Floor joists deflect or bow slightly over time, especially when the joists cover long spans. As a result, the floors atop the joists can develop a bouncy feel when people walk across them. Curing the bounce may be easier than you think. Unless they are splintered, rotted or otherwise damaged, you need not replace joists. Instead, decrease joist deflection with a variety of methods. For added support, combine methods.
Sistering Floor Joists
When you sister a joist, you secure a complementary joist to the original one. You can use dimensional lumber or 3/4-inch plywood. Either material must be as wide as the original joist. For example, select two-by-10 dimensional lumber if the current joists are two-by-10s, or cut sheets of plywood into strips that are the width of the current joist. Cover the joist span with either material. Add a bead of construction adhesive to the existing joist before nailing the sister into place. The adhesive adds strength to the bond between joists, and it prevents squeaking between the two joist members as the floor moves.
Stiffen the Joists
When a floor joist deflects, it bends downward. This effectively stretches the underside of the joist. Nailing a two-by-four to the bottom edge of the joist helps stop some of the deflection. Orient the two-by-four so that its face -- one of the wider sides -- rests against the bottom edge of the joist, I-beam style. Attach the two-by-four with construction adhesive and nails at least an inch longer than the joist's width. Add two-by-fours until you span the entire length of the joist. This fix reduces the overall headroom available in a basement or crawlspace, though. Although not as effective as sistering for stopping a bouncy floor, stiffening the joists does provide an alternative if the joist bays -- the space between each floor joist -- are cluttered with heating ducts, plumbing and wiring. Such obstructions can make sistering joists a difficult endeavor.
Blocking is installing perpendicular wood framing between floor joists. The wood must be two-by lumber of the same width as the joists, and it must fit tightly in each joist bay. Stagger the blocking, driving nails through the joist and into the blocking material. As with sistering, blocking requires that joist bays be free of obstructions. Blocking helps distribute the load each joist bears onto neighboring joists, which reduces bounce.
Beef Up Support
Adding a beam perpendicular to the joists shores up several framing members at once. The beam may be a four-by-four, a pair of two-by-eights, or a pair of two-by-10s. Support the beam with six-by-six pressure-treated posts or a steel column filled with concrete, called a lally. Place the beam about midway along the joist’s span. While a beam and its supporting columns are the most effective means of solving floor bounce, it is also the most intrusive. The beam reduces headroom, and the columns present obstacles for anyone venturing into the basement or crawlspace.