I am new here but found a similar question and liked the kinds of responses and that some are good at thinking outside of the box. I have a 1910 home with 2x4 (full dimension) fir roof rafters 32" on center and with three or four layers of roofing above. We are going to remove the roofing and skip sheathing down to the rafters and fill in the rafters to 16" on center. We will then sheath and re-roof with a standing seam roof, which requires a fairly flat sub straight. I was thinking of trying to "persuade" the old 2x4 back to flat with moisture (either steam or misting on water) and steel tubing or angle iron clamped to the rafters and slowly (over a few days to a week) tightening clamps to remove the sag. From there, we either sister on another ripped 2x6 or screw or screw and glue plywood to the 2x4 to keep it straight. We will probably also put sheetrock up on the underside of the rafters so furring up in the middle on top would still be problematic.
Any ideas or comments on my (possibly crazy) idea are appreciated.
Why bother trying to straighten the sagging 2 x 4's. The simplest solution is to sister new straight 2 x 4's to the sagging ones, level with the new rafters. Only cost is some more lumber, but it adds to the overall strength. Adjust the new rafters to maintain the 16" centers.