The performance of wood-based products under conditions that enable biodegradation are of concern to all who envisage sustainable use of forest based products. This concern is also the basis of classification of wood species on the basis of their biological durability. Engineers are looking for options to transfer such classification into practical service life values for constructions and related service life prediction, but they should be able to process also the variability and reliability of the intrinsic biological durability of the material. Hence this paper mainly focuses on an improved statistical approach of biological durability of wood related to outdoor uses in out of ground contact situations. Starting with Weibull distribution functions of mass loss data obtained from worst case laboratory Basidiomycetes testing some parameters are proposed. Clearly the median mass loss value recorded for the most degrading test fungus is a suitable factor to classify a sample of wood with regard to the resistance to decay, but other quantiles like those for 10% or 90% of the observations are useful to indicate variability in service life that can be expected. Furthermore the use of percentages of the wood material that can be attributed to different durability classes allows a probabilistic approach and might help engineers to develop a better technology for assessing wood product performance and use such factors as service life indicators. For a wide range of wood species statistical factors are presented which could replace current simple durability class approach and which might help to differentiate from the common in ground contact classification on durability. Since a lot of valuable wood species are mainly used for use class 3 applications like window joinery, cladding, decking and garden furniture it is of interest to obtain reliable data for calculating service life of such wood-based products anyhow.

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