The previous owner's installed a ceiling fan without proper bracing. As a result, the ceiling cracked from the center of the room to the wall. I was going to repair the 3/8" drywall, but have come to find that the drywall was installed directly on the original plaster ceiling. How do I repair the ceiling? I think it requires new drywall in the affected area (one edge of the crack hangs down about 3/8" from the other side), but haven't repaired drywall when it's over plaster.