You can contact your local building department for snow load information.

Pitch can affect loads, shedding can be a factor depending on what type of roof material you use (metal vs. shingle), as well as exposure, etc. etc. You might also need to calculate unbalanced snow - what happens when the wind helps pile up snow on 1 side of the roof.

If memory serves there was some intro information in Steve Chappel's book:
http://books.google.com/books/about/A_timber_framer_s_workshop.html?id=pElSAAAAMAAJ

[img]http://bks5.books.google.com/books?id=pE...iUzOHV5kmnJpl9b[/img]

The short answer - pitch, roof material, exposure can affect the loads, and you need to calculate those based on the tributary area that each rafter is picking up, over its span, calculated against its length (a 14:12 rafter is longer than a 6:12 rafter over the same span).