Hardwood over concrete can look perfect on day one, then start to cup, gap, or loosen months later. In Georgia, that risk is higher because our air stays humid for long stretches, and concrete acts like a slow, stubborn sponge.
If the slab feels dry to the touch, it can still hold moisture deeper down, where it matters most. That’s why concrete moisture testing is not a formality, it’s the decision point that protects your flooring, your warranty, and your budget.
Why Georgia slabs keep surprising hardwood installs
Slab-on-grade concrete is always connected to moisture in some way. Ground moisture, rain events, irrigation, and seasonal humidity changes all affect how much vapor moves through a slab. Even when a home has a vapor retarder under the concrete, small breaks, seams, or penetrations can create localized problems.
The biggest trap is the moisture gradient. The top surface can read “dry,” while the interior is still damp. Once hardwood, underlayment, or adhesive goes on top, the slab can no longer “breathe” upward. Moisture then builds at the surface line, right where glue and wood are most sensitive.
This is also why rules of thumb don’t hold up well. “The slab is old, so it must be dry” is not a safe bet. Concrete can continue releasing moisture for years, and newer low-cement mixes can dry more slowly in real jobsite conditions.
When we plan a hardwood install, we treat testing as part of the floor system, the same way we treat flatness and acclimation. If you want a quick overview of flooring options and install basics, our hardwood flooring installation services page is a useful starting point.
RH probes vs calcium chloride, what each test really measures
Two common methods show up on real projects, especially when hardwood is going over a Georgia slab. They are not interchangeable, and the numbers do not “convert” cleanly.
ASTM F2170 in-situ RH probes (internal slab moisture)
ASTM F2170 uses probes placed inside drilled holes to measure relative humidity inside the concrete. For slabs drying from one side (typical on grade), the test depth is 40 percent of slab thickness. That depth targets the moisture level most likely to balance out under a floor covering.
A key practical point for 2026: F2170 allows official readings after 24 hours of equilibration. That speeds decisions without skipping the science, as long as the test is performed correctly and the building conditions are stable.
Why we like RH testing for hardwood: it catches moisture that hasn’t reached the surface yet. For a deeper discussion on method selection, see how to choose a concrete moisture test.
ASTM F1869 calcium chloride (surface vapor emission over time)
ASTM F1869 places a calcium chloride dish under a sealed dome for a timed period, then reports a moisture vapor emission rate (MVER), usually in lb/1000 ft²/24 hr. It measures what leaves the surface during the test window.
Calcium chloride can be useful, but it is sensitive to surface conditions and can miss deeper moisture that has not migrated upward yet. It is also not permitted for some slab types, including lightweight concrete. Industry writers have long pointed out the confusion this creates, and it’s explained well in a breakdown of the two common slab moisture tests.
What the numbers mean (and why we don’t “translate” them)
Here’s the part that frustrates homeowners: you can “pass” one test and still fail the installation. That’s because RH and MVER measure different things, in different units, at different depths.
Most wood flooring and adhesive manufacturers publish limits such as “maximum %RH” for F2170 and “maximum MVER” for F1869. Those limits vary by product and system. We follow the product documentation first, then we decide on mitigation if needed. A manufacturer-oriented discussion of test method preference is outlined in GCP’s overview of concrete moisture test methods.
A simple way to read the reports:
| Test method | What you’ll see | What it indicates | Practical meaning for hardwood |
|---|---|---|---|
| F2170 RH probes | %RH (example: 78%RH) | Internal slab humidity at test depth | Best for predicting moisture conditions once the floor is installed |
| F1869 CaCl | MVER (example: 5 lb) | Surface vapor emitted during test period | Helpful snapshot of surface behavior, not the full slab profile |
Typical expectations on many jobs look like this (always check the product limits): RH thresholds are often in the 75 to 85 percent range, while MVER limits are often around 3 to 5 lb for many adhesives and systems. We do not recommend trying to “convert” 80%RH into an MVER number. It’s like comparing miles per hour to fuel economy. Both relate to driving, but they are not the same measurement.
A practical testing plan for hardwood over a Georgia slab
Before we test, we want the home close to normal living conditions. HVAC should be running, doors and windows closed, and indoor temperature and humidity kept steady. Testing a slab in a “construction climate” often leads to numbers that change once the house is occupied.
We also test in enough locations. A common baseline is at least three test areas per 1,000 square feet, then more for large rooms or known risk zones (exterior walls, plumbing runs, and areas that previously had moisture issues).
If results are high, we don’t guess. We choose a response that fits the flooring system:
- Drying time and dehumidification: Sometimes the correct move is simply more time under controlled conditions.
- Approved moisture mitigation: A manufacturer-approved epoxy or urethane moisture barrier, or an approved adhesive system, can control vapor when drying alone won’t.
- Flooring system adjustment: In certain rooms, engineered wood, a floating system, or a different adhesive can reduce risk, as long as the manufacturer allows it.
This is also where contractor discipline matters. Documented testing protects you if there is ever a warranty claim. If you want a clear, homeowner-friendly primer on what should be included in a proper install scope, read our hardwood flooring installation overview. If you are comparing bids, our guide to choosing a hardwood flooring contractor explains what to verify before you sign.
Many of our clients come to us after searching for the best flooring contractor in alpharetta ga or the best flooring company alpharetta and milton because they want the testing handled correctly, not rushed. We also support full interior projects, so it’s common to pair floors with a bathroom remodeling contractor in alpharetta and Milton, the best local kitchen remodeling contractor in alpharetta, or the best kitchen contractor alpharetta. If your scope includes stairs or tile, we coordinate trades like a tile installation company Alpharetta, a Stair company Alpharetta, or a stair contractor alpharetta, and we can align that with a top hardwood floor sanding contractor in alpharetta when refinishing is part of the plan.
For a free estimate, call us at 470-352-1156. If you show us any existing quote from other contractor, we beat it by 5%.
Hardwood on a slab can perform beautifully in Georgia, but only when moisture is measured and managed. RH probes and calcium chloride tests answer different questions, so we select the method that matches the flooring system and the risk profile of the home. When we treat concrete moisture testing as a required step, we prevent expensive surprises after the furniture is back in place.