How to Test a Garage Floor for Moisture Ahead of Epoxy
Epoxy Floors Craft Antioch explains the three ways a concrete garage slab is tested for moisture before a coating goes on — the plastic sheet check, the calcium chloride test and in-slab humidity probes — and what to do with the readings.
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The floor as it starts
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The finished floor (illustration)
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Why moisture ends coatings
A slab with moisture vapor coming up through it can push a coating off. Concrete is porous, and in a slab-on-grade garage the ground below is the source; many older slabs have no vapor barrier at all. Contra Costa County's building findings describe local soils as clay loam and alluvial fans, which hold water, and a young slab also carries water from curing.
Test one: the plastic sheet check
A taped square of plastic is left on bare concrete for about a day. Condensation underneath, or a dark patch on the slab when the plastic is lifted, suggests moisture. It is cheap and quick and tells yes or no, not how much.
Test two: calcium chloride, ASTM F1869
A dish of calcium chloride is sealed under a dome on the bare slab for 60 to 72 hours. The weight gain gives the moisture vapor emission rate in pounds per 1,000 square feet per 24 hours. Many coating data sheets state a maximum rate; above it, a moisture-mitigating primer is used or the job is reconsidered.
Test three: in-slab humidity, ASTM F2170
A hole is drilled into the slab, sleeved and sealed, and a probe reads relative humidity inside the concrete after the slab has equilibrated. It reads deeper than the surface, so it catches moisture the calcium chloride test can miss. The reading is compared with the data sheet's limit for the product.
What to do with the readings
| Reading | Usual response |
|---|---|
| Under the data-sheet limit | Coat as planned |
| Over the limit | Vapor-barrier primer, priced as an option (about $400–$1,000 for a typical garage) |
| Far over the limit, or a leak at the edge | Fix the source or reconsider the job before any coating |
Ask for the method, the number of test spots and the readings in writing.
Where this leads
- Service page: Garage Floor Prep and Crack Repair — What the slab needs before any coating: grinding, crack and spall repair and moisture testing, explained step by step.
- Local page: Epoxy garage floors in Prewett Ranch and Country Hills
- All services and typical prices: Epoxy garage floors in Antioch
- More illustrated guides: Epoxy Floor Guides
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