In lieu of a real structural engineering calculation, from various reading online, here's how I gather it's done. In my application, I need to calculate the minimum number of jack/trimmer studs to support a 24 ft roof truss span with a 2 ft overhang to get a clear opening of 6 ft. The load factors are the dead load of the roof weight itself and the projected roof snow load. Live load is irrelevant since the snow load will exceed the live load requirements. One concern is that I don't know what kind of wood was used in the construction of the house, so I need to determine that as well as the relative strength with respect to the wood specified in the tables.
The Southern Pine Council has some nice resources to calculate spans and such, of course using southern pine for framing. The Headers and Beams document contains information to calculate the header needed for a given span, as well as the allowable roof load given a header and number of trimmers. The latter is what I was looking for, and the illustration for the load calculation is quite helpful. (This is the ref207.pdf file, numbered page 27, which is 29 of 36 in the PDF.)
eHow has an article on the calculation of roof snow load based on ground snow load and the pitch and area of the roof. The article makes reference to a Cornell University snow load calculator, but doesn't provide a link. A search turned up a course page with a calculator. (Perhaps I should buy the textbook, Structural Elements for Architects and Builders, by Jonathan Ochshorn.)
The other concern is wind load, since we get a lot of wind on the hill. The IRC has something to say about that, which I have yet to research.
Of course I'd feel more comfortable having a professional do the calculation; the conundrum being, am I neglecting my due diligence if I don't?
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