A.The sag is permanent due to creep of the wood fibers when overstressed. You don’t indicate the rafter spacing and size, but in a structure old enough to have four roofs, I would expect them to be full-size 2x4s on 16-inch centers. No current span tables would consider them adequate, although they obviously have not collapsed under a load much heavier than anticipated. Depending upon what your local code official will permit, you could sister the rafters with new 2x6s that are slightly shorter than the original rafters (10 or 12 feet, depending upon the location of the bow). The original rafters have plenty of strength near the ends, and the sistered 2x6s, once nailed tightly to the original rafters, should provide adequate strength. The new members will also provide a good, straight nailing base for the new sheathing, and eliminate the need to shim up the old rafters. I also recommend refastening the rafters to the top plate with metal framing connectors, since the original toenails have probably slipped somewhat.