Concrete Q and A: The questions never stop!

Christopher Capobianco

Just as I was contemplating the topic of this month’s column, three situations came my way that not only gave me inspiration for the topic but also reinforced that there is still a lack of understanding of concrete floor slabs and how they interact with floor coverings, even by people with tremendous knowledge of building design and construction. It’s no wonder that over $1 billion in claims are attributed to moisture other issues related to concrete.

The first question came from a reader on the Coverings website who asked, “I have a potential VCT flooring job that I would like to install over the existing VCT flooring. The only problem is indentations and bubbles in some of the existing tiles. Is that from moisture not being able to escape? What do you recommend to correct this problem because I really do not want to take up all the tile — it’s a really big job.”

I’d recommend picking up some of the affected tiles to see what’s going on beneath. This could be telegraphing of irregularities in the concrete subfloor. Indentations could be in the tile itself because of heavy weight or furniture, or could be telegraphing of dips in the substrate that were not addressed prior to the original installation. Bubbles could be caused by bumps in the concrete, debris left behind, and so on. At the same time that you remove tiles, run some moisture tests throughout the floor to see what the condition of the concrete is. Run the ASTM F 2170 relative humidity test throughout the space, which is easy to do by picking up a single tile, drilling a hole in the slab, placing a probe and coming back the next day for a reading. This is easier than calcium chloride testing that requires you to pick up four tiles, grind the slab clean, wait a day, place the test and then wait three days. If there is a moisture problem, that will need to be addressed, and you certainly won’t be able to go over the existing floor.

Affected tiles could be telegraphing of irregularities in the concrete subfloor, but test for moisture since that is often the culprit causing tile lifting and cracking (below).

Another question came to me from one of the largest architectural firms in the world.

“Are you aware of any issues where concrete sealers applied to concrete slabs interfere with installation of floor coverings? I suspect most concrete pours are cured with the application of curing and sealing compounds, but I seem to recall issues where resilient flooring installations are not compatible with concrete sealers. Any insight from your experiences?”

In the case of sealers or curing compounds applied to new concrete, the instructions from adhesive and floor covering manufacturers have always called for concrete to be clean. The industry standard (ASTM F710 section 4) goes into pretty good detail, suffice to say to that clean concrete slabs are critical before you patch, level or apply adhesive prior to floor coverings of any kind. Many installers I’ve known have a standard practice of giving the floor a light sanding before starting with their regular floor prep — easily done with a buffing machine and 20 grit sanding pad. In previous columns, I’ve shown what happens when installers just skimcoat over old adhesive, dirt and such, which can be a disaster. Water-based adhesive on the floor covered with water-based patching compounds will sometimes soften up lose their bond to the floor. This can cause the new floor covering and everything under it to release, which is never a good thing. In the case of resilient floor coverings, that that layer of adhesive under the patching compound can also provide just enough cushioning that indentations become visible in the finished floor coverings. Again, in the importance of a clean concrete slab cannot be overstated.

On the same subject, somethings the word “sealer” is used to describe a moisture mitigation system. That’s not really the right term, but to be clear, not all mitigation products can have floor covering adhesive applied directly to them. The most common application is that a mitigation coating is applied to clean concrete, and when cured a primer is applied and then a topping such as a leveling/patching compound to provide a smooth substrate and a suitable bonding substrate for the floor covering adhesive.

The third scenario that came my way also involved covering a concrete slab prior to installation of resilient. In this case, it was a below-grade basement that was being finished for a playroom and laundry room. The slab had been trenched in some places to install French drains. The architect said the plan was to fill the trenches with new concrete and then plywood was to be installed on top of the concrete as underlayment for vinyl flooring that I was helping the architect select. I could not throw up a red flag fast enough!

First, with regard to trench cuts, this is a common practice that’s the cause of many flooring failures. In addition to scenarios like the one I just described, we often see this in retail stores where they need to run electric or plumbing out into the middle of the space. These trenches are often filled with concrete that’s of unknown composition and may be mixed with too much water. In addition, if the slab had a vapour retarder beneath it, it’s compromised by this procedure. In both cases, moisture levels are very high at this spot in the floor.

Ideally, assuming the slab has a proper vapour retarder beneath, the way to handle this is to fill with a fast-drying material like a self-leveling underlayment. However, most general contractors will balk at the cost of that procedure. The alternative is to fill with concrete, properly mixed, then apply a moisture mitigation coating over the top, and smooth the whole with a good patching compound.

Plywood over concrete as underlayment for an adhered resilient floor is a bad idea on many levels and you’ll see warnings about that in many flooring manufacturer guidelines. My first time experiencing this was early in my career as a flooring retailer. A customer had converted a garage into living space and installed plywood on the slab with concrete nails. Our installers used patching compound to smooth the floor before installing sheet vinyl. Within a short time, we got a call from the customer complaining about pock marks, or indentations, in the floor. The plywood had absorbed enough moisture from the concrete to swell ever so slightly, except where it was fastened. Those little indentations were all of the nails.

I had discussed this point of covering concrete in spaces that were never designed to be occupied in my column “Recycled Buildings ” in Coverings this past January. It’s important to deal with potential moisture in these cases, regardless of age of concrete. Because the space was not designed to be occupied so it’s unlikely a vapour retarder was used under the slab. That’s not to mention that if you fasten with nails or screws into the concrete, fasteners can crack the concrete and accelerate movement of moisture to the surface.

In both of these scenarios, a far better option would be to use a moisture mitigation coating to block the moisture and then patching or leveling compound to smooth the concrete and increase the height if needed. If sound or thermal insulation is needed, a layer of rubber underlayment can be incorporated.

Christopher Capobianco has been in the floor covering industry since the 1970s in various roles including retail and commercial sales, technical support, consulting, journalism, education and volunteer work. He currently is part of the sales team for Spartan Surfaces in New York City. You can reach him via

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