Surface treatments protecting untreated Douglas-fir timbers from internal decay

The performance of untreated Douglas-fir piers with and without supplemental treatments was assessed over a 16-year period by culturing increment cores for the presence of decay fungi. Kerfing of decking and caps before exposure markedly improved the performance of untreated timbers, although the protective effect declined with time. Application of topical biocides at the time of construction also reduced the incidence of decay fungi present in the timbers, although that effect also declined with time, probably as the chemicals leached from the wood. The water-soluble fungicides fluor-chrome-arsenic-phenol and ammonium bifluoride provided excellent protection, while disodium octaborate tetrahydrate provided slightly less protection. Pentachlorophenol provided the least protection of the chemicals tested. The results illustrate the benefits of supplemental protection even when pressure-treated wood is not used.

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