This report is one of a series covering the long-term testing by BRE of the effectiveness of wood preservatives and the natural durability of timber. It gives the results of 20 years' exposure for those tests started since 1950, and also gives details of new trials established between 1950 and 1970. The test used is the ground contact stake test, which is the normal method used for assessing preservatives intended for use in relatively severe service conditions. 20 pages.