In the following article, manufacturers share guidelines for preparing subfloors for floating floor installations.

Floating floors continue to grow in popularity both for their aesthetics and relative ease of installation. However, just because an installation uses little (in the case of perimeter-adhered LVT) to no adhesive doesn’t mean it’s as simple as an installer going in, unpacking and laying the floor, then leaving. Care still has to be taken to ensure the substrate is ready to receive the flooring—and yes, that includes moisture testing and acclimating the flooring.

For this manufacturer’s roundtable, we spoke with Michael Mayer, business development, Schönox HPS North America; Brett Fleury, USG product marketing manager, performance flooring; David Jackson, DriTac field technical services manager; Pat Cunningham, technical service supervisor, Ardex/W.W. Henry; Arthur Mintie, Laticrete senior technical services director; Renee Tester, QEP product marketing manager; Steve Taylor, director of architecture and technical marketing, Custom Building Products; and David Ford, Stauf USA’s vice president of sales and marketing.

Q: What is the subfloor flatness requirement for installing a floating floor?

Mayer: Always follow ASTM F710-11, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. There are two accepted measurement methods. One is described in Test Method E1155, Determining FF Floor Flatness and FL Floor Levelness Numbers. The other is described in Test Method E1486, Determining Floor Tolerances Using Waviness, Wheel Path and Levelness Criteria.

The American Concrete Institute recommends that flatness and levelness be described using the F-Number System as outlined in ACI 302.1R-06 and ACI 117R. This system identifies two numbers. FF controls local surface bumpiness (or waviness) by limiting the magnitude of successive 1' (300 mm) slope changes. FL controls overall levelness (or pitch) by limiting differences in the average of 10' (3 m) elevations along sample measurement lines.

Jackson: Underlayments are required when working with floating floor applications. Typically, the underlayment will help mask subfloor imperfections. It is important for an installer to understand all of the parameters associated with the installation of the underlayment and the tolerances with which the underlayment can or cannot overcome.

DriTac recently unveiled two new sound abatement underlayment systems: DriTac 8302 Double Impact for wood and floating floor installations and DriTac 8301 Impact for glue down and floating resilient floor installations.

DriTac supports the standards set forth by the National Wood Flooring Association (NWFA) for hardwood flooring installations. The NWFA lists subfloor flatness requirements as follows: “Wood subfloors must be flat, clean, dry, structurally sound, free of squeaks and free of protruding fasteners. For installations using mechanical fasteners of 1 1/2" and longer, the subfloor should be flat to within 1/4" in 10' or 3/16" in 6' radius. For glue-down installations and installations using mechanical fasteners of less than 1 1/2", the subfloor should be flat to within 3/16" in 10' or 1/8" in 6' radius.”

DriTac supports the standards set forth by the Resilient Floor Covering Institute (RFCI) for resilient flooring installations. Resilient flooring manufacturers require that new and existing concrete subfloors meet the requirements of the latest edition of ASTM F 710 Standard Practice for preparing Concrete Floors to Receive Resilient Flooring, which includes E1155 Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers.

It is always highly advisable to check with the flooring manufacturers’ recommendations, prior to proceeding with any installation.

Cunningham: Industry standards specify that the surface of the subfloor must be flat to within 3/16" in a 10' radius. These values are dictated by the manufacturer of the selected flooring and should be verified prior to any installation beginning.

Ford: Most require 3/16" within a 10' radius. For LVT and resilient flooring it’s best to get even tighter if possible.

Q: Are there different considerations an installer should be aware of when installing over a concrete substrate vs. a plywood substrate? What should they keep in mind?

Mayer: Wood subfloors need to be made of solid wood or structural wood panels and must be clean, dry and free of any oil, grease, dirt or other contaminating substances that could affect the performance or act as a bond breaker. Loose boards will need to be mechanically fastened, and badly cupped or warped board subfloors will need to be replaced.

For concrete subfloors, make sure the surface is dry, clean and free of dust, wax, residual adhesives, adhesive removers, curing and sealing compounds, and any other materials as recommended by the manufacturer of the self-leveling underlayment. Mechanical profiling may also be recommended. Steel troweled concrete, concrete sealed with curing compounds and any substance that could act as a bond breaker will need to be shot blasted or scarified. The concrete subfloor moisture vapor emission rate or in-situ relative humidity cannot exceed the maximums recommended by the self-leveling underlayment and flooring manufacturers.

For other substrates, the surface must be clean and free of any contaminants that may reduce performance or act as a bond breaker. Contact the self-leveling underlayment manufacturer for specific recommendations.

Fleury: There are many common conditions a floor covering professional needs to consider before actually installing the floor covering. Identifying the moisture content of the substrate should be top priority for both types of subfloors. Excessive moisture below most floorcoverings often leads to floor covering or adhesive failures. Testing the concrete and identifying the moisture vapor emission rate (MVER) per ASTM F1869 or the relative humidity (RH) of the slab per ASTM F2170 and testing wood substrates for the percentage of moisture content is the best way to prevent moisture issues that lead to a failure of the floor system.

Floor flatness tolerances and structural movement (deflection) will also need to be considered in order to ensure the surface profile of the substrate surface falls within the tolerances required by the specific floor covering product that is specified.

Anything on the surface of the substrate that would interfere with or prevent the adhesion of a repair product or the floor covering system would be considered a contaminant. Substances like oil or grease, sweeping compound, joint treatments or curing compounds may have a negative effect on bonding materials to the surface. Therefore, cleaning, eliminating or mitigating contaminants in preparation for the floor covering installation is important.

In wood framed multi-family structures, either new construction or renovation, fire and sound code requirements must be considered. It is important to note that floor underlayments or patching compounds used in these assemblies must carry an Underwriters Laboratories (UL) type designation in order to be installed in a UL fire rated assembly. If any one of the components in the specified UL design (ie. the ceiling tiles or drywall, ceiling suspension systems, joists, RC channel, insulation subfloor and floor underlayment/patching compound) does not carry the appropriate UL stamp, the entire assembly would not meet the building code.

Jackson: There are numerous differences that come into play with installing over a concrete substrate versus a wood substrate. One of the most notable differences is moisture limitations and testing. This will vary for each substrate. Concrete subfloors must also have high spots ground down and low spots filled.

When dealing with concrete substrates, installers should understand the moisture limitations associated with the underlayment. If the moisture is elevated, are there any additional precautionary steps needed prior to the installation of the underlayment? This information should be ascertained prior to the installation.

When dealing with wood substrates, deflection can come into play and become a primary concern.

Cunningham: One of the first items to address is the need for flooring products to acclimate on site per the manufacturers’ recommendations. Failure to do so will result in an unsatisfactory end result. Excessive expansion and contraction of flooring due to lack of acclimation will negate even the best of substrate preparation.

With concrete, there is always the concern of moisture which can negatively affect all flooring structures. Testing for moisture and subsequent remediation may be required. Additionally, concrete substrates must be flat and smooth enough per flooring manufacturers’ tolerances to assure adherence of the flooring, eliminate gapping of modular constructs and assure acceptable aesthetic end results due to undulation.

Wood substrates must be of an approved grade or quality to assure success. Degrees of gauge thickness and face smoothness will be dictated by the flooring manufacturer. Exterior quality plywoods are designed to receive waterborne products, such as smoothing compounds and adhesives. Moisture in the wood substrates must also be at or below the maximum levels allowable prior to flooring installation.

Mintie: When installing a floating floor over a concrete substrate, it is important for installers to be aware of the moisture vapor emission rate of the slab, as this can adversely affect certain types of finishes. For example, if a finish is sensitive to high moisture vapor emissions, the use of a vapor reduction coating or an isolation membrane for this purpose should be used to help control the moisture vapor emission rate from new or existing concrete slabs prior to installing underlayments and/or any finishes. Without this step, the finishes could be adversely affected via warping, curling, delamination and increased potential for mold and mildew growth.

Typically, plywood substrates will not have the same high vapor emission concerns; however, they tend to have issues with substrate movement. Installers should ensure that the plywood floors are rigid, stable, non-flexing and can support the new flooring finish and installation system.

Tester: While flatness requirements are the same for both types of substrates, there are other considerations that must be taken into account. For plywood substrates, you want to ensure that the subfloor system is properly constructed to avoid any undue deflection/movement and that it is sufficiently fastened to remove noise/squeaks. For concrete substrates, the primary concern is vapor emissions that may damage the floorcovering; all concrete substrates should be covered with 6 mil polyethylene sheeting before installing an underlay pad, or the installer must use an underlay pad that has a vapor barrier incorporated.

Taylor: When installing a floating or unbonded mortar bed over a concrete floor, it is important to note where there are movement joints in the concrete slab and make sure they are carried through the mortar bed. Because these joints can move vertically as well as in plane, it is important that they are brought through the mortar bed and tile assembly. Plywood substrates should have gaps between the individual plywood sheets to accommodate expansion and contraction of the plywood, but these do not have to be brought through the mortar bed.

Ford: Pay attention to the transitions between the sheets of plywood (both the thickness consistency and level consistency). These differences will be magnified in resilient flooring. Keep in mind dynamic joints /score marks in concrete will be exacerbated in resilient flooring as well. “Fill and flatten” [the substrate] is the main thing to remember!