Q.I've heard that in older buildings (at least 70 years old) the effective strength of wood members like joists and rafters increases with age. If that's true, how does one evaluate the increase? Or is the increase negligible?
A.Paul Fisette responds: The general rule is that the mechanical properties of wood show little change over time. In short, the aging of wood, unlike that of cheese, does not make it better. Nor does it improve its strength. It is still possible that old wood joists may in fact be significantly stronger than they were on the day they were installed, because wood does gain strength as it dries. An existing dry joist (around 12 percent moisture content) might be 50 percent stronger than its original "wet-wood" value if it was originally installed as rough-cut, green lumber.
But, typically, age works against you: It's actually more likely that those 70-year-old joists and rafters are now weaker than when they were installed. Except for the drop in moisture content, everything else works to weaken wood. Seventy years is a long time; there's a good chance the lumber has been exposed to fungi, insects, elevated temperature (like that experienced in south-facing roof structures), and excessive loading. These forces can significantly weaken structural wood.
The bottom line is that there is no reliable way to predict design strength beyond what the grade stamp on the lumber indicates (if there even is a grade stamp).
There are some other interesting ideas about what makes wood stronger: During the World Series, I heard of players rubbing their bats with a bone to make them stronger.
Paul Fisette is director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor.
