What Is Under Your Missouri Floor: Plank, Plywood or Slab
The subfloor decides more about how a new floor performs than the new floor does, and in a house this old or this new you can usually work out which kind you have in a few minutes.
Board and diagonal plank
Common in the prewar housing — which in Missouri means a great deal of St. Louis, the older parts of St. Joseph and Kansas City. Solid boards, often laid diagonally across the joists, frequently with gaps between them. They move individually with humidity, so a new floor laid straight onto them can telegraph every joint.
The usual answer is a plywood or hardboard underlayment over the top to give a continuous, flat, stable surface. That is a real cost and it belongs in the quote.
Plywood
The postwar standard and the easiest case. Sheets fastened across the joists, stable, and straightforward to lay almost anything over provided the flatness measures up and nothing is soft.
OSB
Common from the 1980s onward — the strand board that looks like pressed chips. Structurally fine and considerably less tolerant of water than plywood: once it has swelled at an edge it does not recover, and swelled edges telegraph through a new floor.
Slab
Basements, additions and much newer construction. Concrete brings its own question, which is vapor rather than movement: moisture travels up through a slab and has to be managed with a barrier or a suitable underlayment. It also brings the flatness question in sharper form, because a slab that was poured out of flat stays out of flat.
How to tell what you have
- Lift a floor vent or a threshold and look at the cut edge.
- Look up from the basement or crawl space — the underside of the subfloor is usually visible.
- Tap across the floor. Boards sound different from sheet goods, and both sound different from concrete.
- The build year narrows it before you look at anything: board and plank in prewar housing, plywood postwar, OSB from the eighties, slab in additions and newer builds.
Common questions
Does the subfloor type change what I can lay?
It changes what preparation is needed, which changes the price. Almost anything can go over almost anything once the substrate is right.
What if part of it is soft?
Fix it before the new floor goes down. It is cheap while the floor is up and expensive afterwards, and no surface material compensates for decking that flexes.