Exterior Window Trim and Rough-In Sizing

Question

Short version: How much does/should exterior trim overlap window casing?

Long version: I'm trying to figure out how to position a rectangular window for a spot near the peak of the house. I want to make sure that the exterior trim doesn't collide with the soffits (and leaves an aesthetic spacing). The problem I have is that I don't know how much to plan on the trim overlapping the window.

Hypothetical Numbers: Suppose the window I want to position has a rough-in size of 30" wide by 36" high. The trim I'm using is 5" wide. What will be the final exterior width and height of the trimmed window?

Answer 1

Pick out your trim for the window and for the frieze. Decide on head flashing. Decide on the jamb reveal. Make your best guess for the window position. Then frame the rough opening 3" extra wide and 3" extra tall. Then fill the RO down to size by screwing (not nailing) 1x stock to the jacks, header, and sill. Center your sheathing over the RO so that a single 4x8 sheet covers it entirely. Nail it to the 2x framing members, but not to the 1x.