How a wooden staircase is put together, and why stairs creak

A staircase feels like one solid object, but a traditional timber stair is really a precise joinery assembly held together by wedges and blocks, not nails. Knowing how it goes together is the quickest way to understand the most common complaint people have about stairs, which is the creak.

Housed strings and wedges

The two strings, the raking boards up each side, are routed on their inner faces with a shallow tapered housing for every tread and every riser. The treads and risers are slid into those housings from behind, and then a glued, tapered hardwood wedge is driven in behind each one to force it tight against the front of its housing. The wedge is what actually holds the step: it is not the timber sitting in the groove that keeps it firm, it is the wedge locking it there. That is why a stair is built and wedged with the back, the underside, open to work on.

Glue blocks and the tread-to-riser joint

Where each tread meets the riser below it, there is an internal right angle, and that joint is stiffened with glue blocks: small triangular or square blocks glued into the angle so the tread cannot flex down and away from the riser. The tread and riser are usually also tongued or screwed together at the front. Between the wedges holding the ends and the glue blocks holding the angle, a well-made stair has no joint free to move, and a joint that cannot move cannot creak.

So why do stairs creak?

Almost always because one of those joints has started to move. A wedge shrinks a little or works loose, so the tread can drop a hair and rub as you step on it. A glue block lets go, so the tread flexes against the riser. Or a tread and the riser behind it rub where they meet. It is nearly always movement at a joint rather than the timber itself failing, which is also why, on a stair you can get underneath, a creak can often be traced and the loose wedge or block re-fixed. A seasonal creak that comes and goes with the heating is a slightly different story, tied to the timber gaining and losing moisture, which is covered separately.

Why it is a factory job, not a site job

This is the case for a stair being properly made and dry-fitted before it ever reaches your house. Wedging and blocking a stair tight is workshop work, done with the assembly open and accessible. A stair that has been dry-fitted and correctly wedged and blocked arrives quiet and stays quiet. One rushed or fixed down to a price is the one that starts talking to you a year later. There is more on this in what a good staircase fit looks like and what you are actually getting.