Plywood and OSB

Most homeowners have never actually looked at their own subfloor and have no reason to have, since it spends its whole life covered by something else. That is a shame, because it takes almost no effort to identify, and knowing what is under a room tells you something real before a new floor ever gets discussed.

Look first: telling them apart at an exposed edge

Find an exposed edge. A basement ceiling with the subfloor visible from below, an unfinished closet floor, an attic hatch that cuts through the panel, or a rim joist at the edge of a room are the usual places. Plywood shows itself immediately as thin, distinct layers stacked on top of each other, the classic striped edge everyone recognizes from a cut piece of lumberyard sheeting. OSB reads completely differently: a face covered in visible wood strands and flakes, and an edge with no separate veneer plies in it at all. The strands themselves are not random, whatever the surface suggests. They are laid in cross-oriented layers, the face strands running along the panel's long dimension and the core strands running across them, which is what the word oriented in oriented strand board refers to and what lets the panel carry a span rating in the first place.

Somewhere on that same panel, usually stamped in ink near an edge, is a grade stamp. It carries a mill or association mark, a thickness, and a span rating written as two numbers separated by a slash, which tells an installer how far apart the framing underneath can be for that specific panel to perform as designed. You do not need to decode every mark on it. Its presence and legibility alone tell you the panel was manufactured to a published standard rather than improvised, and that stamp is the only thing on the panel that attests to anything at all.

What that tells you

Plywood's layered construction, with the grain alternating ply to ply, gives it slightly more consistent strength in both directions and a modest edge advantage: a cut or exposed edge on plywood tends to hold a screw a little better and resist swelling a little more than the same situation on OSB. OSB's cross-oriented strand construction makes it denser and more uniform panel to panel, and it is the more common choice in new residential construction today largely on consistency and supply. Installed dry, fastened correctly and kept dry afterward, both perform as a subfloor essentially the same way, and neither one is a reason by itself to be concerned about a floor. Both show up under houses of every age, and which one is under yours takes a look rather than a guess.

What the stamp does not tell you

A grade stamp describes how the panel was manufactured. It says nothing about the condition it is in today. It cannot tell you whether a past leak left a soft, delaminated spot somewhere in the room, whether the fasteners holding it to the joists are still tight, or whether the panel has cupped slightly at a seam. Reading a stamp is an identification, not an inspection, and the two are not the same exercise even though they both start with the same exposed edge.

The spring test, and what it actually catches

Walk the room slowly, in socks, paying attention rather than just crossing it. A well-supported panel over properly spaced framing should feel solid and consistent underfoot. Noticeable bounce, give, or a soft feeling concentrated in one area, especially near a wall or a seam, is worth paying attention to. It can point to undersized framing spacing for that panel, a fastener that has let go, or old water damage nobody addressed. It is a red flag worth mentioning, not a diagnosis on its own, and what it usually means and how it gets fixed is covered in full on its own page.

What none of this tells you

Identifying the panel and walking it for soft spots is a useful, honest self-test, and it stops there. It does not tell you whether the subfloor is flat enough for the specific material you are considering, whether a moisture reading matters in that room, or whether anything needs to be leveled before installation. Those are separate questions, covered on the concrete slab and moisture barrier pages when the subfloor in question is over a slab, and answered directly by a professional look at the free in-home measure either way.

What to bring up at the measure

None of this has to be right for it to be useful. Say what you found: where the exposed edge was, whether that edge showed stacked plies or pressed strands, whether a stamp was legible, whether any part of the room felt softer than the rest, and whether there is access to the underside from a basement or a crawlspace. That is a far better starting point than a guess, and it changes what gets looked at first. The same instinct is worth applying to what goes on top, which is why the samples in the Wentzville showroom sit on the floor rather than on a wall. Our installation process puts the subfloor check ahead of the order rather than after it.