Five kinds of wood-based composites (density fiberboard=MDF, hardwood plywood=HP, softwood plywood =SP, particleboard=PB and aspen oriented strand board=OSB) were tested for their resistance against subterranean termites and decay under protected aboveground conditions. The exposure method was previously developed for evaluating performance of sill plates (dodai) in Japanese homes. Ten replicates (10 cm square x board thickness) were prepared from each wood-based composite, allocated into 5 groups so that two each were placed on a concrete block and covered with a PVC box to protect specimens from weather and to minimize site effect. Each specimen was placed on a 19 cm high concrete block with pine sapwood feeder stakes in the hollows of the concrete block. Annual visual inspection had been conducted and the degree of decay and termite attack was rated according to the AWPA standard (10: sound=no attack, 9: trace to slight attack, 7: moderate attack, 4:heavy attack, and 0: failure=no more available for further testing) since the specimens were installed in Kagoshima Prefecture, a southern part of Japan on June 22, 1999. As expected, biological attacks developed with time, although there was no sign of decay on any MDF, HP and PB specimen after 5 years’ exposure. Termites started attacking earlier and higher in terms of severity of damage than decay fungi. This was quite common to all wood-based composites tested. A few specimens showed unforeseen low or high resistance to biodeterioration: 4 SP and one OSB specimens rated 0 due to heavy termite attack. Performance of MDF was ranked first, and followed by HP, PB SP and OSB. These results suggest the necessity of protective treatment to ensure a longer service life of wood-based composites even for an aboveground use, as far as use conditions are considered to be conducive to biodeterioration.

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