How can I find a reliable dry wall contractor close to Fremont CA?

Drywall on the roof?? That I don't understand. But, one trick is to go to a development or two where new construction is taking place. Ask to speak with the General Superintendent. Ask him if he'd supply you with the name(s) of a drywall company that he currently uses that has proven to be reliable. Most "Super's" are pretty good about this, and will give you the names of the companies they've used in the past.