There is a perception that oriented strand board (OSB) is less resistant to fungl than plywood under conditions of moderate exposure to moisture. Douglas fir-faced plywood (DFP) has been suggested as a benchmark for acceptable durability under such conditions. This project was initiated to determine the minimum level of low-toxicity chemical treatment needed to upgrade the fungal resistance of OSB, made from aspen, to equal that of DFP. Against a brown-rot fungus in a soil-block test, 0.2% zinc borate (ZB), added during manufacture, and subsequent spray treatment with 10 µg/cm2 oxine copper provided the required performance. Against a white-rot fungus 0.2% ZB alone was sufficient and between 0.1 and 0.2% may have been adequate. For resistance to mould in a humidity chamber, spray treatment of OSB with 10 µg/cm2 oxine copper provided equivalent performance to DFP. The recommended combination of treatments for moderate moisture exposure is 0.2% zinc borate and 10 µg/cm2 oxine copper.

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