So that squeaky floorboard makes it hard to walk to the bathroom in the middle of the night without waking the entire household? Then you've got to fix it. The cure for an annoying floor squeak depends on what layer is squeaking, what type of finish the flooring has and whether you have access to the floor from below.
Locate the squeak from below by looking for any gap or movement of the subfloor when someone above steps on the squeaky area. If you notice either, you have a couple of options: Spread construction adhesive on a tapered shim and tap it into the gap, just until it's snug (too tight a fit will make things worse). Or put a bead of adhesive on the top edge and one face of a short 2-by-4 with a caulking gun, then fasten the 2-by-4 to the joist, angling 12d common nails upward so you draw the board tight against the subfloor.
If the subfloor is tight under a nailed-down hardwood floor, put as substantial a weight as you can, 200 lbs. (90 kg) or more, directly on the squeaking floorboard(s). From below, drill numerous closely spaced clearance holes, 1 to 2 inches (2.5 to 5 cm) apart, through the subfloor and drill pilot holes into the hardwood flooring. Drive in an appropriate-length sheet-metal screw, 1/4 inch (6 mm) shy of the total thickness of the wood.
Locate the joists. Typically this is easier to do from below. Tap the ceiling while listening for a dull (versus a hollow) sound; use an electronic stud finder or drive exploratory nails to locate one joist. Then, assuming a 16-inch (40-cm) center-to-center joist spacing, locate the others.
Measure the distance from the wall to the center of the relevant joist(s) downstairs. This measurement must be very accurate. Use it to mark the joist location on the floor upstairs with masking tape.
If you have hardwood floors, concentrate a very heavy weight on the loose board, then drive a spiral flooring nail at an angle through the floor and into the joist; or use a special fastening system called Counter Snap, in which the screw breaks off just below the surface for a nearly invisible repair (see Tips).
Drive in additional fasteners as needed; countersink the nails with a nail set. Fill the holes with a color-matched wax-putty stick or crayon, then buff off the extra wax with a dry cloth.
If you have carpeting installed over an underlayment, use a special fastening system called Squeeeeek No More (see Tips). Its screw also snaps off just below the floor surface, so you can install it right through carpeting without damage.
Sheet-metal screws work well for tightening floors from below because the coarse thread grabs well and the head does not allow it to go in too deeply. Spiral flooring nails have superior holding power, though they're not quite as strong as screws. Squeeeeek No More and Counter Snap are unique systems made by O'Berry Enterprises. You can reach them at (800) 459-8428 or www.oberry-enterprises.com.
Don't screw up into a finished floor until you have determined the thickness of the subfloor, any underlayment and the finished flooring.