Floor bounce, when you can feel the floor boards flex under your feet, can be caused by weak floor joists. You can fix a weak or even a cracked floor joist by installing a sister joist. A sister joist is a set of boards that secure to and run parallel to the existing floor joist. This adds strength to the beam and prevents the beam from flexing when you walk over it.
Measure the length, width and the thickness of the existing joist with a tape measure. A common joist measures 2 inches thick by 10 inches wide. The length varies with the length of the house. If you cannot find 2-inch thick boards, use 1-inch-thick boards and place a joist on both sides of the existing joist. Otherwise, use a 2-inch-thick board and attach the joist to only one side.
Cut the sister joist to length with a circular saw.
Place construction adhesive onto the one side of the sister joist and onto one side of the original joist. If you are using joist that measure half the thickness, place glue on one side of each sister joist and on both sides of the original joist.
Place the sister joist against the original joist and butt the sister joist up against the floorboards. Clamp the board in place to secure it. Again, if you are using two boards, duplicate this process on the other side of the original joists.
Secure the sister joist in place with 4-inch drywall screws. Place a screw approximately every 6 inches lengthwise and every 2 inches widthways.
If you need to prop up a sagging joist before you install the sister joist. Place a 4-by-4- board across the bottom of the sagging joist and the two joists beside it. Place a lally column (metal columns used to brace flooring) under the board and position a hydraulic jack under the column. Jack up the sagging joist no more than ¼-inch a day. This will protect the house from damage. Once the joist is jacked up, install the sister joists.