For most of the functional requirements of buildings - loading, heat loss, etc - the basic design data can be expressed in quantitative terms. In considering the exposure of a building to driving rain however, it was hitherto necessary to use rather indefinite terms such as 'sheltered', 'moderate' or 'severe'.

This digest describes the development of a numerical driving-rain index and suggests how it can be used to give these terms a more quantitative basis. Maps are presented, showing how the severity of driving-rain conditions varies over Britain as a whole; variations due to local topography and those occurring over the face of a building are discussed briefly.

Digest no.127 "An index of exposure to driving rain", was based on work done at BRS during the 1960s. A BRE report "Driving-rain index: annual mean driving-rain index in the United Kingdom with proposed revised rules for assessing local exposure" by Lacy was later published by HMSO in 1976.

 The BRE publications were superseded by the British Standard Draft for Development DD93: 1984 "Methods for assessing exposure to wind-driven rain", and then by BS 8104: 1992 "Code of practice for assessing exposure of walls to wind-driven rain".