How to Fix a Squeaky Floor Caused by Gaps between the Subfloor and Joist
Squeaky floors can be extremely irritating. Fixing a squeaky floor caused by gaps between the subfloor and joist is easiest if you have a basement or an access space. Floors get squeaky when wood dries out (either finished wood or a wooden subfloor), causing the subfloor nails to pop loose and the subfloor to separate from the floor joist.
Gather your materials: Chalk, shims, hammer, screws or nails.
You also need a helper who weighs enough to make the floor squeak. A pipsqueak of a kid may not weigh enough to create a squeak.
Have your helper walk across the floor looking for squeaks.
Go to the basement under the room where the floor squeaks. Have your helper walk across the floor and stop when they hear the squeaking.
Mark the spot with chalk.
Your helper may have to bounce up and down a couple times until you identify the exact location from below.
Hammer in a shim between the joist and subfloor where you see gaps.
Look for gaps between the subfloor and the floor joist; drive a shim between the joist and subfloor with a hammer. You might need to put a couple of shims together to fill the space tightly.