When the framing of the roof is put together quite frequently two of the ceiling rafters will be fixed together as this will create a much longer span.
Garage roof trusses sagging.
So you should start by inspecting the roof from the under side or attic.
A sagging roof line may original from the ceiling s horizontal rafters.
If the framing is sound and problems are not found elsewhere but the roof still sags the problem might have originated because of poor installation of the plywood sheathing or shingles.
Half inch drywall can sag if it s hung under roof trusses that are spaced every 24 in.
You may find that several are in need of straightening and that some of the sheathing must be replaced.
Finally make sure stored items are securely locked in place.
Are doors sticking or jambs out of square.
Then slowly allow the trusses down.
This repair will keep the trusses from sagging.
Sagging rafters may be the result of having framed the roof with lumber of dimensions smaller than current building standards such as using 2x4s or 2x6 s or there may be damage to the framing from insects or rot.
As demonstrated by the dent in my pickup s hood vibrations from the garage door opener can cause items to shimmy around and eventually crash down onto your car or you.
Drywall instead of 5 8 in.
If you have traditional site built roof framing your storage capacity depends on the design of the framing.
However it is important to fix the rafters before doing any other work to the roof.
It can be nailed temporarily to each truss as the next truss is put in place.
If your ceiling drywall is sagging between joists sometimes called pillowing it s probably on the top floor and attached to the roof trusses.
If so i bet the installers used 1 2 in.
Even ceiling cracks can be caused by damaged trusses or downward pressures.
However a bigger problem is nailing the new web member to the ridge peak of the truss and there will be 2 members nailed in this location.
To hold them up until the decking of the roof is placed on top a 12 foot 1 inch by 4 inch piece of lumber is used.
The first step is to find out how many rafters are sagging and if anything else is damaged.
Should the bottom cord of one or more trusses be sagging or damaged it may be reflected in a sagging ceiling or if the roof is over loaded to heavy then the downward pressure may cause the sagging.
Usually trusses are put 24 inches apart.