The role of grade and thickness in the degradation of fire-retardant-treated plywood

In some cases, fire-retardant-treated plywood used since 1980 for roof sheathing has rapidly degraded and failed, apparently because of thermally induced acid hydrolysis. This study sought to determine whether plywood grade or thickness influences the manner in which fire-retardant treatment (FRT) and subsequent high-temperature exposure affects the strength properties of plywood. The effects of FRT were evaluated on two thicknesses and three commercial grades of southern pine plywood as well as plywood constructed from nearly defect-free N-grade veneer. Specimens were treated with monoammonium phosphate (MAP), then subjected to exposure at 66 degrees C (150% F) and 75 percent relative humidity for either 30, 60, or 90 days. Modulus of rupture (MOR), work to maximum load (WML), and modulus of elasticity (MOE) of the specimens were evaluated. Results suggest that the rate of plywood degrade resulting from FRT, redrying, and subsequent high-temperature exposure is largely independent of plywood quality or grade. Although the initial strength loss caused by FRT and redrying appeared greater for the thinner plywood, degrade during subsequent temperature exposure appeared to be independent of plywood thickness. Thus, it appears that findings from previous studies on thermal degrade using high quality, N-grade plywood are readily applicable to commercial grades and thicknesses. Evaluation of the effects of knots and voids on MOR revealed that these defects are only partially responsible for the difference in bending strength among specimens