Roof sagging??? Call an expert for an attic & proper roof inspection. Request they take photos and videos of there findings like shown here. This happens when ventilation is over looked, or misunderstood by a roofer that believes it’s just a hoax. Or a weatherization company without proper standards. Maybe Lack of local code enforcement,lacking knowledge of best practices in roofing ventilation to enforce them. If your ridge board is dropping like this! It will cause your walls to be pushed outwards. Will eventually break the rafters. Will Lead to roof leaks. Will promote Mold growth that consumes whatever it grows on. Or wood getting wet and drying out to many times causing Dry Rotting.