Concrete never fully dries — there is always some microscopic moisture in a slab, and it can keep moving for months. A slab that feels dry on top, or that you can walk on without leaving footprints, is not the same as a slab that is dry enough to floor over. That is exactly why slab moisture is measured, not eyeballed. Slab moisture is one of the leading causes of flooring failures — including under waterproof luxury vinyl plank (LVP), where trapped moisture can still cause adhesive problems, odor, and subfloor issues.
Common moisture tests
- Calcium chloride test (MVER): measures how much moisture vapor leaves the slab over a set period.
- Relative humidity (RH) in-situ probe: sensors placed inside the slab measure internal humidity; widely considered the most reliable method.
- Calcium carbide / quick field tests: faster spot checks used to flag obvious problems.
Flooring and adhesive manufacturers publish maximum moisture limits for their products. The test result has to come in under those limits, or the warranty may not apply.
Before you install: let new concrete cure for the time the manufacturer specifies, then test — don't assume a slab is ready because it feels dry on top.
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