Old houses often have floors that bounce, and sag, and squeak. Even floors in new houses do it. That's because the flooring is not getting enough support from the part of the house that are underneath it.

you do not have to learn to live with such a problem. You can solve it.

It's fairly easy to diagnose a problem with a floor. Walk across it and then jump up and down. If the windows and the furniture rattle, if the floor feels like you are going to end up in the basement, you have a problem.

You also have a problem if the floor is not level. That can be tested with a carpenter's level or a rubber ball. Placed on the floor, the ball should stay in one spot, not start rolling, picking up speed as it heads for the corner.

If you know how a floor is put together you can understand both how it can sag or tilt and how you can solve the problem. Take a flashlight to the basement. Look up. You will see that the flooring is supported by joists, that are 2x8-inch wooden planks.

Each end of a joist rests on a sill plate. In old houses a sill plate is a large beam. The plate rests on the foundation wall. Heavy wooden beams called girders are used to support the joists. The girders also rest on the foundation wall and are sometimes supported by posts.

Additional strength is given to a floor by the way the material is laid on the joists. The subflooring, made of rough planks in old houses, and plywood or chipboard in new houses, is laid at right angles to the joists.

The finish flooring is laid at right angles to the subflooring. In new houses, carpeting is often laid directly on the subflooring. This sandwich of layers reinforces itself and makes the whole floor stronger.

Problems occur when any part of this framework is weakened. A floor may sag because the girder is not properly supporting the joists or because there is no girder. Installing a girder is a job that should be done by a contractor.

It is possible for the homeowner to substitute by putting a 4x4-inch beam under the joists where they sag. This makeshift girder is then held in place by jack posts, metal posts with an adjustable metal cyclinder, that work like the jack of a car.

Jackposts can also be used to support a girder that is not doing its job or to supplement posts that are already in place. In any case, the jack posts must be anchored to a concrete footing at least four inches thick.

If the basement floor is dirt, dig a hole one foot deept and one foot square and fill it with concrete. Set the jack post in the wet concrete. When the posts are-in place raise the jack very slowly, no more than an inch every three days. This allows the house to compensate for the movement and prevents plaster walls from cracking.

If the floor bounces, rather than sags, then the joists need to be strengthened. Plumbers and electricans are a common cause of weak joists. They drill holes and cut out parts of a joist to run cables, wires, or pipes. Each time part of the plank is removed, the wood becomes weaker.