Should the bottom cord of one or more trusses be sagging or damaged it may be reflected in a sagging ceiling or if the roof is over loaded to heavy then the downward pressure may cause the sagging.
Roof rafer construction wall sag.
Sagging sheathing between rafters may be due to rot delamination use of fiberboard or other materials that may not perform well over the life of a home or due use of too thin plywood roof decking to start with.
Roof sag or roof collapse forces.
Some roof framing designs include the use of a strut or beam that runs perpendicular across the bottom of the rafters inside the attic.
This article describes explains the directions of forces in roof structures illustrating downwards force horizontal thrust and the causes of rafter sags rafters that disconnect from the ridge or top plate and why a sagging roof pushes the building walls outwards at the wall top.
Decay can occur over time due to a number of reasons including leaks in the roof or leaks from guttering or downpipes wet rot or even insect attacks such as termites damaging the ends of the bearings.
Some roofs sag because of decay to the inside beams and sheathing.
Sagging rafters can lead to serious problems with the entire roofing system.
Sagging ceiling could be an indication of framing issues some of which could be related to the roof.
If you notice that there is a visible dip in your roof chances are you have a problem with one or several rafters and you may soon be facing more issues like water leaks bowed out walls or your roof caving in on you.