BS6367:1983 states that both rectangular and triangular gutters may be considered as special types of trapezoidal gutter; and can therefore be designed in the same way as trapezoidal gutters. The BS acknowledges that the fall to which a gutter is laid will be dependent upon certain considerations-the material of which the gutter is formed, and structural reasons. It suggests that gutters can be laid fl at, particularly in the form of prefabricated gutters of pressed steel and aluminium. It is benefi cial, however, if suffi cient fall is provided to prevent ponding. Where gutters are laid to fall, it has been found that the fl ow capacity of the gutters is increased. This, however, is seen as an additional margin of safety. Flow capacities of level trapezoidal gutters complying with BS569 and discharging freely.