How to Build a Permanent Support for a Sagging Floor in the Basement

Identify sags or irregular slopes by putting a 4-foot level on the floor or by stretching a string across the floor with a line level on it; either technique will show any changes from side to side. Test the flooring in all directions, although the most likely sagging will be along floor joists. Mark the biggest deflections or sags on a rough sketch of the floor plan.

Inspect the joists in the basement for obvious signs of damage; a cracked or broken joist may have to be removed, which may require professional assistance. Use the level or level string line to mark each joist with the angle of sag and the end points where the deflection begins and ends.

Use floor jacks to raise the joists to level. Set the base of the jack on the floor with the top under the joist. Raise it slowly, by turning a screw top with a wrench or with a handle built into the jack. Work slowly and from each side of the sag, with multiple jacks if necessary; don't try to raise a sag in the center with a single jack all at one time. Lift the center of the sag slightly, then raise jacks on either side, working across the joist from end to end. Lift no more than 1/4-inch on any jack at any time. Take your time; a floor that took years to sag can't be restored in minutes, but can be achieved in a day or two.

Keep alternating jack raises until the bottom of the joist is level with the level string line. Start in the center of the floor and work in both directions; a serious sag in two directions may require many jacks to lift several joists to restore the floor to level in all directions. Run a second level string line perpendicular to the joists if the floor sags in two directions.

Check upstairs areas periodically to ensure the lifting is not causing cracks in the walls or shifting doors and windows. Stop and review at any sign of problems; it may not be possible to raise floors to absolute level without creating problems in the wall framing.

Set 4-by4-inch posts under the restored joists once a satisfactory level is reached. Put the bottom of a post in a metal post bracket and slide it in place under the joist; raise the jack at that point slightly to let the post fit in, then lower it to sit on the post. Fasten the joist to the post with 16d framing nails, toenailed diagonally with a hammer.

Install multiple posts if necessary to support a major sag; most floors, however, can be restored with one or two permanent supports in the center of one or two joists. Inspect connections between joists and sill plates around the foundation to ensure the lifting has not loosened any nails or caused any shifting in other foundation and framing.