Subfloor preparation is quite possibly the most important step – the foundational, starting step –of any installation, yet it’s also often performed incorrectly or not done at all. As we here at FCI hear time and time again, “The flooring is only as good as what’s beneath it,” and that extends to the subfloor and how it is prepared to receive a floor covering.

We asked manufacturers about what happens when an installer or contractor walks onto a jobsite and finds asbestos, old cutback adhesive residue, or high levels of moisture. They offered their insights on these conditions, and all of them stressed the same point: “Do not skip or skimp on subfloor preparation to make your schedule work.”

In the next issue (December 2014) we will conclude this two-part series with a look at what to do if a slab exhibits problems such as curling and cracking, as well as what to do if old curing compounds are discovered in the concrete.

Asbestos. Craig Morris, Ardex technical service manager, said the presence of asbestos-containing materials should be identified at the time of the initial job walkthrough so there will be no surprises. “This is not something the installer wants to find out after the job has started as this could be a costly oversight.”

He added that any removal of asbestos-containing material should only be handled by professional abatement companies that understand the federal, state and local regulations. “One important caveat is the abatement company should be strongly discouraged from using chemical adhesive removers. Their use may negatively affect any new adhesive and/or floor covering that will be installed over top. For alternatives to chemical abatement, consult the Resilient Floor Covering Institute’s (RFCI’s) Recommended Work Practices for the Removal of Resilient Floor Coverings. The manufacturer of the finish floor covering and/or adhesive also should be able to provide direction on how to properly remove existing adhesives residues so the bond of their flooring will not be impacted.”

Custom Building Products’ Steve Taylor, director of technical and architectural marketing, agreed that installers should follow all regulations, and also consider the type of flooring to be installed. “In some cases it is acceptable to install ceramic tile over well-bonded flooring that contains asbestos. The installer should contact his installation product manufacturer for installation recommendations.”

InstaFloor’s vice president of sales, Canada, Paul Laporte, noted: “If the asbestos is in a stable state, meaning the tiles are still in good condition and they are not curling up or breaking apart, then simply placing a damp-proof membrane (DPM) barrier of a minimum of 6mil over the area prior to the installation of InstaLay will suffice. Ensure overlap of seams and taping at joints, and run the DPM up the walls about 2-4 inches to allow moisture buildup to escape. As long as the asbestos is not disturbed there is no risk to the environment or the occupants.”

Dan Marvin, MAPEI director of technical services, said any flooring that looks suspicious needs to be tested. “If you suspect an older flooring surface or adhesive is asbestos-containing, it should be tested by an independent lab prior to bidding the job.”

According to Sonny Callaham, Royal Adhesives & Sealants technical product manager, contractors should “learn the local and federal guidelines. The guidelines can vary from city to city. Then determine what needs to be removed and what can stay. Typically asbestos that is well-bonded to the substrate is not a health hazard for the building occupant; it’s only when the asbestos becomes friable [easily crumbled] that it becomes a health concern.”

Added Kirk Kazienko, USG technical sales manager, “USG does not recommend installing our USG Durock Brand floor preparation products over any subfloor, flooring or adhesives that may contain asbestos. Asbestos must be mitigated prior to installation of any USG Durock floor prep products, or any USG products at large.”

Cutback and other adhesive residue. Morris recommends complete mechanical removal, via grinding, shot blasting or some other method, of any water-soluble adhesive before installing patch, underlayment or other subfloor preparation material. If this step isn’t taken, “the tensile pull created by cementitious underlayments can literally pull any residue off the concrete, leading to a disbonding situation with the underlayment.”

If a new adhesive will be applied without leveling or skimming, at the minimum the residue should be razor-scraped, he added. “If the adhesive residue is well-bonded and a similar adhesive will be installed over top, then often times it is sufficient just to razor scrape down any ridges in the existing adhesive. Be aware, though, that the new adhesive bond is only as good as the old adhesive’s bond to the concrete.” For non-water soluble adhesives, “prepare them to a thin, well-bonded residue using the wet-scraping technique as recommended by RFCI.”

Eric Kurtz, Bostik hardwood installation systems market manager, said old adhesives should be mechanically removed whenever possible to ensure a strong bond. “Chemical compatibility, bond strength, and durability will be affected by the presence of old adhesives.”

When working with dry-set mortar for ceramic tile installations, Taylor stated: “The majority of the old adhesive or cutback should be removed. Scraping the old adhesive or cutback off the floor, to the point it is only a surface stain, will allow a polymer modified dry-set mortar to bond to the substrate with an acceptable strength.”

Regarding his company’s InstaLay product, Laporte said, “If the old adhesive is simply low-level trowel marks then using the InstaLay 30HG will cover these marks and no other floor preparation is needed. If the application is over concrete, regardless of the floor level it is on, a 6mil DPM is recommended.”

Laticrete’s Yoni Feldman, underlayment product manager, said installers should refer to manufacturer recommendations when deciding whether a surface is prepared enough for flooring installation. “Refer to the product data sheet for the material being installed for guidelines on surface preparation, including removal of existing adhesives.”

Marvin noted that even adhesives that are designed to work over old cutback residue still require the cutback to be “scraped away until the subfloor can be seen through it, to bond completely.” He added, “Ideally, old adhesives should be completely removed through mechanical means or with an adhesive remover designed to leave no hydrocarbon residue.”

Tim McDonald, Merkete vice president of sales, said several industry companies make primers that are compatible with cutback adhesive residue, including Merkrete. “Installers will still want to get off as much residue as they can, or else it will require more primer. The most important thing is to knock down the high ridges.”

Callaham stated it is always best to consult the manufacturer for recommendations. “Those are the guidelines the installer should follow. They will vary from manufacturer to manufacturer and especially will vary on the type of flooring that is being installed. Sometimes it will be as simple as basic prep with a skimcoat or it could be a complete abatement of the surface of the substrate.”

Mark Long, Stauf technical director, added, “The best thing to do with old adhesive is to remove it. Any old adhesives can cause bonding issues in the future if you just go over the top of it. So it would be in the installer’s best interest to remove the old adhesive entirely if possible.”

Ron Loffredo, Tec/H.B. Fuller Construction Products senior area technical manager, offered this tip: “Some adhesive residue can be very tacky or gummy. The use of floor patching compounds applied over the entire floor can reduce this residual tacky or gummy film to assist the scarifying equipment in the adhesive removal process.”

Regarding his company’s USG Durock Brand floor prep products, Kazienko stated, “The products can be installed over non-water-soluble adhesives on concrete only. The adhesive residue must first be tested to make certain it is non-water-soluble. Any water-soluble adhesive residues must be mechanically removed down to clean concrete. Non-water-soluble adhesive residues should be prepared to a thin, well-bonded residue using the wet-scraping technique. Any existing patches below the adhesive must also be completely removed.”

Moisture testing and mitigation. Morris said that moisture testing should only be performed by technicians certified by the International Concrete Repair Institute (ICRI) to assure the test’s reliability. He added, “If testing shows concrete with a high moisture vapor emission rate (MVER), it must be remediated before installing any moisture-sensitive finish floor covering – period.”

He stressed that if a moisture-sensitive floor over a high-moisture concrete fails, the responsibility will fall on the installer. “Do not let assurances from the GC or architect create the false impression that they take responsibility for any failures that will occur. As the flooring expert on the jobsite, once you install one square foot of material over the substrate, you have acknowledged its suitability to receive that material.”

For concrete with a high MVER, Morris recommends installing a 100% solids epoxy. “Most manufacturers of these types of products offer free training in the correct preparation of the concrete and application of the product. The successful completion of such training usually allows the installer to offer an extended warranty which is often helpful in providing the end-user with an assurance that the installation has been performed correctly.”

Kurtz said moisture testing with relative humidity (RH) in-situ probes (ASTM F2170) or calcium chloride testing (ASTM F1869) can both reveal moisture problems in the slab. “Both take 72 hours or more to perform. If moisture levels are too high for the flooring manufacturer’s warranty, use a moisture mitigation system.”

Taylor said whenever possible, perform both moisture tests. “The floor covering manufacturer may recommend one or the other, but most feel that both numbers are important. Dry-set mortars are not sensitive to excessive moisture, but the moisture can cause other undesirable effects, such as efflorescence. If there is a concern that the moisture vapor emission is too high, there are moisture vapor control membranes available. Many uncoupling membranes will control the moisture vapor emission from the concrete slab for the installation of ceramic tile.”

According to Grete Heimerdinger, Lignomat vice president, all materials in contact with the floor should be tested before installation. “That includes the floor planks, the subfloor, what is underneath the subfloor (usually concrete) and the ambient conditions of the air inside the building.”

She said testing equipment will need to include a moisture meter (either pin or pinless) with correction for species and for types of subfloors, a thermo-hygrometer for the air, and a calcium chloride or RH in-situ probe test for the slabs. “It is absolutely necessary to measure the ambient conditions in the room. Acclimation of the floor and subfloor is only taking place when the ambient conditions are within the relative humidity ranges given by the floor manufacturer and are the same later when the building is occupied.”

Marvin broke down the relative differences between the two ASTM tests. “The ASTM F1869 test is a surface test to see how much moisture is passing through the slab (MVER expressed in pounds per 1,000 feet) in a set amount of time. ASTM F2170 places a probe into a hole bored into the concrete to see how much moisture is contained within the slab itself (relative humidity expressed in %). Most flooring lists limits for MVER and relative humidity.”

When the moisture level exceeds the limits of the flooring, he recommends a moisture reduction barrier. “There are a number of these on the market that act to reduce the amount of moisture the flooring will see regardless of what is present in the slab. This can be a stand-alone product such as a two part epoxy, or it can be a function of the adhesive that is used. Keep in mind that to be effective, any moisture reduction barrier must be continuous – if holes are present in the barrier, moisture will be allowed through.”

Along with ASTM F1869 and F2170 for testing the moisture in the slab, Callaham recommends installers or contractors working with resilient floors to consult ASTM F710 Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. “These standards help guide the flooring contractor in the right direction, outlining the number of moisture tests that are required. No matter if the slab is freshly poured or is considered an older slab, moisture testing must be performed.”

He added, “Once you know the moisture condition of the slab, there are many options. Most adhesive manufacturers have systems that will tolerate 10 lbs. of moisture and 90% RH in certain conditions. When levels exceed these limitations the best and most accepted mitigation system is a two-part epoxy. This requires the surface of the concrete substrate to be mechanically profiled, either by shot or bead blasting, prior to applying the system. The best epoxy systems comply with the new ASTM F3010-13 Standard Practice for Two-Component Resin Based Membrane-Forming Moisture Mitigation Systems for Use Under Resilient Floor Coverings.”

Long believes performing both tests will give installers the most accurate picture of the moisture situation in the slab. “That’s the ideal situation, though on a normal jobsite, especially a new home, installers and contractors often don’t have the time allotted to do both. In that case, go with the manufacturer’s recommendations, and make sure to get an accurate reading.”

When choosing a moisture mitigation product, he added, “Know your moisture level – that will give you a good gauge when looking at the different options out there. Once you find that level, you can pick your best option.”

Loffredo said the most popular test method for finding out moisture content is ASTM F2170. “Because moisture migrates upward from within the slab, measuring moisture at its surface will not accurately portray the subfloor’s relative humidity. Probes placed at specified depths inside the slab more accurately measure its relative humidity levels, and as a result, more reliably measure the risk moisture poses to a particular installation.”

Kazienko said USG recognizes both ASTM tests. “If the moisture vapor emissions rate (MVER) exceeds 5 lbs./1000 sq. ft./24 hours per ASTM F1869 or if the relative humidity exceeds 80% per ASTM F2170  treat the concrete subfloor with USG Durock Brand RH-100 Moisture Vapor Reducer in all areas of use where potential for moisture problems may exist. USG Durock Brand self-leveling underlayments and patches are not vapor or moisture barriers. Transmission of excessive water vapors or moisture from the concrete subfloor through the USG Durock products can interfere with floor-covering adhesives and compromise their performance.”

According to Jason Spangler, Wagner Meters flooring division manager, RH testing allows an installer or contractor to “accurately assess the risk of flooring success or failure. If the moisture levels are too high for the chosen finish, keep in mind, not all finishes have the same moisture threshold.”

He added, “One option is to look for a different finish that is less moisture-sensitive. A second option is to utilize a topically applied moisture mitigation product. In theory, applying such a product encapsulates the moisture in the concrete slab, allowing for installation of the finish. Not all mitigation products are the same so make sure you do your homework for your specific installation.”