The presence of residual preservation in dip-treated and brush-treated shingles of various species from the western United States was assessed 18 years after installation using Aspergillus bioassays. The performance of western redcedar (Thuja plicata) was compared to western hemlock (Tsuga heterophylla), sugar pine (Pinus lambertiana) and western larch (Larix occidentalis). Untreated shingles of all four species were seriously decayed, while those receiving topical treatments continued to maintain their appearance. Bioassays from the shingles indicated that residual pentachlorophenol and chromated copper arsenate were still present on both upper and lower surfaces. The results illustrate the benefits of topical treatments for protecting wood in above ground exposures.

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