How to Build a Roof Soffit Box Start with a frame. A three-piece wood box is fine; it can be a triangle piece, a back piece, and a nailer that is on the small side that will attach the entire thing to the house itself.
Next, trim to fit. Use clamps wherever possible to help hold everything in place. Finally, remove the nubs. You can leave them on if you want, but for a professional finish, they should go.
Throw on a little bit of primer and paint and the soffit box will be finished and ready to install.