Provides recommendations and guidance for the preservative treatment of timber to provide protection against biodeterioration in certain specified end-use situations in the United Kingdom. It also recommends levels of penetration and retention of preservative, from the classes defined in BS EN 351-1, for different timber components and takes into account the natural durability and treatability of the timber employed, the biological use class and the desired service life. It also provides guidance on establishing whether a specified level of treatment has been achieved.