A wooden staircase often has two different kinds of problems as it ages and the wood dries out. The stair treads will either bow or cup. In either case, they can make for annoyingly squeaky stair steps that will eventually need to be replaced.
Bows or Cups?
A bowed step is a step in which the stair tread has dried out and the step has turned into what is basically an upside-down cup. The center of the stair rises upward and the edges of the stair turn downward. A cupped step is just the opposite. As the wood of the tread has dried out, the center of the step has dropped down with the edges of the stair rising upward.
The way to determine what type of wooden staircase damage you have is to actually step down hard on the stair at various places and listen carefully for the tell-tale squeak that indicates a dried tread. If the step squeaks when you step on the edge of the tread, then you have a cupped tread. If, on the other hand, the step makes a noise when you step heavily in the center of the tread, then you have a bowed step.
There are two ways to fix a bowed or a cupped step that squeaks. You can shim the step so that it does not rock or squeak. If the problem is relatively minor, you can make an easy repair to that single step. In many cases, you will only need to make a repair to the step that is squeaking, rather than replace the entire stair tread.
To fix a squeak you will need a small thin screw. Pre-drill a hole into either the center (bowed tread) or two holes – one at the back and one at the front of the tread (cupped tread). Screw in and countersink the screw, then fill in the top with a touch of wood putty. Sand the putty to blend it in with the stair.
Some stairs are so old and dried out that sinking a few screws in them to even out the stair tread will not work. In this case, the reason the stair is squeaking is more likely because the tread is actually rocking back and forth as weight is placed on the stair. Very often, if this is the case, you can actually look closely at the stair and see a gap between the stair tread and the riser. If you discover this, then you will need to place a wooden shim in the gap to stop the rocking. In very bad cases, you may need more than one shim to accomplish your task. Once you have the shim(s) firmly in place, the squeaking problem should be addressed.
Squeaky wooden stairs are usually an indication of wooden stair treads that have either cupped or bowed and make noise when weight is placed on the stair. Replacing a bowed or cupped stair is not usually needed. Instead, determine what kind of problem you have and either use screws to even out the wood tread or shims to fill any gaps that might exist in extremely dry wood stair treads.