How to Level a Bowed Floor

Measure and cut a sister joist the same dimensions as the bowed joist, using a handsaw. If pipes, ducts or other obstructions prevent the sister joist from running the entire length of the bowed joist, cut it so it runs 3 to 4 feet on either side of the bow at the least when installed.

Extend the measuring tape perpendicularly along the span of three floor joists, with the bowed one in the middle, and calculate the length. Measure and cut a 4-inch-by-4-inch post to that length with a saw.

Set a hydraulic jack under each corner of the post and position this assembly under the three joists. The middle joist is the bowed one you want to repair. Crank the hydraulic jacks so the top of the post falls just below the joists. Set the sister joist alongside the bowed joist and continue to crank the jacks until the post falls flush against the undersides of the joists.

Crank the jacks up by just 1/8 inch every day so they gradually straighten out the bowed joist. Because the lifting process is gradual, it will not cause cracks in the flooring or drywall. Periodically hold a level on the floor surface to determine whether the bowed joist is level, so you know when to stop.

Attach the sister joist to the middle floor joist when it is leveled. Insert a tube of construction adhesive to a caulking gun. Insert the tip of a nail through the end of the tube to break the seal.

Spread the adhesive to any side of the middle floor joist in a zigzag pattern, until the entire length is covered.

Press the sister joist next to the glued side of the middle joist before hammering a series of 16d finishing nails through the sides to secure them together, spaced 16 inches apart. Ensure the nail heads are flush with the surrounding wood surface.

Lower and remove the two hydraulic jacks so the sister joist supports the middle joist.