Underlayments, like most subfloor preparation products, are the invisible and often unsung essentials of any professional flooring installation. These crucial products ensure the subfloor is primed and ready to accept further materials – whether that be a crack isolation membrane, a backerboard, flooring underlayment or the flooring itself. In part one of our two-part look at underlayments, we talk with manufacturers about the various patches, primers and self-leveling underlayments, and when to use them.

Patches and Primers. Craig Morris, Ardex technical service manager, said it is important to patch or skim coat a concrete slab prior to flooring installation. “With bonded installations, failure to patch and skim will result in the telegraphing of any defects in the concrete to the floor covering. In unbonded installations, it can bring previously out-of-tolerance concrete into tolerance.”

He said patching compounds can be used to correct cosmetic issues up to 1/2” in depth. “Self-leveling underlayments (SLUs) may go to an inch or slightly more in one application. With the addition of aggregate, self-levelers are often installed at depths of 3” or more. The general rule of thumb is to install the least amount of material necessary to achieve the desired smoothness.”

Bostik’s Chad Bulen, technical service manager, said primers are an essential step of installing a self-leveling underlayment. “They can be liquid-applied by roller, squeegee or broomed into the concrete to promote adhesion of the SLU. Secondly, primers inhibit the absorption into the slab of water that is mixed with the SLU.”

When applying a primer, an installer should look to create a uniform coating across the slab. “Take note of any pinholes or voided areas,” Bulen said. “If these are detected, a second application in these areas is recommended.”

Installers who work with ceramic tile should look for a slab with a variance of no more than 1/4” in 10’ or 1/8” in 10’ for larger tiles, noted Steve Taylor, Custom Building Products director of technical and architectural marketing. When working with patching compounds, “the manufacturer will specify the maximum thickness. Some cement-based patches can be applied up to 4” thick, while others should not be applied more than 1/2” thick.  Some patches also have a minimum thickness, to assure the durability of the patching material.”

Anamaria Cindric, marketing for Dependable/Keene Building Products, said a primer should be used when the bond is weak on a concrete floor or the concrete is highly absorptive. “A highly porous substrate is inclined to absorb water, which will quickly remove the water from a cementitious patching compound. This in turn causes the patch to not develop to its full strength and hardness.” She added the substrate needs to be clean and ready to accept the compound, which includes the removal of “bond breakers such as oils, sealers and dirt, to name a few.”

Ron Loffredo, H.B. Fuller Construction Products/Tec senior area technical manager, said his company’s patches and underlayments can fill up to 1 1/2” in a single application or 5” with proper aggregate. These compounds “can be used for deep fills, ramping and leveling concrete subfloors.”

The company’s primer can be used on a wide variety of substrates. “However, installers need to know that the necessary thickness of the primer varies depending on the substrate. With porous concretes, the primer needs to be diluted to be effective, while [the presence of] concrete curing compounds require the primer to be applied at full strength,” he noted.

Yoni Feldman, Laticrete product manager, said primers are most often applied as a thin layer, with patches often ranging in thickness from 1/8” to 1 1/2”.

According to Jeff Johnson, MAPEI floor covering installation systems business manager, the most important rule to follow when working with patches is adding exactly the right amount of water. “If you overwater a patching compound, it will not cure properly and will not hold up to traffic.”

He said patches are typically good for feather edge up to 1”. “For repairs that are thicker or deeper than 1”, a different type of product such as a screed should be suggested. Additionally, patching compounds are not recommended for use under moisture vapor barriers, because they are generally not moisture-resistant.”

Primers only need to be applied if the next layer of the system calls for it, such as when self-leveling underlayment is being applied, Johnson added. “Other than that, a primer might be applied to dry, dusty concrete to create a more suitable substrate for adhesive bonding.” While most of these types of products are acrylic latex-based, when moving to an epoxy-based formula “you will need to prime it again before you apply self-levelers or patching compounds because it creates a glass-like surface.”

Robert Dickson, Parex USA director of technical services, said “you can always use a primer, but they are not always needed. A primer typically makes the surface that will receive the patch a better surface to bond to. It also helps clean that surface from any debris.”

Patches are typically fast-setting. “Because of this, they can get warm to the touch. Before applying anything on top of them, the applicator needs to wait until they have cured and become cool to the touch.” Dickson added, “If a patch needs to be built up more, you can apply multiple layers to the height needed, provided the first layer is set up and cool to the touch.”

Kelton Glewwe, Roadware Inc. vice president of marketing, recommends repairing and filling any deep areas with a product like the company’s Concrete Mender. “It is excellent for crack repair, spalled joints and surface spalls. Use it to repair structural floor issues before adding self-levelers and underlayment materials. When a self-leveling underlayment is applied at uneven depths, there is a greater potential for cracking. If the underlayment is normally 1/2” thick and is suddenly 2” thick at a joint or spall, the potential for shrinkage cracks is increased.”

Schonox’s Russell Wright, southeast regional business manager, said his company’s leveling compounds can flow thinner than 1/8”, adding “to achieve a truly level floor, an installer would need to pour enough leveling compound to crest over the highest point in the substrate by approximately 1/8”. This would allow the leveling compound to continue to flow beyond the peaks.”

Alan Kin, Texrite sales and technical, said patches with coarse, medium and fine aggregate are used for different types of slab preparation. “The main uses of a fine filler patch system are for fine and ultra-thin applications for use with thin surface-conforming products like VCT tile and epoxy coatings. Coarse and medium aggregate patches are for filling and repairing medium to heavy, deep surface repairs.”

He noted primers create a better bonding surface by promoting Portland cement to hydrate and crystalize. “Rough or porous conditions are great candidates for using primers. Exterior or high-traffic service would be another area for using primers.”

Kirk Kazienko, USG technical sales manager, said his company’s USG Quik-Ramp Patch can be used where an increase in height up to 2” is required in a ramping situation, without the addition of an aggregate or additive. He added, “The Quik-Ramp product can be applied from featheredge up to 2” neat (no aggregate added), and up to 5” with the addition of aggregate.”

He noted any water-soluble adhesives on the concrete must be completely removed before applying a patch. “Additionally, do not use our patches in exterior applications or areas where continuous exposure to moisture is a possibility, whether occurring topically or from within the substrate.”

Self-Leveling Underlayments. Morris said that any self-leveler poured at a depth in excess of 1 1/4” to 1 1/2” needs aggregate added. “As with all cementitious materials, as they cure SLUs will exert a tensile stress against the substrate. The deeper the pour, the higher these stresses get. The addition of aggregate helps relieve these stresses. However, even with the addition of aggregate, SLUs are not structural in design and should not be used to make structural repairs.”

He said installers need to be careful when installing SLUs over a wooden subfloor. “No matter how well-built they are, wood subfloors move. Even something as simple as changes in temperature and humidity can lead to movement within the substrate, which can telegraph into rigid cementitious underlayments.” Morris added most manufacturers will require the wood flooring structure or underlayment be a minimum 3/4” tongue and groove, APA-rated Type 1 exterior exposure plywood. “Mechanical cleaning of the wood to remove contaminants can be accomplished via sanding.”

 Bulen stated some SLUs can achieve deeper pours up to 3” without aggregate or pea gravel. “The typical SLU requires pea gravel in a pour over 1” in depth.” He also cautioned installers on taking the term “self-leveler” too literally. “To an installer, the term self-leveling means they just pour and go. That is the exact opposite of what needs to be done. These products need to be tooled, raked or back-rolled to achieve the desired flatness of the floor.”

Taylor noted primer should always be used with SLU, not only due to helping improve the bond and keep water from leaving the leveler, but “more importantly it will help the flow and installation of the SLU.” He said the addition of aggregate, when needed, “will help control shrinkage in a deeper pour – anything greater than 2” in one pour.”

Portland cement-based SLUs will not bond properly to plywood or wood substrates, he added. “For those instances, it is necessary to incorporate expanded metal lath in the installation. The lath is screwed or stapled to the wood before the SLU is applied. As the SLU cures around the lath, it will be mechanically fastened to the wood substrate.”

Cindric also recommends using a synthetic or metal lath for a wood subfloor. She added, “One thing to take into consideration is that wood absorbs water. Exterior grade plywood is recommended because it is not as porous. Other types of wood subfloors should be approved by the manufacturer. Like any other self-leveling application, the wood subfloor must be primed first.”

When working with SLUs, Cindric recommends mapping out the room first. “For larger areas the product will need to be screed and spread out. It is a good idea to map out the room in order to keep pouring against the freshest materials, giving you a smoother transition.”

Loffredo said Tec SLUs can achieve a depth of up to 5” with proper aggregate. “We recommend the use of a well-graded, washed dry pea gravel 1/8” or larger, and the aggregate and substrate should be completely coated with the underlayment mixture. Using an aggregate is a cost-effective alternative to using multiple pours of an SLU to achieve a desired thickness. It saves money, time and materials.”

He said the difference between a competent and a great pour of SLU lies in the professional’s experience. “Experienced installers know how to place the products so they work the way they’re intended to work. They understand how to strategically place the product for optimum flow rate. Generally, installers should pour the cement in low areas first, and then work their way into the higher areas.”

Feldman stated that Laticrete SLU can be applied up to 3” in a single lift without the need of an aggregate. For thicker applications, aggregates including pea gravel can be used. When pouring onto a wooden subfloor, “the plywood used must be an exterior glue plywood and constructed to meet minimum deflection ratings as required by building code and industry guidelines. The adhesive used must meet ANSI A118.11 requirements and/or have an ISO or EN rating of C2P1.”

Johnson shared these rules of thumb when using an aggregate. “Add aggregate to about 50 percent of the desired thickness of the SLU. The aggregate needs to be washed, dry and nonreactive, such as pea gravel. The SLU is poured into the placed aggregate to a level about 1/4” to 1/2” lower than the final desired thickness. The SLU is aggressively raked into the aggregate bed. A final application of SLU is then placed on top of the wet mix to finish the installation with a smooth surface.”

According to Dickson, when pouring SLU on top of a wood subfloor, in addition to using lath, “it is recommended to tape the joints (masking tape or duct tape) to prevent the material from running through the gaps. Use a primer, too.”

One common misconception about SLU is that it can be used to plug a hole, he added. “Yes and no – the SLU will find the hole and start to drain through it, and level the area below the hole. It might eventually fill the hole when it sets up” but since that is not the primary purpose of the material the practice should be avoided.

Wright stated when working with SLU over wooden substrates, “it is important to review the manufacturer’s technical information to verify recommended and warranted uses. Many wooden substrates undergo a significant amount of movement, and most cement-based products are not tolerant to movement and deflection.”

Kin said accurately measuring the water for the SLU pour is critical. “These products are fluid and very wet by nature, and are prone to severe cracking, surface color change and shrinkage/delamination when too much water is added. The experienced installer should recognize the manufacturers of SLU products have different bag weights and corresponding water ratios. Water measurement is absolutely critical.”

He also cautioned against placing SLU over clear curing compounds, paint overspray and old adhesive residue. “Often the SLU will bond to the residue but the residue/contaminants will later break loose, causing a full installation bond failure. A fully cleaned or prepared surface should readily absorb clean water droplets and darken when they are placed on the bond surface. Make sure to continue the preparation or removal of the surface until this simple surface water absorption is achieved.”

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Kazienko said his company’s UltraCap, Speed and EcoCap SLUs can be poured neat (with no aggregate) up to 2” in one lift, and SSD pea gravel can be added for depths between 2” and up to 5”. He also recommends contractors always use installers who are familiar with the material and best installation practices. “Any self-leveling underlayment is only as good as the applicator and the preparation that went into the subfloor.”


NOTE: See next month’s issue for Part 2 of our look at Underlayments, covering crack isolation membranes, backerboards and flooring underlayments.