Getting It Right: Wood Flooring Over a Concrete Subfloor

Previously published by Construction Specifier magazine

The visual appeal of a wood floor is hard to match. For most of us, there’s a certain joy we experience when entering a building in which wood has been used generously. We may not be able to put our finger on exactly why we react favorably to wood. It may be the touch of elegance or the warm glow that we sense. But whatever it is, the positive impression is indisputable.

Wood’s advantages are more than skin deep

Wood flooring offers many 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 inevitably 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 as much as 10 percent or more.

A wood floor also contributes to better indoor air quality. 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, which is one reason that hardwood gets used frequently in dance or music studios.

Beware of subfloor moisture

On any commercial flooring project where wood will be used, it’s important to consider in advance how moisture could play havoc with what you’re trying to achieve. You’ll want to be sure that you address all possible sources for excess moisture. The National Wood Flooring Association (NWFA) has estimated that at least 75% of all flooring failures are the result of moisture issues.

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 that they hold very little water inside. But in fact, 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.

Indeed, without enough water, a concrete slab won’t 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, you 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 that 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 five-inch concrete slab, expect that it may take five months or more for the slab to dry sufficiently, once the building is enclosed and the environment is conducive for drying, before one can successfully install a finished floor product.

Failure to (1) build this drying time into the overall project and (2) conduct an accurate moisture assessment of the concrete floor slab may mean that one is scrambling later to address one or more serious moisture-related problems. One’s 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

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

Cupping occurs when the edges of a board are higher than the center. Low relative humidity (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 on the edges. 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 without waiting for the moisture level to return to normal.

Buckling is an extreme reaction to moisture in a wood floor and typically occurs in response to prolonged exposure to excess moisture. Buckling is what 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 often all that is needed to address cracking.

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 that 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. When splotches of darker wood appear in the finished floor, this may indicate that the finished floor is in contact with moisture in the concrete subflooring.

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 that one should allow one month for each inch of thickness of the concrete slab and this is once the environmental conditions are conducive for drying.

Keep in mind that 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 that an accurate concrete moisture test be performed prior to any flooring installation. Getting accurate, reliable test results provides the essential information needed for deciding when one can safely install wood flooring over a concrete slab.

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 used most often 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 significant advantages of using another type of moisture test known as the in-situ RH test.

An inherent problem with a surface-based test is that it is unduly influenced by ambient conditions and can easily give false results. Another problem is that 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 that this is all that is needed to make a proper decision about when to install. The test offers no information about the moisture deeper within the slab.

This consideration is critically important. The moisture in a concrete slab typically exists in a gradient, with significantly less moisture at the surface than deeper down. But once a slab is effectively sealed with a finished floor product, such that 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 that 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.

Only the in-situ RH test is reliable

On the other hand, scientific studies in recent decades have shown that a slab’s moisture condition (and how it affects installed flooring) can best be 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, has become increasingly favored as the “gold standard” for moisture testing.

The primary reason the RH test is so useful is that 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 that placement of the RH probe 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 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, 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 a nutshell, no other method of concrete moisture assessment provides the same level of reliability and accuracy as does the in-situ RH test. And 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 with wood floors.

What it all means for your project’s specifications

What does this mean for the architect, engineer, or specifier working on a commercial building project where wood flooring is to be installed over a concrete floor slab? Simply put, it is imperative that 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 that the in-situ RH specification is available for free download at https://www.rhspec.com/download-the-rh-test-specification/.

Recognize that only the in-situ RH test has been shown to give reliable results that lead to consistent, successful project outcomes. And recognize as well that unless this specific test is spelled out for the general contractor and/or the flooring professional, some in the industry may unwittingly choose to employ another test that may place the project at risk. Therefore, always be very specific about which concrete moisture test you want to be conducted.

For more information about what to do to avoid moisture problems when using wood or concrete, contact Wagner Meters by calling worldwide toll-free at 541-291-5123.

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. https://www.constructionspecifier.com/artistry-meets-practicality-wet-slab-concrete/
• Spangler, Jason 2016. Moisture measurement in concrete floor slabs. A specifier’s primer. https://www.constructionspecifier.com/moisture-measurement-in-concrete-floor-slabs-a-specifiers-primer/
• Spangler, Jason. Hardwood floor problems: heed the warning signs. https://www.wagnermeters.com/moisture-meters/wood-info/warning-signs-floor-moisture-problems/
• Wagner Meters. Common wood flooring problems caused by moisture. https://www.wagnermeters.com/moisture-meters/wood-info/common-wood-flooring-problems-moisture/