Leaching of Preservatives in Treated Experimental Shake Roofs

Experimental roof panels, made on 1.2 x 1.2 m plywood squares, were made using preservative-treated barn shakes, exposed to weather at a high rainfall site to study the performance of the treatments. Preservatives were chromated copper arsenate (CCA) type B and C, ammoniacal copper arsenate (ACA and CAA), pentachlorophenol (PCP-LPG and penta in oil), and disodium octaborate tetrahydrate (Timbor). Runoff rainwater, collected periodically from the roof panels and analyzed for the preservatives, showed that the concentration of preservatives decreased to a constant low level after 6 to 8 months exposure, and remained at this level for an additional year. Shakes collected after 8 months exposure showed loss of preservatives of 30 to 40 percent from the butt ends, but only a small amount from the exposed middle parts. Exceptions were PCP-LPG, which had a somewhat higher loss from the middle parts of the shakes, and Timbor-treated shakes, which lost about 90 percent from the butt ends and about 40 percent from the middle parts of the shakes. In general, all shakes after exposure of 8 months still retained preservatives in sufficient concentration to prevent biological degradation.

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