Proper subfloor preparation is key to any flooring installation, but this crucial step is often skipped to accommodate busy schedules. In our last issue, we spoke with a panel of manufacturers about subfloor preparation considerations including what to do when asbestos, cutback/adhesive residue and moisture issues are discovered. We talk to these same manufacturers this month about steps to take when encountering old curing or moisture mitigation compounds and how to handle a slab that’s cracked, curling or not flat.

Old curing compounds. Eric Kurtz, Bostik hardwood installation systems market manager, stated old curing compounds can be difficult to detect but will still pose problems. “Most adhesives will bond to the curing compounds very well, but the compounds do not bond as strongly to the concrete as the adhesives would. As the floor expands and contracts, the curing compound can flake off the slab causing the flooring to debond.”

According to Steve Taylor, Custom Building Products’ director of technical and architectural marketing, not all moisture mitigation compounds will interfere with a dry-set mortar bond. “It is advisable to find out what was used and discuss it with the mortar manufacturer. If there is not any information concerning the additive, it is best to profile the surface by shot blasting or using a similar method to improve the mechanical bond to the surface.”

Laticrete’s Yoni Feldman, underlayment product manager, stated that old curing compounds should be removed wherever possible. “Remove them by mechanical means to help ensure a clean substrate free from any potential bond breakers.”

Dan Marvin, MAPEI director of technical services, recommends performing basic tests to see if there might be a problem. “A droplet test can be conducted to see if water beads on the surface. If it does, chances are good that a topical or integral sealer is present, which will reduce the effectiveness of many flooring adhesives. A pH test can also be used to determine the amount of alkalinity at the surface.”

He added, “Applying a test patch of the adhesive system being considered is always a best practice but keep in mind that different areas of the same slab can behave differently. Shot blasting and scarification can also help adhesives have something to grab onto when the concrete is suspect, especially in the case of topically applied sealers or curing compounds. When in doubt, core samples can be sent to an outside lab to determine exactly what is in the concrete.”

Sonny Callaham, Royal Adhesives & Sealants technical product manager, said curing compounds should always be removed from the slab. “Although, chemically the adhesive and curing compound might be compatible, the mechanical bond will be compromised if the curing compound was not applied correctly. And there is no way to test and see if it was applied correctly.”

Stauf’s technical director, Mark Long, said contractors should take the time to test and evaluate the slab to see if any compounds are present. “From there, you can decide what to do, but it’s hard to do anything unless you get down to the concrete in some of those situations. In reality, the general contractor or builder is not going to want to hear about what has to be done, but it’s important they know about it. If you go through with the installation without telling them what you found, it’s on your shoulders.”

Unsuitable slabs. Craig Morris, Ardex technical service manager, said cracking in concrete can happen for many reasons, including temperature changes and structural movement. “Depending on the type of crack and the proposed flooring structure to be installed over top, they may be ‘chased’ out to remove any contaminants followed by flash patching with a cementitious patching material or injected with a resin filler to ‘stitch’ the concrete. However, if the crack or joint is active, it must be honored up through the flooring structure. For active cracks where displacement exists, a structural engineer should be consulted to confirm concrete integrity.”

He also said to contact a structural engineer when the unevenness of a floor is related to slab curl. “The slab must first be stabilized prior to commencing with crack repair and finish flooring installation.”

Kurtz noted that self-leveling underlayments or polymer-modified Portland cement patches can both be used to create a flat surface for flooring installation. “Significant cracks should be sealed with an elastomeric sealant to prevent moisture intrusion.”

For large format tile, the slab should have variations of no more than 1/8” in 10’, Taylor stated. “If the slab is stable, this may require grinding or leveling with a cement-based leveling compound. If there is still the potential for movement in the slab, it may be necessary to rely on the use of an unbounded mortar bed as described in TCNA F111 to prepare the floor for ceramic tile.”

Ron Loffredo, H.B. Fuller Construction Products senior area technical manager, said while small voids can be filled with patch, “larger cracks should be filled with insulation or backer rod, and then self-leveled.”

InstaFloor’s Paul Laporte, vice president of sales, Canada, noted the subfloor should always be evaluated prior to any installation. “This is critical to prevent problems occurring down the road. With a little care and time you will save thousands of dollars in potential claims.”

Marvin added, “Cracks should be filled prior to any flooring being applied. Slabs with curling or flatness issues are perfect candidates for self levelers. Any cracks that are out of plane (one side higher than the other) or more that 1/8” in width can be an indication of structural issues and should be checked by a structural engineer before proceeding.”

Tim McDonald, Merkrete vice president of sales, said self-leveling products are a great way to bring a slab into tolerance, but they cannot be used in every situation. “If you go into a commercial kitchen, where you are supposed to pitch the floor to the drain, the floor cannot be dead flat. That’s where you need underlayments that can go as high as four inches and can be tapered down to zero while still maintaining a high compressive strength. You do not want a self-leveler in this situation.”

Long said to take time and address cracks properly, not just assume a one-size-fits-all approach will work. “Taking the proper time to fix the cracks will save you some headaches down the road. Don’t just put floor patch over all the cracks and think you’re done with it. Make sure you’re using the proper products.”

Kirk Kazienko, USG technical sales manager, added, “USG Durock brand floor prep products are meant to provide a flat and level subfloor when properly applied. However our products aren’t ‘cure-alls.’ Even properly addressed and prepared cracks can still telegraph through floor prep products” in some instances.