these days. I have talked to many people about it and have received many many answers. Some if not all are "tales".
Some other sights seen in ceilings over a time of usually 7 gears and greater: the rafters show, the actual sheets of 4x8 rock begin to show, waves show. In sinsop. the rock seems to conform to the rafters and the framing.
One thing some people have stated is that the ceiling should be 3/4 rock. Maybe this would work, but who wants to hang 3/4 inch rock especially on the ceilings?
Another thing is that the original rock was screwed in and not nailed. Screwing will compress the rock against the stud or rafter and make an indenture. Humm????
Another one is that the rafters have stretched or contracted or waved causing stress on the rock and actually put stress on it.
Another is humidity and drying out with the seasons. Its contracting and expanding.
Another is that this only happens in the summer and will go away in the fall and winter.
Now, lets see what others will say about this. As you have seen, I do not know the reason. These are the answers I have received. Take it or leave it. Mostly I will leave it.