The most common construction timbers used in the UK are of low natural durability and, generally, resistant to preservative treatment. These include species of spruce, hemlock and fir. However, their characteristically limited and non-uniform uptake of preservatives may still confer sufficient protection to give satisfactory durability performance in terms of biological resistance. This paper describes the results of a study investigating the protective efficacy of treatments defined according to the approach in European Standard EN 351-1 when penetration and retention values are used in timbers showing non- uniform treatment characteristics. The data show that variation between batches of samples is high for preservative uptake, retention and penetration. The biological data have given indications that this leads to differences in performance effectiveness. However, the bioassay method developed does not in its present form, sufficiently discriminate between treatment/species combinations.

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