When a wood floor makes squeaking, creaking or groaning noises every time someone walks across it, chances are the problem is due to one or more loose or warped boards that move up and down when stepped on.

It is the edge of one board rubbing against the edge of another that usually causes the noise, although it may also be caused by partially loosened nails. As the loose board moves up and down it tends to rub against these nails to create a kind of squeaking or groaning sound -- this is often the case when the squeaks occur in a plywood floor, or in plywood subflooring.

Whichever defect is causing the problem, to cure it permanently you will first have to locate the troublesome boards (the ones that are loose or warped), then fasten them down securely so that they can no longer move or flex when stepped on.

To pinpoint the trouble spots, have someone walk over the floor slowly and in different directions until the noise is heard.

If the floor is over a basement with an unfinished ceiling it is often easier to work from below while your "assistant" walks around on top of the floor above you. If possible, narrow down the area where the noise originates by stepping off and on suspected spots with one foot until you have located the specific board -- or the joint between two boards -- that is the source of the trouble.

Once the troublesome boards have been located the next step is fastening them down or bracing them so they will not move up and down when weight is applied. Most wood floors actually consist of two layers of wood flooring: a bottom layer of plywood or wide boards that run diagonally across the joists, and a top layer of narrower finished flooring that is almost always tongue-and-groove boards that often run parallel to the joists (though not necessarily so). Squeaks can be caused by loose or warped boards in either the subflooring or the finished flooring -- or both.

When the finished flooring is loose, the easiest way to fasten the loose boards down after you have located them is to work from above, driving nails down from the top. For maximum holding power use serrated or "threaded" nails, because these grip much better than ordinary nails.Hammer them in at an angle of approximately 45 degrees, slanting them in opposite directions.

To avoid damage to the flooring, use a nailset to finish driving the nails for the last inch or so and to set the nail-head slightly below the surface. The holes that remain will be scarcely visible if you have used finishing nails, but if the holes are objectionable they can be filled with wood plastic tinted to match the finish of the flooring.

When the floor is over a basement with an unfinished ceiling, a neater repair can be made by working from below in most cases. Working from below may also be advantageous when the floor above is carpeted or covered with vinyl flooring.

If the problem is the simple one of finished flooring that has lifted up from the subflooring, the loose boards can be secured from below by driving screws up from the bottom. Have someone stand on the floor directly over the raised spot to hold the boards down, then drive screws up from the bottom through the subflooring. The screws should be long enough to penetrate about three-quarters of the way into the finished flooring after passing completely through the subflooring.

When both layers of flooring have buckled upward -- usually because the subflooring has raised up or worked loose and pushed the finished flooring with it -- nailing from above will not always do the trick. Nails will not pull the flooring down hard enough.