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.
How to repair sagging roof rafters.
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 may be the result of numerous heavy snowfalls or inadequate ceiling joists.
Ceiling joists are placed at the ends of the rafters to keep the rafters from spreading out and sagging.
Roof rafter decking sagging.