The BRE ground contact field trials for the determination of the biological natural durability of different timber species have recently been reviewed. The data obtained from these trials have been used to appraise the different ways in which natural durability may be expressed. It is concluded that the use of the mean as a method of assigning the timber to one of five durability classes may be acceptable where general recommendations are required, for instance in association with the use of wood preservatives to enhance durability, but that the median provides a better indication in this context. Where a specifier requires more detailed information on timber performance, the mean (or median) is inappropriate and a system of percentile values would be more informative.

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