Guide
Moisture testing before coating concrete
Moisture moving up through a slab is one of the most common reasons floor coatings blister and peel. Testing it costs far less than replacing a failed floor.
Why moisture matters
Concrete holds water long after it looks dry, and slabs on grade can draw moisture from the ground, especially where no vapor retarder was installed. Many coatings form a barrier on top. Vapor pressure building under that barrier can push the coating off the slab, causing blisters, bubbles and debonding, sometimes months after installation.
Common test methods
| Method | What it measures | Notes |
|---|---|---|
| ASTM F2170 in-situ relative humidity | Relative humidity inside the slab using probes in drilled holes | Widely referenced by flooring and coating manufacturers; probes need time to equilibrate |
| ASTM F1869 calcium chloride | Moisture vapor emission rate from the surface over a set period | Requires the space to be at service conditions; still referenced on many data sheets |
| ASTM D4263 plastic sheet | Visible moisture under a taped plastic sheet | Qualitative only; useful as a quick indicator, not a pass or fail |
| Electronic meters | Surface or near-surface readings | Useful for mapping areas to test properly; not a substitute for the methods above |
When to test, and what results change
Test at service conditions
Results are most meaningful with the building enclosed and heated or cooled as it will be in use.
Test new slabs late
New concrete continues drying for months. Coating dates are set by results against the product’s limit, not just by slab age.
High readings
Options include waiting, a moisture-mitigating primer, or a more moisture-tolerant system such as urethane cement. Each has a cost and schedule effect.
The product sets the limit
Acceptable readings are defined by the manufacturer of the system being installed.
Questions people ask
Can concrete look dry and still have a moisture problem?
Yes. Moisture vapor can move through a slab with no visible water and build pressure below a low-permeance coating.
Which moisture test is used?
In-situ relative humidity probes and calcium chloride testing are common methods. The specification and coating manufacturer determine which results apply.
What happens if the reading is too high?
The choices may include a tested mitigation primer, a more moisture-tolerant system or postponing the coating until the source is corrected.
Tell us about the floor
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