Just signed up and making my first post. My house was built by Sears and Roebuck back in the late 30's. The ceilings in my living room, and dining room have 12"x 12" tiles on them held up with adhesive. Some of the tiles are coming loose and causing a section of the ceiling to sag. I'm thinking of putting up drywall over the tiles to save on time and mess. Would there be any problems with doing this? I plan on making sure the ceiling is flat, and marking the joists for the drywall screws.