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Roof construction
The construction of a roof is determined by its method of support and how the underneath space is bridged and whether or not the roof is pitched. There are two parts to a roof, its supporting structure and its outer skin, or uppermost weatherproof layer. In a minority of buildings, the outer layer is also a self-supporting structure. The characteristics of a roof are dependent upon the purpose of the building that it covers, the available roofing materials, the local traditions and wider concepts of architectural design and practice. The pitch is the angle at which the roof rises from its lowest to highest point. There is a required minimum slope of 6 inches in 12 inches, a pitch of 30 degrees, is proportional to the amount of precipitation. Houses in areas of low rainfall frequently have roofs of low pitch, while those in areas of high rainfall have steep roofs. Roof pitch is partly dependent upon stylistic factors, but has more to do with practicalities.
The main engineering consideration is to hold the roof down, keep out water and withstand the uplift forces of high wind speeds.There are a number of structural systems employed to facilitate this, including the use of rafters, tie beams and joists serve to transmit the weight of the roof to the walls of the building, wall-plates set at the top of the wall, hammer-beams, which spread the weight down the wall to create an equilibrium between outward and upward thrust, king posts which transfer the weight of the roof ridge and various types of trusses. The large area of a roof repels a lot of water, which must be directed in some suitable way, so that it does not cause damage to the building. If the water expelled from the roof, runs down the walls, it may seep into the mortar or through panels. If it lies around the foundation, it may cause seepage to the interior, rising damp or dry rot. For this reason, most buildings have a system in place to protect the walls of a building. Gutters and drainpipes are use to remove the water from the vicinity of the building. In many areas, rainwater collected from the roof is stored for domestic use.
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