Most Gulf Coast floor failures don't start in the floor. They start in the slab underneath it. Concrete wicks moisture up from the soil, and if the floor going down on top can't handle it, the failure is just a matter of time. Here's what the moisture numbers mean, how we read them, and why a reading can change your whole install.
Why a Florida slab is never really "dry"
A concrete slab sits on soil, and soil holds water. Concrete is porous, so it pulls that moisture upward and releases it as vapor at the surface — continuously, for the life of the slab. In Florida's water table and humidity, that vapor emission is higher and more persistent than in most of the country. The slab looks bone dry. It isn't. A floor that traps that vapor against itself — or an adhesive that can't tolerate it — fails from below, where you can't see it coming.
The two readings we take
Calcium chloride test (MVER)
A small dish of calcium chloride is sealed to the slab for a set window; the weight it gains tells us the Moisture Vapor Emission Rate — pounds of water per 1,000 square feet per 24 hours. It's a surface-emission snapshot. Most glue-down products want to see roughly 3 lbs or below; borderline numbers mean a moisture-mitigation step before adhesive.
In-situ relative humidity (RH) probe
A probe set into a drilled hole reads the humidity inside the slab at depth — a truer picture of how much moisture is actually in the concrete and will eventually reach the surface. Most flooring systems target an in-slab RH at or under 75–80% depending on the product. We use the probe on anything we're gluing down where the stakes are high.
| Reading | What it means | What we do |
|---|---|---|
| Calcium chloride ≤ 3 lbs | Low surface emission | Glue-down is clear |
| Calcium chloride 3–5 lbs | Borderline | Vapor barrier or membrane first |
| Calcium chloride 5+ lbs | High emission | Mitigation required — no exceptions |
| RH probe ≤ 75% | Slab interior acceptable | Proceed per product spec |
| RH probe 80%+ | Wet slab interior | Membrane or rethink the product |
How a reading changes the install
- It picks the product. A wet slab rules out laminate and makes solid hardwood a non-starter; SPC vinyl plank and properly-isolated tile handle slab moisture far better.
- It adds (or removes) a mitigation step. Borderline numbers mean a 6-mil vapor barrier, a poured liquid membrane, or a moisture-mitigating adhesive — real line items that a quote skipping the test will "discover" later.
- It protects the warranty. Manufacturers void warranties for installs over out-of-spec slabs. A documented reading is what keeps your product warranty intact.