The Wood Handbook does not seem to address the differences in strength
between modern lumber and lumber from the old uncut forests. I would get a
copy of the grading rules for the species that you have and grade it
visually myself, or else hire a Grader.
The old lumber often (but not always) looks much better than modern
lumber -- old, really good looking lumber (straight grain, small or no
knots, no splits) could probably be graded as Dense Select Structural or
whatever the Rules call the highest quality timbers. On the other hand, the
rules for slope of grain used to be less strict for some species. I've seen
old trusses damaged in a way that looked like a sort of tension failure
across steeply sloped grain in a bottom chord.
The connections are likely to be the weak links in an old wood truss -- be
sure to check them all.
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