Getting it right: Wood flooring over a concrete subfloor

by sadia_badhon | January 3, 2020 4:50 pm

by Jason Spangler

All photos courtesy of[1]
All photos courtesy of

The visual appeal of a wood floor is hard to match. Most people experience a certain joy when entering a building where wood has been used generously. They may not be able to put their finger on exactly why they react favorably to wood—it may be the touch of elegance or the warm glow they sense, but whatever it is, the positive impression is indisputable.

Wood’s advantages are more than skin deep

Wood flooring offers more advantages than just aesthetic appeal in both homes and commercial settings. It is easy to clean and is significantly more stain-resistant than carpeting. Wood is also strong and durable, and if properly cared for, its hard surface can last for decades. When a wood floor shows signs of wear from years of regular use, many can simply be refinished rather than replaced entirely. Wood floors also add value to any building, increasing the resale value by as much as 10 percent or more.

A wood floor also contributes to better indoor air quality (IAQ). Unlike carpeting, or even the grout lines of a tile floor, wood floors do not trap dust, pollen, particulate matter, or other common allergens. Wood floors can even improve acoustics. For example, hardwood floors can produce a clean, clear, crisp sound as opposed to other types of flooring that may soften or deaden sound, which is one reason why hardwood gets used frequently in dance or music studios. When choosing flooring material, one should consider what type of effect they are trying to produce.

Beware of subfloor moisture

On any commercial flooring project where wood is going to be used, it is important to consider in advance how moisture could play havoc with the design goals. It is crucial to address all possible sources for excess moisture. The National Wood Flooring Association (NWFA) has estimated at least 75 percent of all flooring failures are the result of moisture issues[2].

One of the most common, but sometimes overlooked, sources of moisture is in the subflooring.[3]
One of the most common, but sometimes overlooked, sources of moisture is in the subflooring.


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

In commercial settings, wood flooring is typically installed over a concrete floor slab. Concrete slabs may look perfectly solid and feel dry to the touch, suggesting they hold very little water inside. In reality, water is always an essential ingredient in the concrete mixture, and all concrete slabs, whether freshly poured or even decades old, will harbor surprising amounts of moisture.

It is important to note, the manufacturer’s specification (if applicable) for the wood product should be the guide regarding what is an acceptable moisture condition for the concrete slab. However, the typical limit for wood and wood-based products is about 75 percent relative humidity (RH) in the slab.

Indeed, without enough water, a concrete slab will not cure properly and attain its characteristic hardness. Curing takes relatively little time. Generally, concrete sets in 24 to 48 hours allowing one to walk on it. Within seven days, one can expect the slab to cure to about 70 percent of full strength, and in about 28 days, the concrete should be approaching full strength.

This curing process should never be confused with concrete drying. While curing happens rather quickly, concrete drying does not. Once a slab is poured, moisture begins leaving the slab from the ‘inside out.’ What this means is moisture within the slab migrates to the surface over time and then evaporates. Ambient conditions will affect the speed at which this process occurs, but it is never particularly fast. For a 127-mm (5-in.) concrete slab, it may take five months or more for the slab to dry sufficiently. This would be after the building is enclosed and an environment conducive for drying is established. The concrete must be fully dry before one can successfully install a finished floor product.

Failure to build this drying time into the overall project and conduct an accurate moisture assessment of the concrete floor slab may mean scrambling later to address one or more serious moisture-related problems. The initial excitement over the beauty of a project’s wood flooring may rapidly wane and turn to frustration and headache instead.

Visible signs of a hidden moisture problem

Since wood is a hygroscopic material, wood flooring naturally expands when moisture is present and shrinks when moisture is absent. Excessive moisture in wood flooring can present itself in several different and problematic ways.

Buckling is an extreme reaction to moisture in a wood floor. However, it is not a common occurrence, and if caught early, spot repair and replacement may be possible.[4]
Buckling is an extreme reaction to moisture in a wood floor. However, it is not a common occurrence, and if caught early, spot repair and replacement may be possible.


Cupping occurs when the edges of a board are higher than the center. Low RH in the environment or moisture in the subflooring are common culprits. As the wood expands on the bottom (due to moisture from the subfloor) or shrinks on the top (due to dry environmental conditions), the edges of the boards start to deform and raise. Aside from the visible warping of the floorboards, the floor could eventually fail altogether. The cupping could continue to worsen and create gaps between the boards. Eventually, if the boards do not revert to their original shape, they could buckle away from the subfloor, or even splinter or crack.


The opposite of cupping is crowning when a board’s center is higher than its edges. If a wood floor is exposed to moisture or humid conditions for extended periods of time, the moisture can saturate the wood flooring and cause crowning. Another possible cause of crowning could be when a floor was previously cupping and was sanded before the moisture level returned to a normal state.


Buckling is an extreme reaction to moisture in a wood floor and occurs in response to prolonged exposure to excess moisture. Buckling happens when the flooring gets pulled up from the subfloor, lifting as much as several inches in one or more places. Fortunately, buckling is not a common occurrence, and if caught early, spot repair and replacement may be possible.


Cracking, or the occurrence of separations between floorboards, is another possible sign of a moisture problem. Typically, cracking is more a response to significant (often seasonal) variations in the air’s RH rather than an issue with moisture in the subflooring. Therefore, monitoring a building’s temperature and RH levels and taking corrective action is all that is needed to address cracking.

Mold or mildew

Persistent moisture in the subfloor may also lead to problems with mold or mildew within or under the wood flooring. A musty smell provides an important telltale clue mold may be a serious problem. If present, it should never be ignored because certain types of mold pose a significant risk to human health.


Discoloration of wood flooring due to moisture may also occur. In many types of hardwood flooring, excess moisture may cause the wood to take on a darker appearance than normal. The appearance of splotches of darker wood appearing may indicate the finished floor is in contact with moisture in the concrete subflooring.

Other types of wood flooring

For engineered wood, delamination is a common issue related to moisture. When moisture penetrates the layers of an engineered wood product, the glue holding layers together can fail.

Solid wood can be susceptible to the effects of moisture. Moisture-related issues can include the ones discussed above.

Wood finishes, such as sealers, stains, varnishes, urethanes, etc., may add some degree of moisture protection from above, but they will not address the various problems stemming from moisture in a concrete floor slab.

Applying a sheet material that serves as a moisture barrier, such as a 6-mil thick polyethylene (PE) plastic sheet overlapped with taped seams, can be one way to mitigate concrete moisture. This sheet material does not negate the importance of installing a vapor barrier underneath the slab. It should serve as an additional element of protection.

Prevention is the best remedy

While wood flooring makes a great choice for a variety of commercial applications, its susceptibility to various problems stemming from moisture calls for due diligence prior to any installation over a concrete floor slab. One aspect of this due diligence is simply to wait. It takes time for a concrete slab to dry, even when specific steps are taken to hasten the drying, such as enclosing the building and turning on the HVAC.

The general rule of thumb for concrete drying is one should allow a month for each inch of thickness of the concrete slab, and this is after the establishment of environmental conditions conducive for drying.

The in-situ relative humidity (RH) test method provides reliable and accurate concrete moisture assessment.[5]
The in-situ relative humidity (RH) test method provides reliable and accurate concrete moisture assessment.

It is important to keep in mind this must be thought of as no more than a rule of thumb. It can help guide one’s expectations but should never be used for making an installation decision.

The key step to preventing moisture-related problems is always insisting on the performance of a moisture test prior to any flooring installation. Getting accurate, reliable test results provides the essential information needed for deciding when wood flooring over a concrete slab can be installed safely.

Two types of concrete moisture tests are often used in the United States. Historically, surface-based tests, such as the anhydrous calcium chloride test, were employed to evaluate the moisture condition of the slab. Today, this type of test is becoming less commonly used as more people in the industry learn about the advantages of using another type of moisture test known as the in-situ RH test.

An inherent problem with a surface-based test is it is unduly influenced by ambient conditions and can easily give false results. Another problem is this type of test is at best indicative of the moisture condition at or near the surface of the slab. It is based on the false premise this is the only information needed to make a proper decision about time of installation. The test offers no information about the level of moisture existing deeper within the slab.

This consideration is critically important. The moisture in a concrete slab exists in a gradient, with significantly less moisture at the surface than deeper down. However, once a slab is effectively sealed with a finished floor product, so moisture can no longer evaporate from the slab’s surface, the moisture inside the slab will tend to even out and the moisture gradient will disappear.

The net effect is the moisture at or near the slab’s surface, which is what the finished floor will now be in contact with, becomes higher than what a surface-based test would indicate. For this reason alone, one should not rely on this type of concrete moisture test. Results from the test may mislead and this could end with flooring failure.

In-situ RH test

On the other hand, scientific studies in recent decades have demonstrated a slab’s moisture condition (and how it affects installed flooring) can be best measured by looking at the RH deep down in the concrete using an RH probe set into the floor slab. Since 2002, the in-situ RH moisture test, as standardized in ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes, has become increasingly favored as the ‘gold standard’ for moisture testing.

The RH test is useful as is it is a depth-specific test that fully considers what happens to concrete moisture after a flooring installation seals off the slab’s surface. Studies at Sweden’s Lund University determined the placement of the RH probe[6] at a depth of 40 percent of the overall depth of the slab (when drying from one side) will provide RH readings that accurately predict the moisture[7] that the finished floor will ‘see’ once the flooring is installed.

In growing recognition of this and other major advantages of the RH test (less vulnerable to changing ambient conditions, faster and easier to perform with test results within 24 hours, and ability to easily track and record RH changes over time), a large number of flooring manufacturers now provide specifications for their products based on the RH test’s numeric results.

In the author’s experience, no other method of concrete moisture assessment provides the same level of reliability and accuracy as does the in-situ RH test. Therefore, no other method can give the same level of assurance for avoiding costly moisture-related flooring failures and the many types of damage (such as cupping, crowning, buckling, mold, or mildew) that could otherwise occur in wood floors.

What it all means for the project’s specifications

It is imperative a project’s specifications always identify and require the exact concrete moisture test to be performed. This is an easy addition to the project’s plans given the in-situ RH specification[8] is available for free download.

It is important to recognize the in-situ RH test has been shown to give reliable results leading to consistent, successful project outcomes. Additionally, unless this specific test is spelled out for the general contractor (GC) and/or the flooring professional, some parties in the industry may unwittingly choose to employ another test that may place the project at risk. Therefore, it is crucial to be very specific about which concrete moisture test is desired.

Jason Spangler, Wagner Meters’ flooring division manager, has more than 25 years’ experience in sales and sales management across a broad spectrum of industries. He has successfully launched a variety of products to the construction market, including the original Rapid RH concrete moisture test. Spangler, who received an MBA from West Texas A&M University, has extensive industry involvement, including the National Wood Flooring Association (NWFA) and the International Certified Flooring Installers Association (CFI). Spangler is also vice-chairman of associations for the Flooring Contractors Association (FCICA). Spangler can be reached via e-mail at[9].

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