For roof drainage calculations it is usual to assume a rate of rainfall for design purposes of 75 mm per hour (1.25 mm/min) for the British Isles. Regional differences are not signifi cant except in relation to total rainfall and can be ignored for this purpose. The 75 mm/h rate is exceeded on rare occasions and such severe storms are of comparatively short duration with intensities of 150 mm/h or more. Where ponding or overfl owing cannot be tolerated (except during those very rare storms for which design is impracticable) a design rate of rainfall of up to 150 mm may be used. Such increases in the design rates are advisable for buildings where overfl ow inside would cause serious damage or inconvenience, e.g. museums, power stations and other industrial buildings, particularly if valley gutters are incorporated. An indication of the frequency of various rainfall intensities is as follows:
75 mm/hour may occur for: 5 minutes once in four years 9 minutes once in ten years 20 minutes once in fi fty years 100 mm/hour may occur for: 3 minutes once in fi ve years 4 1/2 minutes once in ten years 6 1/2 minutes once in twenty years
Note: 1 mm on 1 m2=1 litre/hour (÷60 for 1/min) For intensities other than 75 mm/h, substitute 1.25×design intensity (mm/hour)÷75
The following information is taken from BS.6367:1983 Code of Practice for drainage of roofs and paved areas.