It costs a fraction of a percent of the job and it is the difference between a floor that lasts twenty years and one that bubbles off in year two.
Why It Matters
Concrete is porous, and a slab poured on grade without a working vapour barrier acts as a wick, soil moisture moves up through it continuously and evaporates off the surface. You never notice, because bare concrete lets it go. Put an impermeable coating on top and that vapour has nowhere to escape. Pressure builds at the bond line until it lifts the coating, typically in year two, typically in domed bubbles with white efflorescence underneath.
Method 1: Calcium Chloride
A pre-weighed dish of anhydrous calcium chloride is sealed under a dome on the slab for 60-72 hours. The salt absorbs moisture from the air under the dome and is re-weighed. The result is expressed as pounds of moisture emitted per 1,000 square feet per 24 hours. Most coating manufacturers cap this at 3-5 lbs. It measures the surface only, which is its main limitation.
Method 2: In-Situ Relative Humidity
A hole is drilled to 40% of slab depth, a sleeve is inserted, and after a 72-hour equilibration a probe reads the relative humidity inside the slab. This is the better test because it measures the whole slab rather than the surface, and it predicts what will happen once a coating stops surface evaporation. Most systems need readings below 75-80% RH.
What High Readings Mean
A high reading does not mean the floor cannot be coated. It means the system has to start with a 100% solids moisture-vapour-barrier primer engineered to resist the pressure, typically rated to 95%+ RH, with the decorative build going on top as normal. This adds roughly $2-$4 per square foot. It is not an upsell; it is the reason the floor stays down.
Which Slabs Are Highest Risk
Basements, which are in contact with soil on multiple faces. Slabs poured before vapour barriers were routine, which in the Okanagan means most residential construction before the 1990s. New concrete. A fresh pour holds water for months, and 30 days of cure per inch of thickness is the old rule of thumb. Anything at the bottom of a slope where groundwater collects. And any slab where you can put down a taped plastic sheet overnight and find condensation under it in the morning.
The Test Nobody Charges For, and Why
A moisture test costs a contractor a test kit and a return visit. Skipping it makes a quote look cheaper and moves the risk onto the homeowner, who finds out two years later. Apex tests before quoting on every interior slab, and the reading goes on the written quote so you can see what was measured and what was specified because of it.
Want a real number for your floor?
Free site visit with a moisture reading, and a written fixed price. Across the BC Interior.