Plywood Under Concrete — A Westlake Case Study

The homeowner called me out to look at a sunroom floor that was cracking apart. Not the little hairline stuff — real cracks running corner to corner, chunks starting to move under weight. Something underneath wasn’t holding it up anymore.

I’ve seen this pattern before and I had a hunch about what we were going to find once we opened it up. I was right — and it was worse than I expected.

The Floor Before We Started

Big cracks running across the slab, one of them right through the middle of the room. From up here it looks like a bad pour, or maybe settling. But cracks like this don’t come from the concrete itself failing — concrete can hold up plenty of weight when it’s sitting on something solid. These cracks happen when the thing underneath the slab gives out.

Plunge Cuts, Then a Sledgehammer

You can’t swing a saw the whole way through a slab in a room like this — too much dust, too tight to the walls, and the saw doesn’t need to. What I do is drop plunge cuts along the cracks and grid lines, then come back with a sledgehammer and let the concrete break where I told it to break. Cleaner demo, less noise, less dust in the house.

And There It Was — Plywood Under Concrete

This is the whole story right here. Whoever poured this floor originally didn’t fill the hole with stone or clean fill and pour on top like they were supposed to. They rigged up a plywood base, poured the concrete over it, and called it done. You find this a lot.

Here’s the problem: plywood rots. Give it a few winters of moisture cycling and the wood turns to mulch. Once the wood is gone, there’s nothing under the slab except air. Now the concrete is a bridge with nothing holding up the middle, and it starts to fail exactly like you see in the first photo.

No rebar. No wire mesh. No stone base. Honestly, they’re lucky the whole thing didn’t just drop out from under somebody.

Full Demo, All Hauled Out

Every piece of that failed slab out. No point trying to save any of it — it was cracked through and had nothing under it. Once the concrete was gone I could actually see what I was working with underneath, and how deep the void really was.

Over Two Feet of Nothing to Fill In

More than two feet from the top of where the new slab needed to sit down to solid ground. That’s the void the plywood was hiding. Nobody was ever going to see this — you could have walked on that floor for another five years before it fully collapsed. But the day it did, somebody was going to get hurt.

Three Yards of Premium Stone, Compacted in Lifts

Three yards of premium stone hauled in and compacted in lifts. You don’t just dump it in and pour on top — you have to work it down in layers, tamping each one so the base is dense the whole way. That’s what gives the slab something to bear on for the next 30 years instead of the next 30 months.

You’ll also see the rubber expansion joint I ran around the entire perimeter — the dark strip against every wall. In northern Ohio the concrete is going to move. Hot days it expands, cold days it contracts. If the slab is pinned tight against the walls with nowhere to go, it cracks. The expansion joint gives it that somewhere-to-go.

The Finished Floor

Poured, floated, and finished with a swirl pattern — good grip for a room you’re walking through wet or with pets, and it hides traffic wear better than a smooth trowel finish. The expansion joint tucks in clean at the wall so the slab can breathe without anyone noticing it’s there.

This floor is going to last. Not because the concrete is special — it’s the same mix anybody else pours. It’s going to last because what’s under it is compacted stone instead of plywood, and because the perimeter has room to move.

Why This Matters If You’re Buying Concrete Work

The homeowners on this job had no way of knowing what was under their floor. The original crew covered it up with concrete and the new owner inherited the problem years later. That’s the hardest part about concrete — the stuff that decides whether it lasts is the stuff you can’t see once it’s poured.

If somebody quotes you a concrete job and their price seems too good to be true, ask what they’re doing under the slab. If the answer is anything other than compacted stone or clean fill in proper lifts, walk away. Plywood, dirt clods, construction debris, even loose gravel that never got tamped — all of it fails. Compacted stone is the only right answer.

And in this part of Ohio, the expansion joint isn’t optional. Anybody quoting an interior slab that’s butted tight to the walls with nothing between concrete and drywall doesn’t understand what the winters do to concrete. Move on.