Please note that this is a scanned copy of a paper originally published in 1998, so the text is not as clear as in documents created as pdf files. The average number of days each year during which thunder storms may occur across the UK is between 5 and 20. This figure can double over an 11-year period due to the sunspot cycle. The number of lightning strikes to a square kilometre of land each year in England and Wales ranges from 0.1 to 0.6 depending on geographical location. A lightning protection system (LPS) for any structure must be considered to be a sensible precaution, but its installation is particularly important where there is a clear risk to life, the structure or its contents are irreplaceable, or essential services could be affected. This Digest gives guidance on calculating the risk of a building being struck by lightning. It describes the principles of designing LPS for new and existing buildings. However, as stated in BS 6651, even where an LPS is provided, it can never be completely effective in eliminating the risk of damage to a structure or its contents. 8 pages.