What is under the carpet on your stairs

Carpet and underlay off, and underneath: hundreds of staples, tack strip holes at the back of each tread, a painted or bare riser, and a front edge that may be construction-grade lumber rather than anything meant to be seen.

What causes it

In rough order of likelihood. Each one has a tell — the observation that separates it from the others.

1. 1

   The stair was built to be carpeted

   How to tell: Treads in plywood, spruce, or poplar; risers in MDF; and no returned edge on the open side. Standard in almost anything built after about 1980.

2. 2

   A hardwood stair that was carpeted over later

   How to tell: Solid oak or maple treads with a proper nosing profile and a mitred return on the open side. Worth restoring, and more common in pre-1960 houses than people expect.

What it actually means

This is the fork in the road for every carpet-to-hardwood project, and it decides the budget. A stair built to be carpeted cannot be sanded into a finished stair — the material was never chosen to be seen. A hardwood stair that was carpeted over usually can be brought back.

Original hardwood treads: pull the staples, fill, sand, and finish — the least expensive good outcome available. Construction-grade treads: retread with solid hardwood, or rebuild. Anyone who offers to sand and finish a plywood tread is not going to be there in two years.

• Pull one tread's worth of staples and look at the wood at the back where nothing has worn. Species and grain tell you which case you are in.
• Check the open side for a mitred return. Its presence usually means a real hardwood stair.
• Photograph a tread end-on so the layers are visible. It answers the question faster than a description.
• Do not start sanding to find out. If it is construction lumber you have spent a day learning what a photograph would have told you.

• Squeaking or creaking stairs
• Loose or wobbly stair railing
• Loose, lifting, or cracked stair nosing