Making Concrete Substrates More Concrete

Concrete Subfloor
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Installing hardwood over concrete, especially when using a direct glue-down method, can be a challenge. More often than not, the concrete surface appears clean, dry, and sound, but looks can be deceiving. Here are several tips that may help clarify potential substrate issues and avoid problems.

First, recognize the subfloor grade. The grade, or general exterior ground level, is an important first factor to identify as there is a difference in materials appropriate for below- and on-grade installations. Sometimes below-grade slabs can be difficult to determine. One rule-of-thumb is if the soil is 3” or more above the floor of any level, then that level should be considered to be below grade.

In all cases, for ground-supported slabs, it is important to verify the presence of an effective sub-slab vapor retarder. On new construction, ask the owner or the general contractor if a vapor retarder is present. For older projects, whatever vapor retarder that may have been present likely has disintegrated, so proceed cautiously. Coring to verify is an option and note that newer polyolefin plastic sheet materials last much longer.

It’s the surface that matters. Just because the concrete has met the specified compressive strength requirement does not mean it is “sound” concrete. If curing wasn’t performed properly (kept wet long enough), the surface may not have hydrated sufficiently to fully develop its strength potential. This can lead to issues like dusting (Fig. 1, page 56) and mortar flaking (Fig. 2, page 56). The surface should not gouge easily with a hand tool; this is an indication of a soft surface.

The concrete also can have internal delaminations – near-surface (Fig. 3, page 57) or deeper – that aren’t visible. The flooring may bond to the weak surface, but the weak surface may not bond to the material below, causing the flooring installation to fail. Potential delamination can be “sounded” by dragging a chain across the surface. The “ring” of a sound slab to the “rasp” of a delamination is a great audio indication of a potential problem beneath the surface.

The substrate must be clean. Contaminants such as oil, paint, and drywall compound can prevent bond. Other products like wax-based curing compounds also must be removed completely. Many of these surface contaminants may not be visible. If the “contaminant” penetrated, as can occur when solvents are used to clean previous mastic, the concrete surface may require removal.

Check the surface porosity. The porosity/absorption (ASTM F3191) of the prepared substrate surface will impact the amount of adhesive needed, as well as the adhesive open and working times. In some instances, the use of a primer may be advisable – consult with the adhesive manufacturer.

Another factor that affects the amount of adhesive is the concrete surface profile (CSP), which is the texture of the concrete surface. The NWFA Wood Flooring Installation Guidelines has more information on the CSP, as does the adhesive manufacturer.

NWFA Technical Guidelines

Check the flatness of the slab using a straight edge. As stated in the NWFA Guidelines, the substrate should be flat to within 1/8” in 6’ or 3/16” in 10’, unless otherwise specified by the wood flooring manufacturer.

Jointed concrete slabs naturally deform after placement. The amount of deformation may cause the slab to exceed the flatness tolerance. If the substrate is out of tolerance, grinding high spots and/or filling low spots is recommended to reprofile the surface and avoid excess, inadequate, and variable adhesive thicknesses. However, if the deformation was excessive, the reprofiled slab may relax once the flooring is installed, which can buckle the flooring. Slab subsealing may be advisable (see sources).

Making Concrete Substrates More Concrete Figures 1 and 2

Fig. 1 (left above) – Dusting is due to a weak concrete surface either from a lack of curing or finishing in excess water.

Fig. 2 (right above) – Mortar flaking over near-surface coarse aggregate due to a lack of curing.

Verify the moisture condition of the concrete substrate by standard test methods (ASTM F2170 and F1869), both discussed in the NWFA Installation Guidelines. As part of the hydrologic cycle, moisture (as a vapor) rises vertically from the water table, regardless of its depth, toward the clouds in the sky everywhere on Earth regardless of the ambient relative humidity (RH) or climate. And while concrete can resist liquid water (as in dams and water tanks), it is permeable to water vapor. When vapor reaches an area at or near the dewpoint temperature, it condenses to liquid. Ideally, this phase change (vapor to liquid) occurs below a sub-slab vapor retarder, before it enters the concrete, but it also can occur within concrete pores and capillaries.

Gaseous vapor does not transport soluble alkali salts. However, the concrete surface contains abundant soluble salts originally transported and deposited by liquid bleed water during concrete placement. These salts can appear as efflorescence (Fig. 4) on the slab as it dries, especially if the concrete surface is rewetted, prolonging drying, prior to flooring installation. However, after installation, when the building is air-conditioned and the flooring is cool, vapor can condense within the concrete surface and form a high pH solution that can break down adhesives. Therefore, it is imperative to verify the presence of a low-permeance vapor retarder beneath the slab to mitigate water vapor transmission. Moisture testing the slab also is required to determine that the concrete, itself, doesn’t contain sufficient moisture to impact the adhesive in the short- or long-term.

Making Concrete Substrates More Concrete Figures 3 and 4

Fig. 3 (left above) – Delamination can be the result of finishing air-entrained concrete, troweling before the bleed water has finished rising, finishing concrete setting from the top down (crusting), or a poorly bonded topping.

Fig. 4 (right above) – Efflorescence is the deposit of soluble alkalis (mainly potassium, sodium, and calcium) from the cement/concrete left behind after liquid water on or in the surface evaporates. They should be cleaned off the surface.

If the concrete contains an integral admixture that claims to mitigate moisture, verify it reduces the water vapor transmission rate (WVTR) to the same permeance required of vapor retarders (ASTM E1745) and topical mitigation systems (ASTM F3010). To qualify as a vapor mitigation, a maximum permeance of 0.1 perms is required and effective mitigation products reduce the WVTR to less than 0.01 perms. The lower the permeance, the lower the risk of condensation and the development of adverse solution chemistry. If an integral admixture or reactive penetrant does not reduce the WVTR, it does not mitigate moisture vapor. The same is true of ineffective vapor retarders and surface coatings.

Sometimes it’s easy to visually identify issues with concrete substrates, but oftentimes it’s not. It’s these hidden issues that cause the most problems for hardwood flooring installation failures. It pays dividends to spend a little more time on due diligence with concrete substrates. As the skilled craftsman advises, “measure twice and cut once.” Or, as I like to say, be a hero to your client!

Scott Tarr is a consulting engineer and president of North S.Tarr Concrete Consulting, P.C. based out of Dover, New Hampshire. He has presented more than 200 seminars and authored or coauthored more than 250 technical reports and more than 75 technical publications, including the books Concrete Floors on Ground for the Portland Cement Association and Guide to the Design and Construction of Concrete Toppings for Buildings for the American Society of Concrete Contractors.


  1. Wood Flooring Installation Guidelines, National Wood Flooring Association (NWFA), St. Louis, MO, 2019.
  2. Tarr, S.M., Craig, P.A., and Kanare, H.M., “Concrete Slab Repair:
    Getting Flat is One Thing, Staying Flat is Another!” Concrete
    Repair Bulletin, Vol. 19, No. 1, International Concrete Repair Institute,
    Des Plaines, IL, January/February 2006, pp. 2-5.

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