofs exist all over the world, and each area has its own tradition or preference for materials used. In warmer climates, where there is less rainfall and freezing is unlikely to occur, many flat roofs are simply built of masonry or concrete and this is good at keeping out the heat of the sun and cheap and easy to build where timber is not readily available. In areas where the roof could become saturated by rain and 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. Flat roofs are characteristic of the Egyptian, Persian, and Arabian styles of architecture. Around the world, many modern commercial buildings have flat roofs. The roofs are usually clad with a deeper profile roof sheet (usually 40mm deep or greater). This gives the roof sheet very high water carrying capacity and allows the roof sheets to be more than 100 metres long in some cases. The pitch of this type of roof is usually between 1 and 3 degrees depending upon sheet length.
Construction methods Any sheet of material used to cover a flat or low-pitched roof is usually 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, though gutter systems are of advantage in keeping both walls and foundations dry. Gutters on smaller roofs often lead water directly onto the ground, or better, into a specially made soakaway. Gutters on larger roofs usually lead water into the rainwater drainage system of any built up area. Occasionally, however, flat roofs are designed to collect water in a pool, usually for aesthetic purposes, or for rainwater buffering. Traditionally most flat roofs in the western world make use of felt paper applied over roof decking to keep a building watertight. The felt paper is in turn covered with a flood coat of bitumen (asphalt or tar) and then gravel to keep the sun's heat, ultraviolet light and weather off it and helps protect it from cracking or blistering and degradation. Roof decking is usually of plywood, chipboard or oriented strand board (OSB, also known as Sterling board) of around 18mm thickness, steel or concrete. The mopping of bitumen is applied in two or more coats (usually three or four) as a hot liquid, heated in a kettle. A flooded coat of bitumen is applied over the felts and gravel is embedded in the hot bitumen.
A main reason for failure of these traditional roofs is ignorance or lack of maintenance. The gravel coating protects the tar underneath from breaking down under UV rays from the sun. The gravel can shift from wind, heavy rainfall, or people walking on the roof. This exposes the tar to weather and sun. UV rays lead to material failures such as cracking and blistering, and eventually water gets in. Roofing felts are usually a 'paper' or fiber material impregnated in bitumen. As gravel cannot protect tarpaper surfaces where they rise vertically from the roof such as on parapet walls or upstands, the felts are usually coated with bitumen and protected by sheet metal flashings called gravel stops. The gravel stop terminates the roofing, preventing water from running underneath the roofing and preventing the gravel surfacing from washing off in heavy rains. In some microclimates or shaded areas felt roofs can last well in relation to the cost of materials purchase and cost of laying them. The cost of membranes such as EPDM rubber has come down over recent years. If a leak does occur on a flat roof, damage often goes unnoticed for considerable time as water penetrates and soaks the decking and any insulation and/or structure beneath. This can lead to expensive damage from the rot which often develops and if left can weaken the roof structure. There are health risks to people and animals breathing the mold spores: the severity of this health risk remains a debated point. While the insulation is wet, the "R" value is essentially destroyed. If dealing with an organic insulation, the most common solution is removing and replacing the damaged area. If the problem is detected early enough, the insulation may be saved by repairing the leak, but if it has progressed to creating a sunken area, it may be too late. One problem with maintaining flat roofs is that if water does penetrate the barrier covering, it can travel a long way before causing visible damage or leaking into a building where it can be seen. Thus, it is not easy to find the source of the leak in order to repair it. Once underlying roof decking is soaked, it often sags, creating more room for water to accumulate and further worsening the problem. Another common reason for failure of flat roofs is lack of drain maintenance where gravel, leaves and debris block water outlets (be they spigots, drains, downpipes or gutters). This causes a pressure head of water (the deeper the water, the greater the pressure) which can force more water into the smallest hole or crack. In colder climates, puddling water can freeze, breaking up the roof surface as the ice expands. It is therefore important to maintain your flat roof to avoid excessive repair. An important consideration in tarred flat roof quality is knowing that the common term 'tar' applies to rather different products: tar or pitch (which is derived from wood resins), coal tar, asphalt and bitumen. Some of these products appear to have been interchanged in their use and are sometimes used inappropriately, as each has different characteristics, for example whether or not the product can soak into wood, its anti-fungal properties and its reaction to exposure to sun, weather, and varying temperatures.
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