I just recently finish my 14 X 14 Vaulted ceiling sun room. I used tongue and grove ship lap 3/4" Birch. It turned out wonderful. I posted a question on how I could mate 3 pieces of crown molding together at one corner. No one had a answewr so I put on my thinking cap on. So I glued three pieces of scrap birch together. The I cut the top that meets the ceiling at 12 degrees to match the vaulted ceiling. The cut the bottom square. I then cut the face at a 45 degree angle. Now I have a way to meet the three pieces of crown at the corner. Enclosed is my pictures. I also made a simular piece for the peek because the uneven surface of the boards made it impossible to get a goo 6degree miter joint.