The home may not be designed to support the weight of this extra roofing material and it will sag.
Home roof sag.
Then inspect the metal or plywood gusset plates.
Installing new rafters directly alongside the weak or damaged ones is often enough to correct this problem.
Two or more roofs on one structure spell trouble.
Also inspect the metal or plywood gusset plates used where your wood meets on a truss.
Using a flashlight inspect the rafters and decking in your attic especially in the areas where your roof is sagging.
This is the easiest problem to repair.
Cracked rafters and trusses can cause sagging.
Go outside of the house and determine where the sag on the roof would be in your attic.
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.
Grab a flashlight and head up to your attic to inspect your roof rafters and decking specifically those where your roof is sagging.
Additionally if your roofer has used material that is much heavier than your previous roof it can sag for the same reason.
Look for any kind of damage including rot cracks breaks and more.
Determining the problem determine where the sag is in your roof in relation to your attic.