Checking of CCA-treated radiata pine decking timber exposed to natural weathering

A field trial was established at four locations in Australia to determine whether treatment of radiata pine decking timber with chromated-copper-arsenate (CCA) and the use of different types of fixings (nails or screws) increases the tendency of wood to check during exterior exposure. The effect of adding a hydrophobic wax to CCA on the checking of treated timber was also quantified. Checks in CCA-treated timber exposed outdoors for 1 year tended to be slightly longer and wider, but fewer in number than those found in similarly exposed, untreated, timber. The difference in checking between CCA-treated and untreated timber, however, was statistically insignificant. Checking in CCA-treated timber most closely resembled that occurring in timber that had been pressure treated with water and exposed to weathering. This finding suggests that some aspect of the treatment process, possibly redrying following treatment, may have influenced checking of wood during exterior exposure. The type of nail or screw used to fix decking timbers had no overall effect on checking of boards, but checks originating from screws were larger than those associated with nails, and a greater proportion of screws acted as nucleation sites for checks. Timber treated with CCA containing a hydrophobic wax additive had fewer checks, which were significantly smaller in size than those in CCA-treated timber and the untreated controls (except for check length in untreated timber).