How to Replace a Sinking Floor

Look under the sagging floor for damaged joists or rotting wood. The joists can be broken, curved or splitting. Rotting wood will be falling apart and dark in color. Place a level on the bottom of the joist to help locate any wood that is not level.

Attach a 4-by-6-inch beam perpendicular to the underside of the existing joists. Hold the beam in place and screw the beam to the joists with rafter ties.

Place a hydraulic jack under the floor where you have located the sinking. Place a 1/4-inch-thick steel plate underneath the jack at the floor and next to the wood to prevent sinking and to evenly distribute the weight. The plate should be 4 to 6 inches wider than the top and bottom of the jack.

Install one jack on one side of the beam and another jack on the other end of the beam. This will allow you to move all floor joists up at one time evenly.

Raise the jacks up slowly. You will need a helper to assist in raising the jacks at the same time. Raise the joist no more than 1/8 inch per day. Check the floor upstairs for any damage. Place a level on the floor so you know when you have raised the joists high enough.

Install a lally column, which is a steel-framed cylinder reinforced with concrete, to one end of the raised beam. Slide the column into place under the beam. Hold a level to the side of the column until the column is perpendicular to the floor. Install another lally column at the other end of the beam.

Nail a sister joist next to the sagging joist. Measure the joist and cut an identical two-by-four.

Nail the identical joist to the original joist. Apply construction adhesive to the ends of the joist. Clamp the sister joist to the existing joist and wait 24 hours for the adhesive to dry. Remove the clamps.

Lower the jacks slowly and remove them so the floor is now supported with the new beam, sister joists and lally columns.