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1991 NDS Values for Lumber

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Everybody knows that the 1991 NDS published values for lumber which were
significantly less than previously allowed.  I don't have any problems with
this.  However, it has put a wrench into analyzing existing roofs.

Calculations on existing roofs using the 1991 NDS values often don't work
for the existing loads let alone any additional loads.

Some jurisdictions require the use of the 1991 NDS design values on
everything.  Others say that if it was built before 1986 you can use the
"old" values.

I think it would be helpful if there was a mandate made by those
knowledgeable in the subject which indicated when the 1991 NDS values are
required to be used in all jurisdictions.

Any thoughts on this subject would be appreciated.

Joseph M. Otto, P.E.
Ireland Engineering
Fremont, CA