I have nearly the same problem only my roof is stick built with 2x6. However, the space between the drywall and decking allows for no air/moisture movement. There is no eave and no provision at the ridge for moisture escape. My roof has begun to sag so bad that the drywall joints are breaking where the ceiling and wall come together. I to am looking for the best approach to this problem. Shoddy contractor work is the problem. I will have to remove my roof and start from scratch.. I have a 5/12 pitch and rafters span 15 feet. I hope we get some help with our similar problems.