A squeak in a stair is generally caused as the lumber dries out over time. Hardwoods are regulated by industry standards for a moisture content of 6-8%. This matches the moisture content inside the average home and allows the wood to remain stable under normal conditions.
Pine (grades used for basement stairs and stairs to be fully covered with carpet) has a typical high moisture content of 12-16%. Over time as pine shrinks built stairs will begin to squeak.
If your stair is squeaking, check the humidity level in your home as a first step. Verify where the squeak is coming from. You may hear a squeak walking up the stair, but it could be coming from the framing material rubbing against the stair, and not the stair itself.
You can typically fix the squeak, as simply as "toe nailing"* a long nail or two in the area that is squeaking. That is not to say more squeaks won't develop over time, as the lumber continues to dry out to equal the humidity level of your home.
*Toenailing: driving a nail at an angle into a board. It makes a strong joint while it also pulls joining boards into position.
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