Water in concrete
How does moisture in a slab affect a floor coating?
Concrete on grade moves water up through itself, and the pressure of that vapor under a coating is what lifts the coating off the slab.
The short version
- Concrete poured on the ground carries water up through itself and gives it off at the top.
- Vapor pressure under a coating is what lifts it, and it shows up as blisters rather than as wear.
- A slab is checked with a moisture test taped to bare concrete. It cannot be judged by eye or from a photograph.
- A slab that reads high needs a different system, and that shows up in the price. It does not always rule out a coated floor.
- A basement that has never had standing water in it can still read high.
Where the water comes from
A slab poured on the ground sits on damp soil, and concrete moves water. It wicks up from underneath and travels through the slab toward the dry side, which is the room you are standing in. On an uncoated floor you never notice, because the water comes out of the top and goes into the air of the room.
Put a film over that and the water still arrives, it just has nowhere left to go. It collects under the coating as vapor and pushes. The push is small in any one spot and constant everywhere, and it works on the bond until it finds the weakest place and lifts it.
What it looks like when it happens
Blisters, usually. Small domes that turn up well after the floor went down, often in the same part of the room, often where the slab is coldest or where a downspout empties outside that wall. Sometimes it shows up as an area that has gone cloudy underneath instead, which is the same thing on a smaller scale.
The underside is what separates it from the other failures. When moisture lifts a coating, the concrete under it is clean and the back of the film is smooth. Wear does not look like that, and a coating that came off dirty concrete brings the dirt with it. So a floor that blisters in one area and holds everywhere else usually points at that part of the slab.
How a slab gets checked
This gets measured. A moisture test goes down on bare concrete and gets left to sit, and the reading comes off your slab rather than off a rule of thumb. Nobody can tell you what a floor reads from a photograph, from the age of the house, or from the fact that it looks dry. A basement that has never had standing water in it can still read high.
A high reading changes what goes down rather than whether anything can. There are systems built for a slab that is still giving off vapor, and they are a different material and a different price than the same floor would be on dry concrete. What the reading does not change is the need to know first, because a system picked for a dry slab and installed on a damp one is the failure this page is about.