How to Repair Floor Joist Deflection

Updated May 25, 2017

Joist deflection refers to sags that sometimes develop in floor joists over time. Sagging joists will result in an uneven floor and often a bouncy floor. Getting rid of the deflection means bringing the joist back to level and adding stiffness with framing lumber or engineered wood like plywood, a process called "sistering joists." It takes several days to level a joist without causing damage to the home. It's also a labour-intensive job; adding stiffeners is heavy work, but the result is a more solid floor without sagging. The work is more difficult if a basement or crawl space affords little headroom.

Examine where joists will need to be sistered. Look for ductwork, electrical wires, cables, pipes and hangers. New joists may need to be cut to work around any of these fixtures, or the fixtures will need to be moved. Contact a plumber or an electrician to reroute pipes and electrical wires before proceeding.

Choose sister joists of the same height as existing joists. For example, a floor joist made from 2-by-10 material will require engineered lumber or dimensional lumber that is at least 10 inches wide.

Cut a length of 4-by-4 post with a saw, making it long enough to span three floor joists. Wear safety glasses when operating any power tools and when hammering.

Place two hydraulic jacks under the 4-by-4 post and position the post to span three joists with the middle joist being the one to be sistered. Raise the jacks a little at a time, one then the other, until the beam is snug against the bottoms of all three floor joists. To avoid cracks in plaster or drywall and damage to flooring, raise the joists just 1/8 inch per day. This may take a few days for large deflections. Check progress with a carpenter's level held along the 4-by-4 beam.

Apply a bead of construction adhesive to the existing joist where the sister will be added. With a tube of adhesive in a caulk gun, cut the tip with a utility knife and puncture the foil seal with a nail. Run the bead in a continual "S" pattern the length of the joist. The adhesive has a dual purpose: It not only bonds the two joists together, it also prevents squeaking since the two boards will not rub against each other.

Choose a sister joist that is the same size span as the original joist if possible. If pipes, wires and other obstructions make this impossible, choose a sister joist long enough to reach at least 3 feet on either side of the deflected joist. Lift the sister in place until it contacts the subfloor and apply quick clamps to hold it.

Drive 10d or 16d common nails through the two joist members with a hammer. Create two rows running parallel to each other, spaced a maximum of 16 inches apart. One row should be near the top of the joist, one near the bottom.

Remove the clamps after nailing the sister joist in place. Let the jacks down slowly, allowing the joists to settle into position.


Sister joints may be installed on either side of a deflected floor joist to give the existing piece even stronger bracing.

Things You'll Need

  • Framing lumber or engineered lumber
  • 4-by-4 post
  • Saw
  • Safety glasses
  • Hydraulic floor jacks
  • Carpenter's level
  • Construction adhesive
  • Caulk gun
  • Utility knife
  • 10d or 16d common nails
  • Quick clamps
  • Hammer
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About the Author

Robert Korpella has been writing professionally since 2000. He is a certified Master Naturalist, regularly monitors stream water quality and is the editor of, a site exploring the Ozarks outdoors. Korpella's work has appeared in a variety of publications. He holds a bachelor's degree from the University of Arkansas.