Decking is the largest market for residential preserved wood in the United States. Preserved decking must be resistant to colonization by decay fungi initiating from spores, and occasionally from mycelia due to elevated soil levels or adjacent untreated wood. The most vulnerable parts of a preserved wood deck are untreated, or under-treated, wood that becomes exposed by field cuts and checking. Field cuts can be protected by applying field-cut preservatives. Checks can be protected by mobile preservatives present in the treated wood. Copper ions from chromated copper arsenate and particulate copper preservatives can protect untreated checks; however, copper-ethanolamine leached from copper-amine systems has lower efficacy against basidiospore germination. DDAC and tebuconazole have limited abilities to protect checks. The potential contribution of borates has not been adequately studied. Protection against mycelial attack depends largely on the toxic thresholds of the actives and their distribution within the wood. Toxic thresholds determined by soil block decay testing are higher than currently standardized retentions for the major systems currently used to treat decking in the United States. Improved data are needed to determine toxic thresholds from tests on shell-treated material exposed to realistic above-ground conditions. Further research on a number of fronts is needed to understand and optimize the decay resistance of preserved wood decking.

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