Construction Moisture Before a Floor Goes Down

Curing and drying are not the same, and conflating them is where the optimism starts. A cementitious layer gains strength by hydration, which is a chemical process. Drying is the separate business of the remaining water leaving the layer, and it is drying rather than curing that most floor coverings and adhesives depend on.

That is why a topping is not dry when it is walkable. Drying to a state that will accept an impermeable finish takes far longer than reaching a surface that bears foot traffic, and the difference between those two moments is where a great many failures are created.

Pumped flowing screeds are attractive precisely because large areas can be placed in a single operation, and that speed is real. The drying it does not shorten is then squeezed, because the programme was rewritten around the placing time rather than around the waiting.

Dry board floors are the honest answer to that pressure, because they introduce no water and nothing downstream waits for drying at all. That advantage is easily lost if the rest of the sequence has not been planned around it, which is the same planning failure in a different direction.

A cement screed and a sulfate-bound flowing screed are different substrates chemically, and the difference decides both how the drying behaves and which adhesives, primers and compounds may go over the result. A sulfate binder is also sensitive to sustained moisture in a way a cement one is not.

The practical problem is that nobody records which screed was used, so the next contractor to work on the floor treats it as cement based. Writing down what was laid, and keeping that record with the building, is the cheapest thing on this entire page.

Whether a floor is dry enough for a covering is a specialist judgement made by the flooring specialist, and for a sulfate-bound screed it is verified by testing rather than judged by time or appearance. The decision is theirs because the consequence of getting it wrong lands on their work.

What an owner can do is make sure the assessment happens, that it is recorded, and that whoever is asked to lay the covering has been given the information they need: what the base is, when it was laid, what conditions it has dried under and what any test showed.

Drying conditions, including ventilation and heating, are managed rather than assumed, and forcing drying too quickly can cause its own problems. The building being weathertight before a wet layer is placed, and staying that way, is part of the same question.

Water left standing on the floor during construction matters too, and it does more harm to a sulfate-bound screed than it would to a cement one. So does damage from following trades, which is a common reason repairs are needed at all.

The adhesive sits between a damp base and an impermeable covering, which is why residual moisture is the usual cause of bond failure. A solid timber board exchanges moisture with whatever lies beneath it, so a floor laid before wet trades have dried out can absorb moisture it will spend a long time giving back.

Where a ground floor has no functioning membrane beneath it, smoothing a floor over it does not deal with the underlying moisture, and the assessment belongs to a specialist. A membrane is a layer rather than a product, and it performs only where it is continuous and correctly terminated.

There is time to be found, and it is not in the waiting. It is in identifying the base early, in getting the covering manufacturer's requirements before the base is specified rather than after, in preparing the surface as a planned operation rather than a discovery, and in protecting the floor so that repairs are not needed.

On a sulfate-bound screed the weak surface skin has to be removed and the surface prepared and primed before anything is bonded to it. That is a separate operation with its own slot in the programme, and it is the one most often discovered late.