*Are you absolutely sure that it is only the drywall sagging? I did several jobs for one customer over a two year period who had a sagging spot in her dining room ceiling. Everyone thought the drywall had warped from a drip that she had a few years before in that ceiling. It turned out that the trusses in that area had started to separate from the gusset plates holding them together. The trusses were sagging and bringing the drywall down. The water from the drip weakened the areas that the gussets were nailed into.