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A flat roof is a type of covering for a building. In contrast to the more sloped form of roof, a flat roof is horizontal or nearly horizontal.
The slope of a roof is properly known as its pitch: Roofs which are nearly flat are known as low-pitch roofs (as the angle of slope increases, so the pitch is said to increase).
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 flat roofs though 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 climes where rainfall is less 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.
Traditionally most flat roofs in the western world make use of tar or more usually tar paper applied over roof decking to keep a building watertight. The tar or tarpaper is in turn covered in gravel to keep the sun's heat, UV rays and weather off it and helps protect it from cracking or blistering and degradation. Roof decking is usually of plywood, chipboard or OSB boards (OSB = Oriented Strand Board, also known as Sterling board) of around 18mm thickness. The tar coating is applied in one or more coats as a hot liquid, heated in a tar kettle, or as sheets of tar impregnated 'paper' glued down and sealed together at joints by hot tar - once it has cooled, the gravel is spread evenly over it.
A main reason for failure of these traditional roofs is ignorance or lack of maintenance where people or events cause the gravel to be moved or removed from the tar or tarpaper waterproofing, thus exposing it to weather and sun whereupon cracking and blistering occurs over time and eventually water gets in.
Tarpaper is usually a 'paper' or fibre material soaked or impregnated in tar. As gravel cannot protect tarpaper surfaces where they rise vertically from the roof such as on parapet walls or upstands, tarpaper variants are produced with fine gravel applied to the hot tar during the process of manufacture such that a permanent layer of gravel is stuck to it in order to give it ongoing protection.
Tarpapers vary in quality as does the workmanship and training of those applying it to produce more or less successful results. In some micro climates or shaded areas these rather 'basic' tarpaper roofs can last well in relation to the cost of materials purchase and cost of laying them, however the cost of modern membranes such as EPDM has come down over recent years to make them more and more affordable. There are now more firms supplying modern alternatives and there is generally a growing awareness of their performance and availability.
If a leak does occur on a flat roof then damage often goes unnoticed for long periods of time during which water penetrates and soaks the decking and any insulation and/or structure beneath it. This can lead to expensive damage from the rot which often develops and if left can weaken the roof structure. There are also various health risks to people and animals breathing the mould spores: the severity of this health risk remains a debated point.
One problem with maintaining flat roofs is that if water does penetrate the barrier covering (be it traditional or a modern membrane) 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 see where it is leaking in order to repair it. Once underlying roof decking is soaked it often sags causing a further collection or puddling of water in that area and the problem worsens.
Another common reason for failure of flat roofs is lack of maintenance of drains whereby gravel, leaves and debris block water outlets (be they spiggots, drains, downpipes or gutters), thus water cannot drain away and builds up. This causes a pressure head of water (the deeper the water the greater the pressure) which can 'push' more water into the smallest hole or crack. In colder climates, puddling water can freeze into ice which breaks up the roof surface as it expands whilst it freezes.
An important consideration in tarred flat roof quality is differentiating what many call simply 'tar' into 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 their ability or not soak into wood/boards, their anti-funghal properties and their reaction to exposure to sun and weather and varying temperatures.
Modern flat roofs can use single large factory made sheets (e.g. EPDM synthetic rubber) and these are usually of excellent quality, though unfortunately one-piece membranes are seldom seen used in practice. Rather, strips of EPDM, PVC, tar paper etc. which are widely available in rolls (typically of 900 mm or 1200mm width) are bonded together in either hot or cold seaming processes on the roof during the fitting process where labour skill and training play a large part in determining the quality of roof protection attained. Reasons for not using excellent one-piece membranes include practicality and cost-saving:- on all but the smallest of roofs it can be difficult to lift a huge and heavy membrane to the roof (a crane or lift is required) and if there is any wind at all it can be difficult to control and bond the membrane smoothly and properly to the roof.
Detailing of these various systems also plays a part in success or failure: In some systems ready-made details (such as internal and external corners, through-roof pipe flashings, cable or rooflight flashings etc.) are available from the membrane manufacturer and can be expediently and well bonded to the main sheet, whereas with materials such as tar papers this is usually not the case - a fitter has to make these shapes up on-site or on the roof and success depends largely on their levels of skill, enthusiasm and training - results can vary hugely.
Metals are also used for flat roofs, these can be made from lead (welded or folded-seamed), tin (folded, soldered or folded-seamed) or copper. These are often expensive options and vulnerable to being stolen and sold as scrap metal.
Flat roofs tend to be sensitive to human traffic. Anything which produces a crack or puncture in the waterproofing membrane can quite readily lead to leaks. Flat roofs can fail, for example; when subsequent work is carried out on the roof, when new through-roof service pipes/cables are installed or when plant such as Air Conditioning Units are installed. A good roofer should be called to make sure the roof is left properly watertight before it is left. In trafficked areas, proper advisory/warning signs should be put up and walkways of rubber matting, wooden or plastic duck-boarding etc. should be installed to protect the roof membrane. On some membranes even stone or concrete paving can be fitted. For one-off works, old carpet or smooth wooden planks for workers to walk or stand on will usually provide reasonable protection.
Traditionally the smelly, hot, physically demanding and sometimes dangerous work of tarring flat roofs has often meant that uneducated fitters of doubtful reputation have done work to a poor standard: This together with a lack of regular inspection and maintenance has meant that flat roofs have a poor reputation and there is an unwillingness to retain or to build them which is unfortunate given the potential usefulness of flat areas, the more so with the excellent performance of modern membranes, many of which come with long warranties and which provide an excellent roof covering.
Modernist architecture often viewed the flat roof as a living area. Le Corbusier's theoretical works, particularly Vers une Architecture, and the influential Villa Savoye and Unité d'Habitation prominently feature rooftop terraces. That said, Villa Savoye's roof commenced leaking almost immediately after the Savoye family moved in. Le Corbusier only narrowly avoided a lawsuit from the family due to the fact they had to flee the country as France succumbed to the German Army in WWII.

From Wikipedia.