If the sagging roof is due to rotting or warping roof decking the repair will be much easier.
Fixing saging roof obver garage.
Sagging ceiling could be an indication of framing issues some of which could be related to the roof.
I prefer the term mid span cross tie to collar tie because i associate collar ties with placement high closer to the ridge.
If necessary use something to force the roof up while you work the beams into place and fasten them.
First perform step 2 except keep the string under your beam and off to the side a bit to make room for some framing.
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.
Gradually work along all the beams pushing the roof up as you go.
With the joist back in its original position place one section of the angle iron on the uppermost section of the rafter just before the area where it is sagging or broken.
Any sagging will gradually be lifted and by the time you have finished the garage roof repair your roof should but upright again.
Now add collar ties from one side of the rafter to the other.
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.
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.
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.