This paper reports on a joint study by BRE, the Forest Products Research Institute and TRADA Technology into the feasibility of grading UK grown larch to maximise its natural durability. The research was undertaken to assist sawmills and timber specifiers in the selection of larch timber suitable for external cladding. The findings indicate that the heartwood of larch has variable durability due to a range of factors. Of these, position within the stem is particularly important, with the inner ‘juvenile’ wood being less durable than the outer ‘mature’ heartwood. Although there is insufficient data available to accurately determine the extent of this low durability core, preliminary evidence suggests it occupies the inner 10 to 15 growth rings. The study has proposed practical grading rules that will enable most of the low durability timber to be selected out, though further work is required before this guidance can be definitive.

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