No one likes the feel of a supposedly solid floor bouncing or sagging underfoot. A springy or spongy feeling could indicate a minor issue, a major problem or no problem at all. Once you identify the problem, you might be able to fix it yourself. Some corrections, however, require the services of construction and engineering professionals.
A spongy feel underfoot might indicate that the subfloor is not securely attached to the floor joists. This condition allows the floor to give slightly when you step. If the problem is beneath carpeting, you can roll back the carpet and pad, insert 2-inch decking screws 8 inches apart in the subfloor over each joist and then reinstall the carpeting. If the problem is under a linoleum floor, you can’t run screws into the flooring surface or you’ll damage the linoleum. If you can access the joists from the crawl space, fill the gap between the top of the joists and the underside of the subfloor completely with subfloor adhesive and then place heavy weight on the floor above for at least 48 hours. If that doesn’t fix the problem, the next step is to screw the subfloor down from above and install new flooring.
Overextended Joist Span
Floor joists that span a distance greater than they should can also cause a springy feeling when you walk across the room. Because the floor joists are part of the house’s structural weight-bearing system, consult an engineer before making alterations. One of the simplest ways to fix the joist span is by “sistering” the floor joists from beneath. This involves cutting new joists and attaching them to the existing joists to increase their stiffness factor. Another method is to install bridging between the existing joists. This entails cutting dimensional lumber to fit perpendicularly between the existing joists. The “bridges” run from one end of the joists to the other, usually in the center of the joist span. When you walk on the floor above, the bridging transfers your weight to the adjacent joists and somewhat stabilizes the bounce. A last resort, but perhaps the best solution, is to install a perpendicular load-bearing beam in the crawl space. The location, beam size and method of supporting the beam must be determined by a structural engineer.
Floor trusses bounce; there’s just no way around it. Floor truss manufacturers call it “deflection,” and they claim it’s normal. Unlike floor joists, which are solid, trusses are engineered with smaller members in a triangulated design that allows the relatively lightweight trusses to bear as much or more load as traditional floor joists. Any alterations you make to a floor truss system can void the manufacturer's warranty and can conflict with the system's pre-engineered load distribution. Because the floor’s not going anywhere, the best solution is to get used to the bounce. The only alternative is to ask the truss manufacturer for an engineered support solution, but in most cases you’re unlikely to receive one.
Subflooring typically consists of plywood or oriented strand board (OSB) sheets. Both of these products make solid subfloors, but if they become saturated, through leaking plumbing or flooding, the glue that holds them together releases. Called “delamination,” this causes the sheet to swell and become spongy underfoot. The only solution for delamination is complete replacement of the damaged subfloor sheets. Once delaminated, plywood and OSB lose their structural integrity and are no longer safe or effective.
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