In my case, moisture trapped in the vaulted ceiling has caused the rafters to warp. My roof decking has rotted away along the ridge. My room is 12 feet wide and the 2x6 rafters span 15 feet. The rest of the roof on my house is fine. The rooms roofline with the vaulted ceiling is good 6 to 8 inches lower in the middle of the span. It is level at the top and at the bottom. The load bearing wall remains plumb.