Hello, Phillip.
It all depends... My first concern would be why the roof is sagging to begin with? Are the walls spreading, joints failing, the foundation settling, the rafters rotted or undersized, etc.? A sagging roofline can begin in the basement...
Look for clues. A bit of careful measurement,inspection and analysis before jacking anything would suggest what the specific source of the trouble is, and then the best answers to correct it.
Once the underlying reason is understood, the process can be sequentially reversed, then fixed permanently. Might be easier than you expect, if you are lucky...
Pete