Additionally if your roofer has used material that is much heavier than your previous roof it can sag for the same reason.
Roof sags in middle.
Rotten or saturated sheathing can also cause roof lines to sag.
Installing new rafters directly alongside the weak or damaged ones is often enough to correct this problem.
Some roofs sag because of decay to the inside beams and sheathing.
The home may not be designed to support the weight of this extra roofing material and it will sag.
Use your string line to determine how much the roof need to go up.
Non foundational reasons for a sagging roof.
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.
This is the easiest problem to repair.
If the framing is sound and problems are not found elsewhere but the roof still sags the.
Two or more roofs on one structure spell trouble.
A ridge that droops in the middle forming that unintentional western design feature likely is due to undersized rafters and or missing or inadequate internal bracing rafter or collar ties.