Performance of flat roof outlets

HR Wallingford was commissioned by the Construction Sponsorship Directorate of the Department of the Environment (DOE) to carry out a laboratory study on the performance of grated and plain outlets used for drainage of flat roofs. The study involved the testing of four different conventional outlets and of four siphonic systems. Four main different types of outlet were identified based on the study of shapes and sizes of the outlets, gratings and leafguards and of their relative positions. Guidelines were produced to determine the effective diameter of an outlet for each of these types. Design equations were developed for weir-type flow and orifice-type flow; the effect of gratings or leafguards on the capacity of the outlets tested was also determined.