Moisture Testing & Substrate Profile

If you only fix two things in your coating process, make them moisture and profile. They’re the two tests that separate a floor that lasts a decade from one that peels in a year.

Moisture: why it fails

Concrete is hygroscopic — it always contains some moisture, and it wicks it up from the ground. When you lay a moisture-impermeable coating (epoxy, PU) over a slab that’s still releasing moisture, the vapour pressure has nowhere to go. The result: blistering, delamination, loss of adhesion. The coating fails, and it looks like the applicator’s fault — but it was the substrate all along.

The two accepted tests

Calcium carbide (CM) test. A carbide disk is sealed to the slab surface for a set period; the CO₂ released (from moisture reacting with the carbide) is measured and converted to a moisture emission rate, typically in lb/100 ft²/24h or g/m²/24h. Most coating systems specify a maximum (commonly around 3.0 lb/100 ft²/24h or 575 g/m²/24h). Above that, you need mitigation.

In-situ / probe test. A meter is placed on the surface and measures relative humidity within the slab at depth. Gives a faster, repeatable reading of the slab’s internal moisture state.

Rule: Test in the worst spots (near external walls, over drains, in the wettest area), in the conditions the slab will actually live in. One test in a dry corner tells you nothing about the wet corner.

Surface profile

A coating grips by mechanical key. If the surface is too smooth, there’s nothing to bite into and the coating delaminates.

• Measure with a profile gauge or a tape test (press a strip of pressure-sensitive tape onto a ground area, peel it off, and compare the exposed pattern to a reference profile).
• Most systems want a profile of 2–3 (CSP 2–3). Too low → poor adhesion. Too high → you waste material and the coating thins into the peaks.
• Achieve it with the right grinding grit, then verify.

When moisture is too high

You have three options:

1. Wait and dry — if the slab is young or the conditions are unusual.
2. Mitigate — a moisture vapour barrier (a mid-coat or a penetrating barrier) under the coating system.
3. Choose a moisture-tolerant system — some epoxies and 100% solids systems tolerate higher moisture.

Checklist: Test moisture in the worst spots under real conditions. Measure profile with a gauge or tape test. Confirm both are within the system’s spec before a single litre is mixed.

Key takeaways

• Moisture is the #1 cause of coating failure — always test before you coat.
• Use the right test: CM (calcium carbide) or in-situ probe, and read the result correctly.
• Profile matters as much as moisture — a coating needs teeth to grip.