Flat roofs construction

Flat roofs construction are usually covered with two or three layers of roofing felt. The top layer has embedded on it a covering of stone chippings, which may be added either at manufacture or during installation. The chippings reflect ultraviolet rays, which could damage the felt. Usually the first layer of the three layers of felt is laid lengthwise in the direction of the slope. The second layer is laid across, and the final one in the direction of the first. Some experts say it is better for all three layers to run lengthwise, provided the joins do not coincide. To ensure that they do not, the flat roofs construction begins laying at one edge and works across to the other, where the final piece is cut to width. The second layer begins at this edge with a full-width roll until the first edge is reached, where the last piece will once again have to be cut to width. The third layer begins at the initial first edge, so the joins in this are kept apart from those of the first and second. In two layer coverings, the first usually goes across the slope of the roof, and the other one lengthwise. Alternatively, both can be laid lengthwise, using the method already described to ensure that the joins do not coincide.


Flat roof constructionThe felt is laid on some form of decking, which can be made up of either planks (tongued and grooved or square-edged), exterior-grade plywood or chipboard, compressed strawboard, or a concrete screed on top of some other form of board. The decking is supported on rafters at centres between 400 and 450 mm. Lengths of tapering small section timber, called firring, are fixed to the top of the rafters to ensure the slope needed to allow the roof to shed rainwater. Insulation and a vapour barrier are incorporated in modern constructions. For general construction of a flat roofs construction, the rafters may be fixed in one of several ways. They can rest on, and be skew nailed to, timber wall plates on the top of the external walls. Where the rafters meet a parapet wall, or where the walls of a lean to join up with the house wall, the rafters can be fixed to a wall plate resting on a brickwork corbelled out of the main wall. They can be supported on a joist hanger, or let into a socket formed by the omission of bricks here and there. On extra-lightweight structures, such as garages and conservatories, they may merely be supported by wall plates bolted to the wall. As with pitched roofs, the ceiling is fixed to the underside of the joists. Plat roofs are often used on single-storey buildings. Reaching them is not much of a problem, and does not require such equipment as access to a pitched roof. For instance, you might climb on top of a single storey extension from a large stepladder.


flat_roofNevertheless, where appropriate, all the safety precautions outlined at the start of this chapter should be observed strictly. The flat roof of, say, a modern town house may not seem as daunting as the slate covered pitched roof of, for example, a turn of the century building, but do not be lulled into a false sense of complacency; a fall from a flat roof can be just as serious. The equipment for reaching the flat roofs construction of a house is the same as for a pitched roof: extending ladder or access tower. Roof ladders are not required, however, and complex scaffolding is seldom necessary, but you will need sturdy boards across the roof.

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