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.
Home roof sag.
Some roofs sag because of decay to the inside beams and sheathing.
If the framing is sound and problems are not found elsewhere but the roof still sags the.
This is the easiest problem to repair.
Two or more roofs on one structure spell trouble.
Cracked rafters and trusses can cause sagging.
Determining the problem determine where the sag is in your roof in relation to your attic.
In a stick frame non truss sloped roof these issues usually cause the roof ridge to sag in the middle giving it a swayback look but can also cause one side of the roof to sag.
The home may not be designed to support the weight of this extra roofing material and it will sag.
Grab a flashlight and head up to your attic to inspect your roof rafters and decking specifically those where your roof is sagging.
Then inspect the metal or plywood gusset plates.
Go outside of the house and determine where the sag on the roof would be in your attic.
Additionally if your roofer has used material that is much heavier than your previous roof it can sag for the same reason.
Using a flashlight inspect the rafters and decking in your attic especially in the areas where your roof is sagging.
Rotten or saturated sheathing can also cause roof lines to sag.