A sagging floor may indicate a structural problem. Call an engineer if the sagging is serious or appears to involve the foundation or a central girder that supports a floor’s joists.
A floor that feels bouncy may not have adequate bridging-the diagonal braces between joists. Nail any loose bridging back in place.
Then install solid wood blocking, using lumber the same size as the joists. Apply construction adhesive to each joint and stagger the pieces so that you can nail into them through the joists.
If a floor sags, determine the extent of the sagging with a level and a long straightedge. Measure in from the walls to the sag; then use the measurements to locate the joist involved. To correct a slight sag, drive a hard-wood wedge or shingle as a shim between the joist and the sub flooring.
If a joist is warped or cracked, jack it up and double it, that is, nail a new joist of the same size to it. Remove the bridging on both sides of the joist. Then install a jack post-or a contractor’s jack in a tight crawl space -on a solid, weight-distributing footing, such as a scrap 2 x 10. Lock the jack’s telescoping tubes in place with the steel pin; then rotate the jack’s screw against a hardwood scrap under the joist. Tighten the jack no more than a quarter turn every 4 days until the joist is straight.
Prepare the new joist by chiseling 1/4 inch off its bottom at each end; this will let you slip it easily over the foundation sill and the central girder. Set it in place and shim the ends. Then attach it with 16d nails staggered top and bottom at 12-inch intervals. In-stall solid blocking to replace the bridging. Lower the jack at the same gradual rate that you put it up.
If the sagging involves two or three joists, put a 4 x 6 on top of the jack and raise them together; use two jacks if it involves more joists. Put spacing blocks under the joists so that you can double them more easily.