I'm a little confused by the span tables for rafter sizes. The spans they list, are they for the total span horizontally, or the diagonal from the bird's mouth to the ridge board?

Terry_Worde

Here's my understanding.
Dead loads such as sheathing, shingles, etc., are calculated based on the true length of the rafter (Rafter Length/cos Common Rafter Pitch Angle or Square Root(Rise² + Run²).
Snow loads calcs are based on the rafter run or span. This seeming difference in method used to bug me years ago but the reasoning behind it is easy to understand. The depth of snow on the roof is measured on the plumb, but this doesn't reflect the true weight of the snow on the roof. For example, say we have a 9/12 pitch roof with 5" of snow. If we base our load calculations on the actual rafter length we find that the depth of the snow should be decreased to 4" (i.e. the 5" plumb measurement is only 4" in section).
So we can just use the span and save ourselves the extra math work. The end result is the same.