One of the most, if not the most, efficient methods of extending our existing forest resource is to prolong the service life of wood currently in-service by using those existing structures to meet our future needs (Hamilton and Winandy 1998). It is currently estimated that over 7 x 109 m3 (3 trillion bd. ft) of wood is currently in service within the United States of America (PATH 1999). Research programs throughout North America are increasingly focusing on understanding and defining the salient issues of wood durability and by maintaining and extending the serviceability of these existing wood structures. This report presents the findings and implications of a major 10-year research program carried on at the U.S. Forest Products Laboratory. This research program developed serviceability models for fire-retardant (FR)-treated plywood roof sheathing exposed to elevated in-service temperatures and experiencing thermal degrade. FR-treated plywood roof sheathing is often required by U.S. Building Codes in roof systems for multifamily dwellings having common property walls. This 10-year research program found many important facts. Qualitatively, the mechanism of thermal degrade in FR-treated plywood was acid-hydrolysis. The magnitude of strength loss could be cumulatively related to FR chemistry, thermal exposure during pre-treatment, treatment, and post-treatment processing and in-service exposure. The effects of FR chemistry could be mitigated by use of pH buffers. The strength effects were similar for many levels of plywood quality. Quantitatively, a kinetics-based approach could be used to predict strength loss based on its time-temperature history. This research program then developed models with which to assess current condition, predict future hazard based on past service life, and then predict residual serviceability of untreated and FR-treated plywood used as structural roof sheathing. Each of these findings is briefly described in this report. There are many opportunities for extending the useful service life of wood by better maintenance, remedial treatment, or enhanced serviceability assessment to predict both residual strength and residual utility. Results of research programs such as this can be used to extend service-life by providing the engineer with a estimate of residual serviceability and thereby avoiding premature removal. Many of the concepts employed in the development of these FR-plywood serviceability models are directly applicable to the development of predictive durability models for wood as affected by decay. When such a durability-based service-life model is developed, that serviceability model will aid building code officials, regulators, contractors, and engineers in determining replacement time schedules for wood undergoing biological attack.

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