squeaks occur when subfloor separates from joists
fit scrap wood against floor joist to fix squeak
Fasten a Piece of Wood Against the Floor Joist
Squeaks are caused when the subfloor begins to separate from the floor joists. The nails squeak as they slide in and out of the joists.
To fix squeaks caused by large gaps from beneath the floor, fasten a piece of scrap wood against the floor joist so that it fits snugly against the subfloor. The wood will support the subfloor, preventing it from moving down to the joist.