I am having a similar problem locating information on how to fix a sagging ceiling in my 1972 24 x 60 home. There is no water damage, but it appears the ceiling framing is not attached to the joists. finding the joists, i think, is the issue at hand. I would rather not start pulling down ceiling panels, (which are no longer sold, by the way...) just to have a look around.
Any info will be appreciated.