The annual mean driving-rain index for Great Britain is presented on 29 sectional maps at a scale of 1:625000. To meet the urgent need for a more precise determination of the driving-rain index at individual localities, rules are proposed for adjusting the 'map value' to allow for local topographic variation and roughness of the terrain. These are intended to give local values of the index which correspond to the behaviour in practice of cavity walls filled with insulating foam.

The 'driving-rain index' was devised to help in assessing the degree of exposure of buildings to driving rain. The map of annual mean driving-rain index used up to now came into use in 1962, at first in Building Research Station Digest 23, followed in 1971 by the Building Research Establishment Digest 127 'An index of exposure to driving rain’ with a revised text. In conjunction with rules for modifying the value read from the map to allow for peculiarities of local exposure or shelter, it has been found useful for comparing one area of the United Kingdom with another.

Increases in the cost of fuel led to a dramatic rise in the use of cavity-fill, the most popular fill material being chemical foam. Where this is done correctly with a suitable material, and the design, material and workmanship of the wall are good, the result should be satisfactory. However, with the foam material the risk of water crossing the cavity is increased and experience has now shown that it is necessary to have a more precise estimate of exposure to driving rain than is possible with the small-scale map in BRE Digest 127, with corrections to allow for local conditions, if a rational decision is to be made on whether cavity-fill is to be permitted. Because of this, a new map has been prepared to a larger scale, sufficiently large to enable the 'map value' of the driving-rain index to be read with confidence.

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