Rain gauges do not take into account the driving effect of wind current with rain. Allowance for this is not required when designing drainage for horizontal surfaces or other surfaces protected from the wind, but should be considered where sloping or vertical surfaces occur that are freely exposed to the wind. Relevant information is sparse but at times of peak rainfall an angle of descent of wind-driven rain of one unit horizontal for each two units of descent should be allowed. Allowance for this may be made by adding to the run-off calculated from the plan area of that part of the roof draining to the gutter, an additional run-off from an area equal to half the maximum elevation area of that part of the roof. Use
Walls and other vertical surfaces unprotected by nearby structures of similar height will produce rainwater run-off. Where a wall drains onto a lower roof, provision should be made in the drainage of the latter for the additional fl ow. If only one wall is involved, allow for
a catchment of half the actual vertical area at the full design rate of rainfall. No allowance should be made for absorption. If an angle or bay is formed by two or more walls, the direction from which these walls, considered as one unit, present the greatest projected area in elevation should be found and half that area taken at the design rate of rainfall to determine the run-off.