Self-leveling underlayment being poured out in preparation for spreading. Ease of application is an important property for these materials.

As anyone who has ever laid resilient, wood, or ceramic tile flooring can attest, the flatness and smoothness of the substrate the flooring is applied to is critical for a successful flooring application. Uneven substrates can lead to a wavy appearance in the finished floor. Gouges and bug holes in concrete, seams, knots, and nail heads in wood underlayment will telegraph through resilient flooring and cause an unacceptable final appearance to the flooring application.

In a perfect world, the concrete slab would be smooth and flat, the wood underlayment would be knot free, with the seams all filled and smooth when you arrive to lay the flooring. Unfortunately, we do not live a perfect world, so we often have to make some minor repairs to the substrate, or request the general contractor or owner make repairs to the floor substrate to allow for proper application of the flooring. Fortunately, there are products available to help make repairs to problem substrates and provide a suitable surface for applying various flooring types.

There are three types of cementitious floor prep products currently available to help the contractor and installer remedy substrate flaws.

The first type of product is the self-leveling cementitious underlayment. This material is a flowable, self-leveling mix that is generally able to be applied over existing substrates to level uneven areas and to fill small surface defects. Most underlayments are also pumpable for ease of placement over large areas; contact manufacturers’ technical representative for specific pumping recommendations. Self-leveling underlayments can be applied unextended over large areas at thicknesses between 1/8-inch to 1/2-inch. When extended with the proper aggregate, depths between 1/2-inch and several inches are possible. It is a recommended that the manufacturer’s technical department be contacted for specific information when deep pours are planned.

Because these products are self-leveling very little effort is required to get a flat floor. Some light troweling or floating may be necessary to maintain a flat transition between pours and the use of a porcupine roller is useful to help avoid pinholes on thin applications. Also, self-leveling underlayments are generally fast setting, so application of many flooring products can take place in as little as 24 hours. Be sure to test for moisture content before applying flooring that is moisture sensitive, such as vinyl sheet goods. These materials may require a little extra time for the underlayment to reach the proper moisture level.

The second type of product is a trowelable cementitious patch suitable for filling and leveling small areas of rough concrete, as well as other types of masonry flooring, and for filling and smoothing seams and defects in wood underlayments. This type of product can go from a 1/16”- up to 1”-thick and is fast-setting and non-shrinking to allow earlier placement of floor coverings, often in as little as 30 minutes up to 24 hours, depending on the thickness of the patch and curing conditions. These patches are generally polymer modified to give them better adhesion. For maximum adhesion and additional flexibility to concrete and wood, additional polymer additives may be used. With the addition of the liquid polymer additives mentioned above, these products can even be used over properly prepared ceramic tile, quarry tile, terrazzo, or cutback adhesive residues.

Thin patch and embossing leveler products can be “feather-edged” to allow smooth transitions and filling of vinyl flooring embossing prior to applying a new flooring system. Due to their high levels of polymer modification, these products can go over cutback adhesive residue in many instances.

The third type is the cementitious, aggregate-free skim coat/embossing leveler. This material has no aggregate in it, which allows for very thin applications with a resulting smooth surface. It has a high level of polymer modification to allow it to be used without a primer on many difficult surfaces, such as ceramic tile, terrazzo, resilient flooring and adhesive residue. These materials can generally be applied from feather edge up to 1/2-inch in thickness. Since these materials are aggregate-free, they work very well as embossing levelers for smoothing out the texture patterns of embossed vinyl flooring.

Although these types of products are made with the latest technology in cement chemistry, certain basic rules apply for their successful use. Regardless of the type of product, surface preparation is critical for a successful application of these products. Substrates should be clean, dry and free of any contaminants that might interfere with adhesion. Very dense or slick surfaces should be mechanically roughened to allow for better grab. Primer may be necessary for certain substrates and conditions (be sure to check with the manufacturer if you are unsure about the necessary surface prep for your particular substrate). Overwatering is another common error when using these products, which can lead to longer cure times, low strengths, excessive shrinkage and ultimately failure of the application.

These products can successfully be applied over cutback adhesive residue, however, good practice suggests that the best performance of these products over cutback adhesive residue is to remove as much of the cutback adhesive as possible prior to applying these products.

Any products’ performance is only as good as the substrate it is bonded to, and our experience shows that the thicker the layer of cutback adhesive left on the substrate, the more likely you are to have a problem with delamination of the flooring, usually due to an internal failure of the adhesive residue. Ideally, the amount of residue left after cleaning should be to the point where it looks like paint or stain residue. Use of the proper primer will also help to reduce the likelihood of problems over cutback adhesive.

Many flooring substrate problems can be repaired and/or smoothed over using the proper product or combination of products covered in this article. As with any product application, reasonable care in surface preparation and application will produce a substrate suitable for applying today’s flooring products.

Contact the manufacturer’s technical representatives with any questions on the products before beginning an application to avoid any misunderstanding of the proper products and applications for your particular situation.