A hardwood floor is a thing of beauty and a joy forever - at least until it squeaks when you walk across it. Newly installed floors don't squeak much, because the boards are tight; but they can become noisy as the floor ages.
To understand why squeaks occur and what can be done about them, it's helpful to know a little about how hardwood floors are constructed. Today most wood floors are made of 3/4-inch thick, tongue-and-groove hardwood boards (called "flooring strips") that are nailed to a subfloor. You can't see the nails because they're driven at an angle through the tongue. Carpenters refer to this nailing technique as "blind nailing."During Victorian times builders used thinner boards - 5/8-inch thick - they anchored these by driving nails through the top of the boards (this technique is called "face-nailing") to the subfloor. In some areas this method of flooring continued well into 20th century.
Before 1960, builders used planks of 1 x 6 nailed at a 45-degree angle across the joists (the beams that support the floor). After 1960 they switched to using 1/2-, 5/8-, or 3/4-inch plywood or structural particle-board for the subfloor.
When a wood floor is first installed, it is a tight, seamless surface extending from wall to wall. During the humid days of summer, the wood absorbs moisture from the air and the floor expands. In winter, when the air is relatively dry, the wood releases that moisture and the floor contracts. This cycle of expansion and contraction causes the floor boards to loosen up and work away from the sub-floor.
In some parts of the United States, like the West and Midwest, the relative humidity remains fairly stable and the floor remains dimensionally stable. In the South-east and in states along the East Coast the humidity fluctuations are more intense. Homeowners in these areas have more floor problems.
You can't stop the expansion-contraction cycle, but by controlling the relative humidity in your home you can keep it within acceptable limits. Ideally the humidity level should be in the 40 percent to 60 percent range. This may necessitate running a humidifier in the winter and a dehumidifier in the summer.
A damp basement or a crawl space can allow moisture to penetrate a wood floor directly above and cause excessive expansion. Keep the basement dry by installing a dehumidifier. In the crawl space lay a vapor barrier of 6-mil polyethylene film over the ground. This barrier will keep ground moisture from entering the house.
These measures will help to keep a new floor trouble-free for a number of years, but eventually even the best floors shrink. Then the boards loosen up and rub against each other whenever someone walks on the floor. It's the friction between loose boards that produces the noise. You can eliminate some of the noise by lubricating the boards with graphite or talcum powder. Sprinkle the powder on the boards and work it into the cracks. This method will work for a while, but be prepared to lubricate the trouble spots periodically.
A more permanent solution involves securing the boards so they can't move. Usually the boards have warped and pulled away from the subfloor. Ideally it's best to re-anchor them from below. Of course, this is only possible with the first floor, where the subfloor is visible between the joists in the basement. First you have to pinpoint the loose boards. You can do this by accurately measuring the trouble spot above then transferring your measurements below. Another way to pinpoint the spot is by having an assistant generate the squeaks by pressing on the boards. You can listen below and trace the sound to its source.
A good way to anchor the boards is by driving woodscrews through the subfloor until they grip the boards above. Tightening the screws will pull those boards into place. The screw must be long enough to pass through the subfloor and grip the boards above. Assuming that the subfloor is 3/4-inch thick you can use a 11/4-inch screw (for a thinner subfloor, say, 1/2-inch thick use a 11/8-inch screw).
Start by placing a weight on the floor. This will press the boards against the subfloor. Working from below, drill pilot holes through the subfloor into the flooring strips (be careful not to drill through the floor). Squirt some wood glue into the holes so that it enters the gap between the floor layers. Insert the wood screws and tighten them so they pull the loose boards in place.