What is under your stair carpet is probably not a staircase

What a carpeted stair is actually made of

Plain utility grade treads, no nosing, and often a stringer at the wall that is nothing more than framing, because none of it was ever going to be looked at.

The nosing is the rounded front edge that a finished tread projects past the riser. A stair built for carpet does not have one, because the carpet wraps the edge and does the job. That single missing detail is the clearest way to tell from a lifted corner which stair you have.

The treads themselves are usually plywood or a soft utility grade. They are structurally fine and cosmetically not a floor. And the wall stringer, the diagonal board that closes the gap between the treads and the plaster, is frequently absent entirely on a carpeted stair.

So the honest answer to can we just sand it is almost always no. There is nothing finished there to sand.

The exception, and how often it actually happens

In the oldest housing here there sometimes is a real finished stair under the carpet, usually oak and occasionally heart pine, and it happens often enough to be worth checking and rarely enough not to plan on.

This is where the age of the stock matters. Our dataset at /data/ puts Philadelphia at 40.0 percent pre-1940, and that pre-1940 band is where original finished stairs turn up under carpet that went down in the 1970s.

When it happens it is the best outcome available, because a sound original stair can be refinished rather than rebuilt, and the difference in scope is large.

It is also the reason we look before quoting rather than after. Building an estimate around hoping for it would be dishonest; refusing to check for it would be lazy.

Where the code stops being a suggestion

Adding finished treads raises every step, and Pennsylvania allows no more than 3/8 inch of variation in riser height within a flight, which is exactly what a retread threatens.

Philadelphia adopts the 2021 International Residential Code as Subcode R of the Building Construction and Occupancy Code, and Pennsylvania amends the model: the maximum riser here is 8 1/4 inches, not the 7 3/4 inches the base IRC sets. Section R311.7.5.1 also limits variation within a flight to 3/8 inch.

That variation limit is what a retread runs into. Every riser rises by the thickness of the new material, except the top and the bottom, which change by a different amount because the floor above and the floor below did not move with them. Two steps out of fourteen change differently, and they are the two people trip on.

So the thickness gets decided before material is ordered, not discovered after it is fitted. It is one of the reasons stair work is priced by the stair rather than by the square foot.

The railing has its own numbers, in their own sections, and they are laid out with citations on our stair code page.

The moment to fix the squeaks

With the carpet off and before new treads go down, the structure is open, and it is the only moment when a squeak can be reached from above without dismantling finished work.

Most stair squeaks are movement between the tread and the stringer, or a riser separated from the back of the tread. Once a finished tread is on, reaching either means removing what was just installed.

So the sequence is carpet off, structure inspected and corrected, then finished material. A crew that skips the middle step delivers a stair that looks right and talks to you every morning.

It costs almost nothing while the stair is open, and it cannot be bought back afterwards. If you take one thing from this article, take that.