Realignment of the rafters may solve the sagging problem.
How to repair sagging roof rafters.
This article describes a diy repair project using a site built spreader to straighten sagged roof rafters and decking.
Rafters support the weight of the roof and any rain or snow that falls on it.
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.
Sagging rafters can lead to serious problems with the entire roofing system.
Ceiling joists are placed at the ends of the rafters to keep the rafters from spreading out and sagging.