Once you have established how much the roof sag needs to be jacked up lay a 2x4 plate across the ceiling joists and nail directly under the ridge of the roof.
Fixing saging roof obver garage.
Step 4 strengthen the beams.
Attach it to the beam with carriage bolts but do not tighten completely.
Fixing a sagging roof by bolstering the beams bolstering the beams of your roof is another solution to repairing a sagging roof however it is often something that requires assistance from another able bodied person since beams can be extremely heavy.
Any sagging will gradually be lifted and by the time you have finished the garage roof repair your roof should but upright again.
This may be problematic in winter months but allowing the roof to continue to sag can lead to greater problems such as a cave in.
First perform step 2 except keep the string under your beam and off to the side a bit to make room for some framing.
Now add collar ties from one side of the rafter to the other.
I prefer the term mid span cross tie to collar tie because i associate collar ties with placement high closer to the ridge.
If the sagging roof is due to rotting or warping roof decking the repair will be much easier.
Sagging ceiling could be an indication of framing issues some of which could be related to the roof.
If necessary use something to force the roof up while you work the beams into place and fasten them.
You will need to remove the shingles and the old decking however which means there will be an exposed hole in the roof.
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.