A: You actually have a couple of options that don’t involve ripping out your flooring.
Floors squeak when various layers of wood have gaps between them and aren’t securely fastened. This causes the wood to slide up and down on the shafts of nails as someone walks across the floor, creating the piercing sounds.
Sometimes it’s not clear whether a floor squeak is coming from gaps between the finish flooring and the subfloor, or between the subfloor and the joists. But in this case, there is no mystery. The picture you sent confirms what you wrote: There is a visible gap between the joist and the subfloor.
Plugging this kind of gap with shims often solves the problem, provided it’s done correctly. The trick is to set the shims so they completely fill the gap but don’t go in farther. Especially when there are multiple shims, pressing some in too far can loosen up the others, defeating their purpose. So one option is to remove all of the shims and reset them carefully, with glue. Press them in by pairs, one on top of the other from each side of the joist, unless there is a gap only on one side. When you’re done, test each shim to make sure it is still snug. With a pencil, trace along the joist, onto the shims, to show just how far they are pushed in. Then take out the shims, keeping them in order. Squirt a bead of construction adhesive on both sides of the first shim or shims, and reinsert them just to the pencil lines. Repeat this process until you fill the whole gap. Then stay off the floor above until the adhesive cures.
Construction adhesive, sold in tubes that fit into a caulking gun, is the best glue for this. It fills gaps and remains slightly flexible after it dries, which helps it stand up to the repeated flexing that floors undergo as people walk across them. Try to get all the shims in place before the adhesive sets, or stop when it is no longer workable and resume after that adhesive is fully cured. The working time and cure time might not be what you expect, so read the label. Liquid Nails Heavy Duty Construction Adhesive, for example, stays workable for 20 minutes but takes 24 hours to cure ($2.57 for 10 ounces at Home Depot).
When a gap between a subfloor and a joist is as long and pronounced as the one shown in the picture you sent, there is another alternative: Rather than plugging the gap a few inches at a time with numerous shims side by side, screw a 2-by-4 or a straight piece of plywood to the joist so the top edge of the add-on fits tightly against the back of the subfloor. Make the piece about a foot longer on each end than the area with the gap, assuming you have space for this. If plumbing or wiring is in the way, make the support piece out of plywood — the strip can be up to as wide as the joist is deep — and cut notches as needed to avoid the obstructions. Hold the piece in place, and predrill for screws every foot or so, a few inches down from the subfloor. Drill at a slight upward angle to ensure a tight fit against the subfloor, but predrill only deep enough to go through the add-on. Take the piece down, and spread a bead of construction adhesive along the top edge and make a squiggly line of adhesive on the side that fits against the joist. Then reposition the piece and screw it in place. Use screws long enough to go through the add-on and one or 1¼ inch into the joist.
If the squeaks persist, perhaps the joist has so many knots or other defects that it flexes too much. Try stiffening it by adding blocking — framing that fits sideways between the problematic joist and one of its neighbors. Use lumber sold as 2 by 8 inches (actually 1½ by 7 1/4 inches), and cut it to the length that matches the gap between the joists. Check to make sure it fits snugly, then spread construction adhesive on the top edge. Press the blocking into place and secure with three-inch screws angled upward from the bottom edge of the blocking into the joists.
More from Lifestyle: