How to Repair a Sagging Floor

When a floor sags at any point or feels bouncy when you walk over it, part of the understructure is probably weak. This presents a potentially dangerous condition that should be corrected without delay. Because the condition usually is found on the lower floors of the house, thereby providing you with ready access to the understructure, you can often make the necessary repairs and adjustments yourself.

The cause of the sagging may be that the joists are spaced too far apart to provide rigid support for weakened subflooring. One or more joists may have warped or sagged. The beam that supports the joists may not be making contact all the way across.

Use a long straight edge and level as you inspect the subflooring, joists, and supporting beams. If a joist has warped or sagged but is otherwise sound, you can raise the floor by driving hardwood wedges between the joist and subflooring. Additional sup¬port is gained by toenailing a sturdy cross-piece between the joists under the floor.

Check to see that all joists are resting on the main beam. It often happens that a beam sags at the center-particularly if it is made of wood. Support posts may be spaced too far apart, or they may have buckled or rotted or sunk into the concrete floor. Or they may be absent altogether, in which case you will have to add one or more new posts.

Adjustable metal posts that have a screw-type jack at the end can be used to raise the beam into position. These can be left in place as permanent supports, if need¬ed. The base of the jack must rest on a solid cement footing. If the cement floor is less than 4 inches thick or shows signs of deterioration, you must put in a new footing.

Because of potential damage to the frame and walls of the house, you must never attempt to raise the beam more than a slight amount at a time. Put the jack post into position under the beam so that it just touches. Make sure the jack is perfectly vertical, then give the screw a half turn and stop. Wait several days to a week, and then give it another half turn. Do not be in a hurry - chances are it took quite a while for the beam to sag that much, so it can wait to be straightened out. Continue a little at a time until the floor overhead is level, then either insert a chock between the existing post and the beam or leave the jack post in position permanently.

You can check the level of the floor with a long straightedge as you go along. Or you can tack a string from wall to wall across the floor above the sag. If the string is kept taut, you will be able to observe the distance between it and the floor as this distance gradually diminishes.