Wood, engineered, or vinyl over a concrete slab. What the moisture decides for you

Concrete looks inert. It is not. A slab on grade sits in contact with soil, and soil holds water, and water moves upward through the slab as vapor continuously for as long as the building stands.

Understand that one mechanism and most flooring decisions over a slab make themselves.

What is actually happening in the slab

Two separate sources of moisture matter, and they behave differently.

The first is residual water from the original pour. Concrete gains strength through a chemical reaction that does not consume all the mixing water, and the excess leaves slowly through the surface. A new slab keeps doing this for months, and the drying rate depends on thickness, mix, temperature and whether both faces are exposed.

The second is ongoing vapor drive from the ground, which never stops. A slab with an intact vapor retarder under it drives much less. A slab poured before that was standard practice, or one where the sheeting was punctured during construction, drives considerably more.

Put a floor covering on top and you change the equilibrium. The vapor that was leaving through the surface now accumulates under the covering, and whatever is sensitive to moisture is now sitting in it.

How the moisture actually gets measured

Not by touch, and not by taping a plastic sheet down overnight, which is a rough indicator at best.

Two standard methods exist and a flooring contractor should be using one of them.

• In-slab relative humidity probes. Holes are drilled to a specified depth, sleeves inserted, and the assembly left to equilibrate before reading. This measures the condition inside the slab, which is what will eventually reach the surface, and it is the method most manufacturers now specify.
• Calcium chloride testing. A measured dish of desiccant sealed to the surface for a set period, weighed before and after to give a vapor emission rate. Surface-focused, older, and still referenced in many specifications.

Both have limits published in their standards, and the flooring manufacturer's warranty states which method it accepts and what result it requires. That document, not the contractor's opinion, is the operative one.

Solid wood, and why it is usually the wrong answer

Solid hardwood is a single piece of wood that moves across its width with moisture content. Over a slab it faces two problems at once.

The fastening problem: solid wood is normally nailed to a wood subfloor. Over a slab that means building a plywood subfloor or a sleeper system first, which raises the floor height, complicates every doorway and stair, and adds real cost.

The movement problem: even installed correctly, solid wood over a slab is sitting above a continuous moisture source. Elevated moisture from below and dry indoor air above produces cupping, where board edges rise above their centers.

It can be done. It requires a full vapor barrier, a properly built subfloor assembly, and acceptance of the height change. In most houses the reason to do it is that the rest of the floor is already solid wood and matching matters.

Engineered wood, and the layer that solves it

Engineered flooring is a wood wear layer bonded to a core of plies laid at alternating grain directions, or in some products a composite core. Cross-lamination is the whole point: layers that would each expand in one direction restrain each other, so the assembly moves far less than solid wood.

That dimensional stability is what makes it the default choice over a slab. It can be glued directly down with a moisture-control adhesive, or floated over an underlayment with a vapor retarder, and neither approach requires building a second floor first.

The specification to check is the wear layer thickness, because it determines whether the floor can ever be sanded and refinished. A thin veneer can be recoated but not sanded. A thick sawn wear layer can take a full refinish, sometimes more than once.

Luxury vinyl plank, and what waterproof does not mean

Vinyl plank is not damaged by water, which leads to a reasonable but incorrect conclusion that slab moisture is irrelevant to it.

The plank survives. What can fail is what is around it:

• The adhesive on a glue-down installation, which has its own moisture limit and can release, producing movement and edge lifting.
• Whatever is trapped underneath. A vapor-impermeable covering over a wet slab holds moisture at the interface, and that interface can grow mold even though the plank itself is unaffected.
• The perimeter and the subfloor edges, where moisture accumulates and finds materials that are not waterproof.

Click-together floating vinyl over a slab with a suitable underlayment tolerates more moisture than any other option here, which is why it dominates basements. Tolerates is not the same as ignores, and the manufacturer still publishes a limit.

Side by side

The sequence that avoids the failure

1. Test the slab with a standard method before selecting a product, not after.
2. Read the chosen manufacturer's moisture limit and the method it requires. That is the number the warranty depends on.
3. If the reading exceeds the limit, the options are a moisture mitigation coating applied to the slab, a different product, or waiting on a new slab.
4. Confirm in writing which adhesive or underlayment the estimate includes, because substitutions here are where warranties are lost.

Slab floors that fail almost always failed the first step. Slab floors that last were chosen against a number, and the number cost less to obtain than a single box of the flooring.