The high natural durability of western redcedar (Thuja plicata Donn) heartwood is often attributed to the presence of the thujaplicins, which are toxic to fungi. However, sound, twenty-five year old western redcedar shakes and shingles were found to contain only traces of the thujaplicins, but significant quantities of plicatic acid (which is only weakly toxic to fungi). This suggests that factors other than the presence of thujaplicins, such as western redcedar’s low equilibrium moisture content (EMC), contribute to its natural durability. Western redcedar’s low EMC was confirmed to be attributable to the presence of extractives and through a series of solvent extractions it was found that the lignans, including plicatic acid, were associated with western redcedar’s low EMC. The presence of tropolones did not correspond with lower EMC. Extractives, such as plicatic acid, may contribute to durability through the reduction of EMC.

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