Getting It Right: Wood Flooring Over a Concrete Subfloor

Wood flooring over a concrete subfloor offers a timeless, elegant look that can transform any space. But before you commit to it, it’s important to understand the challenges involved.

This article covers the advantages of wood flooring, why subfloor moisture matters, the signs of hidden moisture problems, how to prevent them, and the essential role of accurate concrete moisture testing.

• Wood’s advantages are more than skin deep
• Beware of subfloor moisture
• Visible signs of a hidden moisture problem
• Prevention is the best remedy
• Why the in-situ RH test is the most reliable
• What it all means for your project’s specifications

The visual appeal of a wood floor is hard to match. Many of us feel a certain joy walking into a building where wood has been used generously. We may not be able to say exactly why. It may be the touch of elegance or the warm glow. But the positive impression is hard to deny.

Wood’s advantages are more than skin deep

Wood flooring offers more than good looks in both homes and commercial settings. It’s easy to clean and resists stains far better than carpet. Wood is also strong and durable, and with proper care, a wood floor can last for decades.

When a wood floor eventually shows wear from years of use, it can often be refinished rather than replaced.

A wood floor can also contribute to better indoor air quality. Unlike carpet, a wood floor doesn’t hold onto dust, pollen, and other common allergens, so they’re easier to clean away. Wood floors can also improve acoustics, which is one reason hardwood is so common in dance and music studios.

Beware of subfloor moisture

On any commercial flooring project that uses wood, think ahead about how moisture could undermine your work, and address every possible source of excess moisture. Moisture-related flooring failures are costly: North American commercial property owners spend an estimated $2.4 billion a year remediating them.

One of the most common, but sometimes overlooked, sources of moisture is the subfloor.

In commercial settings, wood flooring is typically installed over a concrete slab. A slab may look solid and feel dry to the touch, suggesting it holds very little water.

But water is an essential ingredient in concrete, and all concrete slabs, whether freshly poured or decades old, can hold surprising amounts of moisture.

In fact, without enough water, a slab won’t cure properly and reach its full hardness. Curing takes relatively little time. Concrete generally sets enough to walk on in 24 to 48 hours.

Within seven days, a slab typically reaches about 70% of its full strength, and in about 28 days, it should be approaching full strength.

Curing should never be confused with drying. While curing happens fairly quickly, drying doesn’t. Once a slab is poured, moisture inside it gradually migrates to the surface and evaporates.

Ambient conditions affect how fast this happens, but it’s never particularly fast. A five-inch slab may take five months or more to dry enough for a finished floor, once the building is enclosed and conditions support drying.

If you don’t (1) build this drying time into the project schedule and (2) accurately assess the slab’s moisture, you may end up scrambling to fix serious moisture-related problems later, and the excitement over a beautiful wood floor can quickly turn to frustration.

Visible signs of a hidden moisture problem

Because wood is hygroscopic, wood flooring swells as it absorbs moisture and shrinks as it dries out. Excess moisture in wood flooring can show up in several very problematic ways.

To learn about the visible signs of hidden moisture problems in hardwood floors, read our in-depth article.

Prevention is the best remedy

Wood flooring is a great choice for many commercial applications, but its sensitivity to moisture calls for due diligence before any installation over a concrete slab. Part of that due diligence is simply waiting.

It takes time for a slab to dry, even when you take steps to speed it up, such as enclosing the building and running the HVAC.

A common rule of thumb is to allow one month of drying for each inch of slab thickness, once conditions support drying.

Keep in mind that this is only a rule of thumb. It can help set expectations, but it should never be used to make an installation decision.

The key to preventing moisture problems is to insist on an accurate concrete moisture test before any flooring installation. Accurate, reliable results give you the information you need to decide when wood flooring can safely go over a concrete slab.

Two types of concrete moisture tests are commonly used in the United States. Historically, surface-based tests such as the anhydrous calcium chloride test were used most often.

Today, that type of test is becoming less common as more people in the industry learn about the advantages of the in-situ RH test.

One problem with a surface-based test is that it’s heavily influenced by ambient conditions and can easily give misleading results. Another is that it indicates, at best, the moisture condition at or near the surface of the slab.

It assumes that’s all you need to know to decide when to install, but it tells you nothing about the moisture deeper in the slab.

That matters a great deal. Moisture in a concrete slab typically exists in a gradient, with much less moisture at the surface than deeper down.

But once the slab is sealed with a finished floor and moisture can no longer evaporate from the surface, the moisture inside the slab evens out and the gradient disappears.

As a result, the moisture at or near the surface, which is what the finished floor is in contact with, ends up higher than a surface-based test would indicate. For this reason alone, you shouldn’t rely on that type of test. Its results can mislead you and lead to flooring failure.

Why the in-situ RH test is the most reliable

Scientific studies in recent decades have shown that a slab’s moisture condition, and how it will affect installed flooring, is best measured by the RH deep in the concrete, using an RH probe set into the slab.

Since 2002, when it was standardized as ASTM F2170, the in-situ RH test has become increasingly favored as the “gold standard” for concrete moisture testing.

The main reason the RH test is so useful is that it’s a depth-specific test that accounts for what happens to moisture in the slab after the flooring seals off its surface.

Studies at Sweden’s Lund University found that placing the RH probe at 40% of the slab’s depth (for a slab drying from one side) gives readings that accurately predict the moisture the finished floor will “see” once it’s installed.

Because of this and the RH test’s other advantages (it’s less affected by changing ambient conditions, it’s faster and easier to perform, with results in as little as 24 hours, and it makes it easy to track and record RH changes over time), many flooring manufacturers now base their product specifications on RH test results.

In short, the in-situ RH test is widely considered the most reliable and accurate method of concrete moisture assessment, and it gives strong assurance against costly moisture-related flooring failures and the damage that can follow, such as cupping, crowning, buckling, mold, or mildew.

Learn how to avoid these 3 dangers when installing wood floors over concrete.

What it all means for your project’s specifications

What does this mean for an architect, engineer, or specifier on a commercial project with wood flooring over a concrete slab? Simply put, the project specifications should always identify and require the exact concrete moisture test to be performed.

That’s easy to add, because an in-situ RH test specification is available as a free download.

Recognize that the in-situ RH test is the method shown to give reliable results that lead to consistent, successful outcomes. And unless that test is spelled out for the general contractor and flooring professional, someone may choose a different test that puts the project at risk.

So always be specific about which concrete moisture test you want performed.

For more information about avoiding moisture problems with wood or concrete, call Wagner Meters at (541) 291-5123.

Shop Rapid RH

References for more information:

• Kanare, Howard. 2008. Concrete floors and moisture. Engineering bulletin 119, Portland Cement Association.
• Spangler, Jason. 2018. Artistry meets practicality on a wet slab of concrete.
• Spangler, Jason. 2016. Moisture measurement in concrete floor slabs. A specifier’s primer.
• Spangler, Jason. Hardwood floor problems: heed the warning signs.

Previously published by Construction Specifier magazine

Last updated on October 21st, 2026