We can’t really talk about “self-leveling” underlayment without first clarifying the technical part of that phrase. While the term commonly is used, by the purest definition, most of the compounds that are poured out are not truly “self-leveling.” They start off as very liquid products that, because of gravity, seek to find a “leveled” state, but generally, they need some help moving into place (gauge rakes, trowels, etc.). If you can pump enough of a very liquid product into place, it is more likely that it will “self-level.” Outside of that, a better description of most products would be “smoothing or flattening products that need a little help getting into place.”
For clarification, I am making more of a 30,000-foot statement and not addressing all the chemistry and technical aspects of the products being produced today. There are both vast and subtle differences between the countless formulations. There are products designed to accomplish virtually every task and scenario we can think of. My point is to talk quite generically about the practical aspects of “self-leveling” underlayment materials and when they should or should not be used.
That said, we also need to talk about patching/skim coating. I need to be clear on this point: floor patch is just that – a “patch” for small areas. Patching material is not intended to be used to “level” or even “flatten” a substrate.
If you have some seams, “bird baths,” or slight irregularities, then a patching material generally is appropriate. Application of a “skim coat,” while a common practice, is not accomplishing much more than a weak primer. If that material is not formulated to hold moisture, the application of a water-based adhesive can deteriorate and dis-bond that skim coat. You will need to perform two or three applications with sanding in between to get to a layer thickness close to 1/8” to 1/4”. At that point, you have expended significant amounts of labor, material, and money to accomplish very little.
Simply put, use a patching material to patch a floor and a leveling compound to flatten (or level) a floor. If you are looking to flatten (or level) a floor completely, you will need to pour a leveling compound. It will be faster, more cost-effective, and leave you with a much better surface on which to install flooring.
Self-leveling underlayment (SLU) is going to be either cementitious (Portland) or gypsum (natural and synthetic) based. Portland cement originally derived its name from its resemblance to Portland stone, a type of building stone from the Isle of Portland in England. Portland cement today is mainly a combination of different limestones and a variety of different additives. Gypsum occurs naturally, but can be produced synthetically. Synthetic gypsum also is called flue gas desulfurization (FGD) gypsum. It is produced through a chemical reaction in the chemical scrubbers that remove sulfur from the flue gases of coal-fired power plants.
Gypsum underlayment traditionally has been considered a “no-go” for hardwood installation, but the technology drastically has improved, and they certainly could be approved for hardwood. Some gypsum based products are formulated to have even higher compressive strength than concrete. It is important to confirm that the gypsum underlay is approved for hardwood installation (solid or engineered) and the flooring manufacturer will approve installation of their product. Most will have a minimum compressive strength requirement.
SLUs can be designed with virtually limitless differences between them. No single product will “do it all.” Some differences could include how it flows, heals, dries, builds strength, what final strength it has, whether it is suitable for exterior applications, how thick it can be poured, etc. This is true for cementitious and gypsum-based products. So, it is important to fully understand what each product is designed for and its limitations. As always, refer to the technical information from the manufacturer and check with your local representative or their technical department. And, of course, attend manufacturer and industry-provided training.
Don Jewell is the head of technical for Loba-Wakol LLC in Wadesboro, North Carolina. He can be reached at firstname.lastname@example.org.