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.
Ceiling joists are placed at the ends of the rafters to keep the rafters from spreading out and sagging.
When considering what to do about sags in a roof start by noticing if the sag is in sheathing between rafters page top photo0 or sagging along the length of the rafters.
Sagging rafters can lead to serious problems with the entire roofing system.
Roof rafter decking sagging.