The real answer?
Never.
That may sound dramatic, but from a scientific standpoint, concrete is never completely dry. Like most building materials, it retains a certain amount of moisture when in equilibrium with its surrounding environment. However, compared to many other materials in a structure, concrete is particularly wet—even when considered “dry enough.”
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Using ASTM F2170 in-situ probe testing, often referred to as the Equilibrium Relative Humidity (ERH) test, most flooring manufacturers require concrete moisture levels to be between 70% and 75% ERH before installation warranties apply.
To put that in perspective:
That makes concrete one of the wettest materials we routinely call “dry” in the built environment.
Concrete is porous. It absorbs, retains, and releases moisture continuously in response to surrounding temperature and humidity conditions. It is dynamic—not static.
In water damage restoration, understanding how concrete interacts with water is critical.
Except in cases of prolonged flooding, most water losses:
In many typical water losses, moisture is limited to the surface layer of the slab. With proper environmental drying—especially in timber-framed structures—surface moisture often evaporates during the overall structural drying process.
This is why concrete sometimes “dries” at the same time the rest of the structure returns to normal equilibrium.
If testing shows the slab remains above manufacturer thresholds after the building is otherwise dry, there is usually an underlying issue.
Common causes of persistent slab moisture include:
If excess moisture remains, the solution is not simply more drying time.
The true solution is identifying and correcting the source.
Flooring failures can be expensive and disruptive. If flooring is installed over concrete that exceeds manufacturer moisture limits, the results may include:
Using proper ERH testing procedures helps prevent these outcomes and protects all stakeholders—contractors, owners, and insurers alike.
From a water damage restoration standpoint, the key lessons are:
Concrete is never truly dry. It simply reaches equilibrium.
And when it doesn’t, there is always a reason.
Concrete is one of the wettest materials we routinely work with, even when it meets installation standards. Recognizing that fact helps restoration professionals avoid costly mistakes and misdiagnosed drying issues.
When a slab remains too wet for flooring, the question isn’t “Why won’t it dry?”
The better question is:
“What is the real source of the moisture?”
Click here to read Chuck’s entire article: https://www.randrmagonline.com/articles/83738-when-is-concrete-really-dry