How to Level a Bowed Floor Joist

Measure the length of the bowed joist and cut a sister joist identical in size. Ensure the sister joist is as the same height as the bowed joist. For instance, if the joist measures 2 inches by 10 inches, ensure the sister joist is at least 10 inches in width.

Measure and cut the 4-by-4-inch post so it spans three joists, with the center joist being the one you want to repair.

Set the 4-by-4 post perpendicularly under the three joists. Place a hydraulic jack under each end of this post. Lift the jacks, in turn, to raise the post so it fits snugly against the undersides of the three joists. Because raising the joists suddenly may cause cracks in drywall or flooring damage, lift the joists by 1/8 inch daily. This process may take a few days, especially if the sag in the joist is severe. Hold a carpenter’s level flush along the 4-by-4 post to check the progress.

Insert a tube of construction adhesive in a caulking gun. Cut the tip of the tube with utility knife and insert the pointed tip of a nail through the opening of the tube to puncture the seal.

Position the caulk gun against any side of the bowed joist and run it in a zag-zag pattern all along the length. The adhesive holds the support joist to the bowed joist and prevents the floor from squeaking when someone walks over it.

Hold a sister joist next to the caulked joist. If obstructions such as ducts or pipe work make it impossible to run the sister joist all along the span of the damaged joist, ensure the sister joist runs at least 3 feet on each side of the bowed joist section.

Drive 16d nails through the sides of the joists to hold them together. Space the nails 16 inches apart along the length of the joists.

Let the two hydraulic joists down slowly, so the sister joist props the bowed joist up and flush against the subfloor.