Field durability of CCA- and ACA-treated plywood composed of hardwood and softwood veneers

Current American Plywood Association standards for plywood panel products are based on structural performance and allow hardwood veneers to be included in southern pine plywood. However, the American Wood Preservers' Association's (AWPA) standard for permanent wood foundations requires that plywood be made only of softwood plies. Hardwoods are generally more difficult to protect from decay using waterborne preservatives than are softwoods. The objective of the study reported herein was to document field performance of plywood composed of both hardwood and softwood veneers treated with waterborne preservatives and to determine if AWPA requirements are warranted. Results are given for hardwood- and softwood-treated plywood tests after 10 years of ground exposure in southern Mississippi. With reference to southern pine plywood and the current AWPA standard for preservatively treated wood used in permanent foundations, we conclude that the requirement that plywood be composed only of softwood plies is prudent.