Q. As part of a remodel, I have to replace the carpet on a floor framed with 2x10s on 12-inch centers. The joist span is 20 feet, and the floor bounces so badly that things on the table shake when someone walks through the room. Is there a good way to fix this problem before recarpeting?

A.Dan Dolan, P.E., Associate Professor of Wood Engineering at Virginia Tech, responds:A possible solution, which I have used successfully in several cases, is to add a layer of 3/4-inch or thicker OSB over the entire floor, glued and screwed 6 inches on-center in both directions. Make sure the screws are long enough to fully penetrate the new OSB and the underlying subfloor. When possible, using longer screws to penetrate the joists will help some, but the important thing is to make sure the new subfloor is well "clamped" to the existing. Stagger the joints with respect to the joints below. The main problem with this approach is that you gain an inch or more of floor height, which may not be acceptable.

This technique works for one of two reasons, depending on the individual floor. One is that the extra OSB increases the stiffness of the floor, making it vibrate at a higher frequency that is not perceived as annoying by most people (For more on floor vibration, seePractical Engineering, 11/98). In some cases, however, the increased mass of the OSB actually lowers the frequency of vibration, but it doesn’t tend to matter because the increase in mass also increases the inertia of the floor. In other words, the heavier floor is harder to start vibrating, so the problem goes away.