How to Fix a Sagging Joist


Slightly sagging floor joists create an incredible nuisance, causing floors to creak and furniture to rest unevenly on the floor. Many homeowners consider repairing sagging joists to be beyond their skill level, so they either live with the problem or call a contractor. Although in some cases a sagging joist may be a sign of a greater problem requiring professional attention, some slightly sagging joists can be repaired by installing a simple support wall in the basement. However, this fix is only recommended for single story homes -- raising the floor in a two-story home requires more equipment and expertise.

Things You'll Need

  • Measuring tape
  • 4-inch by 6-inch lumber
  • Circular saw
  • Metal joist straps
  • 3-inch wood screws
  • Screwdriver
  • 2-inch by 6-inch lumber
  • Rubber mallet
  • 3-inch galvanized nails
  • Hammer
  • Identify the sagging joist by measuring the distance from the walls to the low point from the inside of the room, and then from the underside of the floor. Ask a helper to tap on the low point of the floor with a hammer so that you can confirm the location.

  • Cut and install a 4-inch by 6-inch beam perpendicularly across the sagging joist, so that the beam spans the distance between the joist and its adjacent neighbor on either side -- use joist hangers and wood screws to secure the beam into place.

  • Cut six 2-inch by 6-inch brace beams approximately one foot longer than the distance between the basement floor and the bottom of the joist. Wedge a brace beam into place under the sagging joist on either side of the low point -- set the brace beams on opposite sides. Use the rubber mallet to hammer the bottom of the brace beam firmly into place against the concrete floor, with the upper end supporting the joist. Install additional braces to support the adjacent joists. Check the floor to see if it the creaking or sagging has been eliminated. If not, continue to hammer the braces into place more tightly each day, slowly raising the floor joist.

  • Cut two 2-inch by 6-inch boards to the length of the 4-inch by 6-inch beam attached to the bottom of the joists to serve as the base and top plates of the supporting wall, and cut one additional board three inches shorter to create a cross support. Measure the distance between the bottom of the beam and the floor, subtract three inches, and cut two boards to this length to form the vertical supports for the supporting wall.

  • Frame out the supporting wall that will hold your raised joist in place. Nail the base and top plates of the supporting wall to the vertical supports with galvanized nails. Nail the cross support between the vertical supports halfway between the top and bottom plates.

  • Wedge the supporting wall into place, so that the top plate rests against the underside of the beam you installed across the joists. Tap the bottom of the wall into place with the rubber mallet. Nail the top plate of the supporting wall to the beam across the joists.

  • Remove the supporting braces slowly by tapping them with the rubber mallet, allowing the sagging joist and its neighbors to rest on the new supporting wall.

Tips & Warnings

  • If your home does not have a basement, use a piece of 1-inch thick plywood to support the brace beams. You will also need to install a concrete pier to serve as a base for the new supporting wall.
  • Raising even a small section of the floor of a two-story house is a much more serious undertaking that will require hydraulic jacks and support posts. (See References 1.)
  • If the entire floor slants steeply from one side to the other, your house may be settling or subsiding--consult a contractor for help raising the house.

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