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Re: A question about evaluating an existing wood structure

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I would start with the code span tables which should provide you with a start 
for determining what size members will work. You need to know the snow load 
for your area (if there is one) and the dead load of the materials. If you 
can inspect the lumber, look for a grade stamp. Most likely it is a grade 2 
or better as this is fairly common for roof rafters.
Use the current code data to test the members. If you run into a problem, you 
can then search out older codes and check the span against the load tables in 
the older codes.
The species of wood are also a concern. Douglas Fir is used throughout the 
Western US while Pine (I believe) is common in the Midwest and east. There is 
a difference in stresses, but not a great amount. 

This should get you started.