Inadequate redrying linked to dimensional instability of CCA-treated southern pine lumber

Some instances of severe warping of siding treated with a commercial waterborne preservative have been reported to the authors. We investigated this problem by evaluating 25-mm-, 51-mm-, and 102-mm- (1-, 2-, and 4-in.-) thick chromated copper arsenate (CCA) treated No. 1 southern yellow pine lumber shipped from three suppliers to a secondary manufacturer in Louisiana. The moisture content (MC), CCA retention, and adsorption/desorption ratio was determined for specimens cut from the lumber. For all lumber thicknesses and target retention levels, including untreated wood, the MC of many boards exceeded the secondary manufacturer’s specification of a maximum of 16 percent MC. No consistent relationship was found between CCA retention and the adsorption/desorption ratio, indicating that the presence of the preservative oxides was not causing excessive fluctuations in wood MC. The results suggest that the source of the dimensional instability is the initial high MC due to inadequate redrying.

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