Depending upon the conditions it is possible to strengthen or repair existing framing members such as floor joists or roof rafters by adding reinforcing material sandwiching the member on either side with plywood is sometimes worthwhile but the plywood must be installed correctly for greatest strength.
How to jack up a sagging floor joist.
Jack up the ceiling with the bottle jack.
Lay a solid wood block below the jack to avoid damaging the floor.
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Jack up the ceiling slowly about a quarter of an inch per day.
Set a hydraulic jack and post under the beam and jack up the joists about 1 8 in.
Thre ways to fix a sagging or sloping floor.
Tack a beam under the sagging joists.
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Nailing two 2x4s together will work to span about three joists unless the sag is under a weight bearing wall.
Then set new joists same height as existing ones alongside the old.