Most epoxy floor failures trace back to one of three causes: skipped diamond grinding, wrong product spec, or moisture in the slab. Moisture is the most-overlooked of the three and the most consistently fatal.
This is what moisture testing actually measures, why it matters, and why most installers skip it.
What moisture in a slab actually means
Concrete is porous. Water moves through it — both the original mix water that has not finished evaporating and groundwater migrating up from below. The amount of moisture in the slab at any given time depends on:
- Slab age (newer slabs retain more original mix water)
- Subgrade conditions (clay vs. sand, water table depth)
- Vapor barrier presence and condition (older slabs often have failed or missing barriers)
- Ambient humidity (Texas summer humidity drives moisture into the slab)
- Recent rainfall or watering near the foundation
A slab that looks dry on the surface can have 4-6% relative humidity below the surface. That moisture rises through the slab as vapor and tries to escape through whatever is on top.
If what is on top is bare concrete, the moisture evaporates harmlessly. If what is on top is a non-vapor-tolerant epoxy coating, the moisture is trapped — and it pushes the coating off the slab.
How moisture-driven coating failure looks
Moisture-driven failure has a distinctive pattern:
- Bubbles or blisters form under the coating, typically 6-18 months after install.
- White haze or efflorescence appears at the bubble locations as moisture carries minerals from the slab to the underside of the coating.
- Coating lifts in patches. Where the bubbles burst or the bond breaks completely, sections of coating peel up.
- Pattern is bottom-up. Unlike wear or impact damage (which is surface-down), moisture failure starts at the slab interface and pushes outward. You see it on top, but the cause is below.
Once moisture failure starts, it spreads. Each bubble or peeled section gives moisture another easy escape path, accelerating the rest. A small bubble field at year 1 becomes large peeled sections by year 2.
Two methods we use to test moisture
1. Calcium chloride test (ASTM F1869)
A small dish of anhydrous calcium chloride is placed on the slab, sealed under a plastic dome, and left for 60-72 hours. The calcium chloride absorbs moisture from the slab; we weigh it before and after to measure the moisture rate (expressed as pounds of moisture per 1000 sq ft per 24 hours).
Results < 3 lb: low moisture, almost any system will bond.
Results 3-5 lb: moderate, requires moisture-tolerant primer.
Results > 5 lb: high, requires moisture-mitigation membrane before standard epoxy.
Results > 8 lb: very high, may require alternative systems entirely.
2. In-situ relative humidity test (ASTM F2170)
A small probe is inserted into a hole drilled into the slab, then sealed. After 72 hours, we read the relative humidity (RH) inside the slab.
RH < 75%: low moisture, standard systems work.
RH 75-85%: moderate, moisture-tolerant primer required.
RH > 85%: high, moisture mitigation required.
RH > 95%: very high, may need to wait for slab to dry or use specialty system.
Both methods are recognized by epoxy manufacturers and by ASTM. We typically run the calcium chloride test on every install (faster, cheaper) and the RH test on suspect slabs or commercial projects where the manufacturer warranty requires it.
Why most installers skip moisture testing
Three reasons:
- It takes time. The test itself is 60-72 hours. That is 60-72 hours between the quote walk and the actual install start. 1-day installers cannot accommodate that timeline.
- It costs money. Test materials run $30-60 per test point. A typical residential garage gets one or two test points. Commercial spaces get more. The cost is small but real.
- It exposes problem slabs. If the test comes back high, the installer has to either spec a moisture-mitigation system (more expensive) or walk away from the job. 1-day installers prefer to install on the assumption that everything is fine and let the floor fail later under someone else's problem.
The honest answer: skipping moisture testing is a profit-center decision. The shop saves $30-60 plus 72 hours of schedule. The homeowner pays $5,000-10,000 for a floor that fails in 18 months.
What we do at Sherwood's
Every install includes calcium chloride testing as standard scope. The cost is built into the quote — we do not nickel-and-dime the prep stage.
If the test comes back high, we either:
- Spec a moisture-tolerant primer (works for moderate readings).
- Install a moisture-mitigation membrane (Schluter Ditra Heat or equivalent) for high readings.
- Recommend waiting for the slab to dry (newer construction) before pour.
- Decline the job if the moisture is so high no compatible system will hold.
Walking away from a job is rare but it happens. Better to lose a $5,000 install than to take it and have the floor fail under our warranty in 18 months.
Want a quote that includes real moisture testing? Call (972) 369-6302 or use the form on any page. We respond within one business day.