If you don't object to European standards, the Eurocodes (Eurocode 5 specifically and associated standards) define various "standard" values for different timbers and give appropriate scalars to ensure that your building doesn't fall down on you. I don't know how the standards compare to those in the US, but having run through some of the maths, my feeling is that they're stricter. I could (one day) talk you through the maths with the obvious caveats that you should have everything checked by an engineer. Calculating snow loads would be a "local" problem though. My tables for snow loads are for France and not applicable to Canada.