I'm at the stage of deciding the trim for my dormers. I had the framers cut the rafter tails to conform with a standard spring angle of 52/38 (and I plan on using a simple 3 5/8 size), this way I have a lot of backing for my crown molding. The 38 wall angle will then sit against a piece of pine below (for a little cornice). I'm not sure if that's an unusual way of framing for the trim (and if people just cut the tails plumb, and then build up support), but I thought it would be a good thing to do.
Anyway, the question I have is is it okay to mount the crown directly to the rafter tails, since they are cut to conform the the crown's spring angle? I've read that a fascia should be supported with a subfascia board in order to avoid warping. Is this really necessary?
I've already had the plywood cut without taking into account a subfascia, so adding one would throw my measurements off. I should add that the bottom of the crown (the part that would be sitting against the wall normally) will be moutned to a piece of pine, and this will be backed probably by a ripped 2x4, either standing upright or sitting flat. Would the solid backing at the bottom be enough, wihout worrying about a subfascia?
Or is a subfascia not really necessary anyway? If it is, though, could a piece of 1/2 inch ply across the tails be enough? Half an inch should throw the plywood roof to crown space off too much--and I'll probably be adding a drip cap anway if there is a little space.