It is one of the basic requirements of flat roofs. The rain water should be drained off quickly so as to avoid leakage of roof. In order to achieve this satisfactorily watertight roof surface, adequate slope and drain outlets have to be provided.
The slopes in flat roofs vary from 1 in 20 to 1 in 40. The rain water outlets should be evenly spaced round the building some possible arrangements of drain outlets are shown in Fig. 25.13.
Figure 25.13 Different arrangements of drain-outlets
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