· Michael Blair
Concrete releases moisture for a long time after it is poured, and a slab on grade in Middle Tennessee also draws it up from the ground beneath. Almost every slab flooring failure traces back to a moisture test that was skipped.
Why the test matters
Moisture moving up through a slab has to go somewhere. Under an impermeable floor it collects at the bond line, which breaks adhesives, lifts planks and grows mould underneath.
The two standard tests are a calcium chloride test measuring vapour emission and an in-situ relative humidity probe. Manufacturers specify limits, and installing outside them voids the warranty.
Which floors tolerate what
Tile with a suitable mortar handles slab moisture best. LVP is generally tolerant and many products carry a stated limit. Engineered wood needs a vapour barrier and a tested slab. Solid hardwood is not appropriate directly on slab.
Carpet is more sensitive than people expect, because moisture at the pad is where odour and mould develop.
- Tile: most tolerant, with the right mortar
- LVP: generally tolerant, check the stated limit
- Engineered wood: vapour barrier plus a passing test
- Solid hardwood: needs a built subfloor assembly
- Carpet: check pad specification carefully
Flatness is the other slab problem
Slabs are poured flat to a tolerance, not to a plane. Floating floors bridge the low spots and flex, which produces the hollow click that is the most common complaint after installation.
Self-levelling compound corrects it, and it is a genuine cost. A quote that assumes the slab is flat without checking is a quote that will produce a callback.
Basements specifically
A Nashville basement adds hydrostatic pressure and cooler surfaces, which means condensation as well as vapour transmission. Address drainage outside before choosing a floor inside.
Where the basement has any history of water, the safest assemblies are tile or a click LVP that can be lifted and dried rather than anything glued or organic.