Taking Your Subfloor Game to the Next Level

When I started in the flooring business more than 20 years ago, my dad would send us on all kinds of missions to learn new skills. Self-leveling floors was one of those memorable adventures. We would load up the truck with 30 large bags of leveler, whatever primer we had at the shop (if any), a mixing drill, and a stack of buckets. We didn’t have cleats, gauge rakes, spike rollers, or any of the fancy tools we use today. He told us to mix up the bags and keep adding water until we saw light brown swirls. That’s how you knew you had enough water. We would dump the mixture out, stretch and push it with a trowel as far as we could reach, repeating that process until all the bags were gone, and the floor was self-leveled. Well, pretty much.

I tell that story to installers when I do a training workshop because, I think for many of them, the idea of self-leveling is either risky, overwhelming, or simply unknown. In the past, I’ve been there and felt that way too. There are a few keys to a successful self-leveling job, and getting these points right can make the process less intimidating.

Substrate Preparation (prep for the prep). One of the most-important things to understand before self-leveling is the substrate requirements. Substrate moisture conditions should be addressed in advance when needed, and the dry floor prepared. I ask several questions when someone calls me, and one of the first is:

“What are you going over?”

Some answers to that question could include concrete, gypsum, plywood, or adhesive residue. Technical data sheets might say, for example: “Subfloors must be smooth, sound, clean, dry, and free of any contaminants that may hinder adhesion.” That means any loose debris, contaminants including dust, or any other bond breakers must be removed and the surface should be clean so primer can bond well. Pay attention to any concrete surface profile (CSP) requirements also, in case you need to abrade the floor. It is important to note that most self-leveling underlayments, especially cement-based levelers, should not be applied over existing adhesive residues. However, there are exceptions, which I will cover a bit later.

Primer. In most cases, a self-leveling underlayment will require the application of a primer. The importance of a primer cannot be overstated. Primers perform many functions including binding residual dust, reducing absorbency, and improving adhesion. The most-important of those, primers, facilitate a good bond between the self-leveler and the substrate. It’s important to make certain you are using the correct primer for the job. Typically, primers are classified based on the porosity and type of substrate. If you are going over a porous floor (such as gypsum or porous concrete), you’re going to use a different primer or dilution ratio than you would over non-porous floors (such as adhesive residue or existing tile floors).

An important thing to note about concrete is that you cannot automatically assume that a floor is porous because it is concrete. We run into many cases where a concrete floor has either been finish-troweled too tight and smooth, or where an additive or sealer has been introduced into the slab that makes it non-porous. This could potentially cause a failure if the primer cannot be absorbed into the capillaries of the concrete. It is recommended to perform a water drop test. If a small droplet of water absorbs into the substrate within about 60 seconds, it is considered porous. If the droplet remains after 60-90 seconds, it is non-porous and may require either a different primer or some profiling (refer to ASTM F3191 for more detail on porosity).

Self-leveling technology has come a very long way in a short time, and just like any other tool in your truck, you need the right one for the job. Cement-based self-leveling underlayments typically can be applied from about 1/8” up to 3” deep in one pour, and are designed for use over concrete substrates.

Type of self-leveler. Self-leveling technology has come a very long way in a short time, and just like any other tool in your truck, you need the right one for the job. Cement-based self-leveling underlayments typically can be applied from about 1/8” up to 3” deep in one pour, and are designed for use over concrete substrates. In some cases, it can be a challenge to achieve a smooth and flat surface if you’re trying to apply just 1/8”, so be sure you’re allowing for enough material to get a nice smooth flow during application. Other types of levelers like synthetic gypsum levelers, and some patented hybrid technology have very low shrinkage and almost no tensile pull during curing. That means they won’t pull on the substrate as they dry, which makes them ideal for use over more-challenging substrates like gypsum, non-water-soluble adhesive residue, and even plywood and OSB without the use of metal lath. Additionally, some synthetic hybrid levelers are climate-irrelevant, which means they will cure regardless of HVAC conditions and can be ready for finished flooring the very same day. Simply put – cementitious levelers are best over concrete, and synthetics and hybrids are best over everything else.

Jeremy Waldorf is Schönox’s technical sales representative for Michigan and Toledo, Ohio. He can be reached at 517.316.5380 or jwaldorf@hpsubfloors.com. Learn more about Schönox by visiting hpsubfloors.com.

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