Ok, looking at AWC 2021
https://awc.org/codes-and-standards/span-tables/
4” ice ~ 43 psf. I’ve never seen more than 2” accumulate here, at that point trees or their big limbs are coming down, all over my neighborhood. So 4” feels like a reasonable worst case.
Using values for northern Doug fir rafters no 1/2.
At 50 psf live load I can span the required distance using 2x10 on 12” centers or 2x12 on 16” centers. At 40 psf, 2x10 on 16” centers.
I have an idea for the construction of the roof covering to reliably shed snow and ice rather than accumulate it.