The biological durability of wood is a relative and unitless size, and therefore wood species and wood-based materials are assigned to durability classes (DC). In Europe, such classes are based on a variety of measures, which can be absolute or relative to a non-durable reference species. The aim of efforts to develop and standardise methods for determining biological durability should be to ensure transparency and comparability and to create test scenarios adapted to real exposure conditions. Whether this is achieved by the current portfolio of European standards may be questioned. In this paper, selected adjustment screws are examined and re-assessed through analysis of published data from comparative durability studies. In particular, the use of absolute and relative measures (i.e. x-values) for durability classification was addressed and the following conclusions were drawn from the analysis. The use of relative measures with non-durable wood species as reference materials, i.e. x-values, has potential to enhance the transparency, comparability and thus the acceptance of durability data and the underlying methods. The concern that the use of relative values for the assignment to durability classes would result in increased variation between testing laboratories was not confirmed. If the initial durability classification in EN 350, which was based on graveyard test data and followed the scale of EN 252, is considered to be authoritative, this assignment of x-values to durability classes for other test methods should also be applied. The limited data available supports this option as it resulted in a durability classification that was as similar as possible.

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