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...
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