Sistering is the process of adding additional material to the sides of existing floor joists to help strengthen and straighten them. Although framing lumber is usually chosen as a sistered wood, engineered lumber is also a good choice because it can add more stiffness than standard dimensional lumber. The job can be more difficult in basements or crawl spaces where little head room is available.
Make certain electrical wires, pipes, ductwork and strapping won't get in your way. New joists may need to be notched with a saw to accommodate existing fixtures. Contact an electrician or plumber if wires or piping appear to need removal or disconnection before proceeding.
Choose new joists that are at least the same height as existing material. For example, if 2-by-10 joists were used, choose the same size lumber for a sister joist.
Cut a length of post with a saw so that it will easily span across three joists. Wear safety glasses when cutting wood and throughout the rest of the process.
Set a pair of hydraulic floor jacks underneath the post and position it to span the joists. Raise the jacks until the beam fits snuggly under the joists. Raise the joists only about 1/8 inch per day to allow the house to settle and to avoid creating cracks in walls and flooring. Continue to raise the joists until the floor is level. Check it with a carpenter's level.
Run a bead of construction adhesive along the length of the existing joist. The bead should run in a continuous "S" pattern spaced about 6 to 8 inches apart so that the adhesive covers the joist well. Construction adhesive will help hold the boards together and reduce future squeaking as the boards move.
Lift a sister joist into place alongside one of the floor joists above the post. The sister joist works best if it is the same size span as the original joist. If this is not possible because of obstructions or other issues, the sister should extend a minimum of 3 feet on either side of the sagging joist. Use quick clamps to temporarily hold the sister joist in place.
Hammer common nails in two rows spaced no more than 16 inches apart along the entire sister joint. The rows should run parallel to one another, 1 or 2 inches from the top of the sister joist and 1 or 2 inches from the bottom of the sister joist. Old hard wood on existing joists can be tough to hammer through. If so, drill 3/8-inch pilot holes in the sister joist and drive in lag bolts to secure the boards together.
Remove the quick clamps once the sister is secured to the existing joist with nails or lag bolts.
For added support and firmness, sister joints can be added on either side of the existing joist. Blocking (adding a short length of framing lumber between joists) or cross-bracing (adding 2-by-4s in an "X" shape between joists) can also help stiffen joists and keep the floor level and bounce-free.