Twenty-Year Performance of Decking with Two Levels of Preservative Penetration

In 1991 a field test was established at two locations in Canada to assess the longevity of deck boards that were either untreated or pressure treated with two levels of preservative penetration. These penetration levels were (1) single incised with 80 percent ≥ 5 mm and (2) unincised. Minidecks were prepared from each species/treatment variable and visually inspected for decay after 5, 9, 15, and 20 years of exposure. After 9 years the treated boards were virtually free of fungal attack, regardless of the preservative penetration, while decay of the untreated boards was moderate to severe. After 15 years there was still almost no decay in the treated boards, while decay in untreated boards had progressed significantly. At the 20-year inspection all untreated decks, with the exception of western red cedar ( Thuja plicata ), would have had to be replaced due to decay of multiple boards, while all chromated copper arsenate–treated decks remained serviceable regardless of preservative penetration. Based on published work showing how shell treatments with copper-containing preservatives protect decking even with checks penetrating the treated zone, these data are expected to be also relevant to newer preservatives with low levels of mobile copper.