There is one failure that ruins an otherwise perfect floor coating, and it has nothing to do with the coating.
Water comes up through the concrete, collects underneath the film, and pushes it off. The floor looks flawless for a year, sometimes two, and then blisters or lifts in sheets. Every step of the installation can have been done correctly and it still happens, because the problem was in the slab before anyone arrived.
Concrete is not waterproof
This surprises people, and it is the whole basis of the problem. Concrete is porous. A slab poured directly on grade, without an intact polyethylene vapour barrier beneath it, is in permanent contact with the ground and wicks water up through itself continuously. That water leaves as vapour at the surface — invisibly, constantly, forever.
An uncoated slab does not care. The vapour evaporates off and nobody notices.
Put a sealed, impermeable coating on top and the vapour has nowhere to go. It accumulates at the bond line between concrete and coating, and vapour pressure is more than sufficient to break that bond. The result is blistering, then peeling, usually starting in the middle of the floor rather than at the edges.
Which slabs in this area are at risk
Four categories, and between them they cover a lot of South Georgian Bay housing stock.
Wasaga Beach, and sandy shoreline ground generally. The town sits on sand, close to both the bay and the Nottawasaga River, and the water table is high and mobile. Sand drains quickly at the surface, but a high table means there is often water not far under the slab regardless. This is the single highest-risk area we work in.
Shoreline property near Collingwood and Meaford. Different soils, same consequence — a table sitting close to the underside of the slab.
Anything poured before roughly 1980. Under-slab vapour barriers were not standard residential practice for a long time. The older streets around the Collingwood core, the village lots in Thornbury and Clarksburg, and rural outbuildings across Clearview are full of slabs that simply never had one.
Cottage conversions. A great deal of housing in Wasaga Beach and along the shoreline began as seasonal cottage and became year-round housing in stages. Garages and additions came with it, built at different times to different standards, sometimes poured in sections. Both the barrier and the thickness are a lottery.
The tests
The screening test you can do this afternoon
Tape a piece of clear polyethylene, about 18 inches square, flat to the concrete. Seal all four edges with tape so it is airtight. Leave it 24 to 72 hours, then look.
- Condensation on the underside of the plastic, or concrete that has visibly darkened beneath it: vapour is moving.
- Bone dry, concrete unchanged: encouraging, but not conclusive.
This is ASTM’s old plastic-sheet method. It is genuinely useful as a first look and it costs nothing. What it will not do is give you a number, and the decision between “coat it normally” and “coat it over a mitigating primer” needs a number.
Do it in more than one spot. Moisture is rarely uniform across a floor — the middle of a slab is often wetter than the perimeter, and the area against an exterior wall on the uphill side of a lot is frequently worse than the rest.
The tests that produce a number
Calcium chloride (ASTM F1869). A pre-weighed dish of anhydrous calcium chloride sits sealed under a dome on the bare slab for 60 to 72 hours and is weighed again. The result is a moisture vapour emission rate in pounds per 1,000 square feet per 24 hours. It measures the top half-inch or so of the slab.
In-slab relative humidity (ASTM F2170). A probe is set into a drilled hole at 40 percent of slab depth, capped, and left to equilibrate. It reads the humidity inside the concrete rather than at its face, which makes it the better predictor of what happens after you seal the surface.
We run one or both before quoting, not after. The result changes the quote — a slab over threshold needs a mitigating primer at $2 to $4 per square foot — and finding that out afterwards means either an awkward conversation or a floor that fails.
The other signals we look for
Before any test, a walk around the floor tells us a lot:
- Efflorescence. White powdery mineral deposit. Water moved through the slab, evaporated, and left its dissolved salts behind. Direct evidence of transport.
- Dark patches that never dry. Particularly in the centre of a slab or along one wall.
- A previous coating that failed. If something was on this floor and let go, we want to know where it lifted and what the underside looked like. Clean concrete on the back of a peeled flake means it never bonded; concrete dust and mineral bloom means moisture pushed it off.
- Musty smell in an enclosed space. Especially in a basement.
- The lot itself. A garage cut into a slope, with ground uphill on one side, is receiving water that a flat lot is not.
What we do about it
Test under threshold: coat normally.
Test over threshold: a moisture-mitigating primer — typically a high-solids epoxy formulated to tolerate substantially higher vapour pressure — goes down first, and the system is built on that. It works, it is well-established, and it is a real cost rather than an upsell.
Active water, not vapour: this is a different problem and a coating is not the answer to it. A basement that takes water in the spring, or a garage with standing water after a thaw, needs the water dealt with first. Coating over an active water problem hides it for a season and then fails, and you have paid for the coating twice.
That last case is the one where we will tell you not to hire us yet. It costs us the job. It is also the reason the warranty on the floors we do install is worth something — it is issued against a written moisture reading, and if we did not take one there would be nothing behind it.