Fill the gap and stop the squeak! You can do this fix in minutes, as long as you can get at the floor framing.
Locate the floor squeak and inject polyurethane construction adhesive into gaps between joists and plywood. Ventilate the area while using this adhesive.
Push the bead of polyurethane construction adhesive deeper into the flooring gap with a plastic spoon instead of your finger. This adhesive is extremely sticky. Wear gloves when handling it and expect that any tools you use to work the adhesive will have to be cleaned with paint thinner, or in the case of the spoon, just thrown away.
Squeaky floors are one of those irritants that homeowners endure for a long time before undertaking the repair. There’s a whole hardware doodad industry devoted to solving this problem. Each set of floor conditions favors a particular remedy. Here’s one fix: When the floor framing in your house is accessible and there’s a slight gap between the top of the floor joists and the subfloor, try using polyurethane construction adhesive (about $4 per tube) to silence the squeak (Photo 1).
Inject the constructive adhesive into the gap and work it deep with a plastic spoon so it flows toward the nail shanks that are causing the squeaks (Photo 2).