Lumber Concerns In High Temperature Low Humidity Climates
Question:
I have been told that the high temperatures and low humidity present in attics can cause deterioration of wood, leading to truss failure. Is this true? If so, what length of time are we looking at and can we see the deterioration in the wood by visual inspection?
Answer:
The conditions you describe will not cause truss failure.
Trusses are made of dimensional lumber, as are conventional frame rafters. If low humidity and high temperatures caused lumber deterioration, then failures would occur in all wood-framed roofs, not just in trussed roofs.
The fact is that low humidity does not cause lumber deterioration – high humidity does. Once the moisture content of lumber exceeds 19%, decay can begin. The average yearly moisture content of standard use in-service lumber in the Southwest is only about 8%.
High temperatures (over 150 degrees Fahrenheit) can cause lumber to weaken but not decay. Sections 6.4.6 and 6.4.7 of the Truss Plate Institute's ANSI/TPI 1 includes a wet service and a temperature factor.