If the framing is sound and problems are not found elsewhere but the roof still sags the.
Roof sags in middle.
Non foundational reasons for a sagging roof.
The home may not be designed to support the weight of this extra roofing material and it will sag.
Two or more roofs on one structure spell trouble.
A sagging roof can come in one of a few classic forms each with its own cause.
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.
Rotten or saturated sheathing can also cause roof lines to sag.
Additionally if your roofer has used material that is much heavier than your previous roof it can sag for the same reason.
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.
Some roofs sag because of decay to the inside beams and sheathing.
If your rafters are still sagging a bit and it is causing your roof to dish out a little in the center of the roof sides add a 2x4 that goes all the way across the rafters in the center of the length of the rafters.
This is the easiest problem to repair.