A garage slab can look dry and still contain enough moisture to matter to a low-permeability resin floor. Moisture testing is not one universal number: surface meters, in-situ relative-humidity probes and calcium-chloride tests measure different things, and the acceptable limit comes from the exact coating or moisture-mitigation system.
Why Moisture Becomes More Important After You Coat
Bare concrete can release moisture to the air. A resin floor greatly reduces that drying path. Moisture and soluble salts moving toward the coating can contribute to blistering, osmotic pressure and loss of adhesion. Sherwin-Williams’ resin-flooring guidance specifically warns that excess moisture or soluble salts can cause osmotic blistering, delamination and premature failure.
Three Common Ways Moisture Is Discussed
| Method | What it tells you | Limitation |
|---|---|---|
| Concrete moisture meter | Fast screening or product-specific surface/substrate reading. | Thresholds are meter/product specific; it does not automatically replace an in-slab test. |
| ASTM F2170 in-situ RH | Relative humidity inside drilled test holes in the slab. | Requires proper placement, conditioning and interpretation. |
| ASTM F1869 calcium chloride | Moisture-vapor emission rate at the slab surface. | More affected by surface/environmental conditions and not the same measurement as in-situ RH. |
Different manufacturers publish different limits. Sika systems, for example, may require 80% or 85% RH for particular products, while Sika also sells moisture-tolerant systems intended for higher-moisture slabs. PPG offers a separate moisture-mitigating epoxy primer with its own ASTM F1869 vapor-emission limit. The lesson is not to memorize one number; it is to match the test result to the exact system.
What a Homeowner Should Ask a Contractor
- What moisture test will you perform?
- How many locations will be tested?
- What is the product manufacturer’s maximum result for this primer?
- What happens if the slab is above that limit?
- Is a moisture-mitigation primer being included or merely assumed unnecessary?
- Will the results be written on the proposal/job record?
A Plastic-Sheet Test Is Not the Same Thing
Taping plastic to the slab can reveal obvious condensation or darkening, but it is not a substitute for the test method required by a resin-floor manufacturer. It can be a warning sign, not a quantitative acceptance test.