The floor joists under the first floor of my 20-year-old house are warping. Is it possible to repair the problem without replacing the joists?
Yes. First, in the basement, jack the joists up to a level that is slightly above what is considered normal. Line up pieces of 19 mm (3/4 in.) thick plywood flush with the either side of the warped joists. They must be flush with the joists, but they must also be flush with the plywood flooring above. The best way to attach the pieces of wood to the joists is using screws and construction adhesive. If you decide to use nails, make sure that you hammer in the nails at different angles so that the nails will not pull out due to pressure. If you can, make sure that the pieces of wood installed are the same width as the joists. This will add to their strength. Another thing that can be done to improve the strength of the joists and to prevent curving of the floor is to add blocks between, and perpendicular to, the joists. Once this is completed, the jacks can be removed.