Adhesive Considerations for Wood Subfloors

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Photos courtesy of Sika-DriTac

When installing new hardwood flooring over an existing wood subfloor, installers have several options, each with their own set of benefits and concerns. As with a concrete subfloor, a successful installation may be achieved by floating, nailing, or gluing down the floor covering over a plywood or oriented strand board (OSB) subfloor, but the natural tendencies of a wood substrate may demand a greater level of consideration than concrete. Wood substrates have a greater propensity for reacting to moisture that may result in squeaking and creaking. While there are many preparation measures and installation requirements that should be understood when dealing with a wood subfloor, the latest adhesive technologies can offer the greatest peace of mind in the long-term.

Preparing for an installation over a wood subfloor necessitates understanding the inherent challenges wood presents. Wood subfloors may be installed directly on top of concrete slabs or crawlspaces. Each of these scenarios may present moisture concerns. During dry winter months, moisture may escape wood and cause the planks to contract and result in gapping. Conversely, during summer months with a greater level of humidity, wood planks may absorb moisture and cup, warp, or buckle. This can be even worse over unconditioned spaces, which are disconnected from the heating and cooling systems benefitting the rest of the building’s interior. Ambient conditions changing from season to season particularly are likely to have an impact in these spaces, presenting problems such as higher relative humidity and a general lack of climate control. Installers should plan the installation with added caution in these scenarios to avoid mold growth, rot, and other concerns.

The long-term performance of any glue-down installation usually can be traced back to the thoroughness of the installation preparation. An unfortunate fact is that many of the industry’s standard preparation techniques and installation requirements are in some cases more-readily dismissed than they should be. Due to the nature of wood subfloors, it could prove beneficial to research the history of the building for prior damage, drainage, or leaks. The subfloor should be dry, sufficiently flat, and clean of any debris or dust to ensure a quiet and stable finished floor. Moreover, installers always should check for moisture content. NWFA guidelines encourage taking 20 readings per 1,000 square feet and then deducing an average reading across the entire installation space. The hardwood floor covering should be allowed time to properly acclimate to the same environment as the wood subfloor, preventing any variance in moisture content between the two woods. These steps may seem time-consuming, but ultimately could save substantial project costs and damages in the long-run.

Recent industry innovations, notably in adhesive, moisture, and sound abatement technologies, could help to cut additional time and expenses. For instance, installers can minimize unwanted moisture transfers from the ground into the hardwood flooring by implementing a Class II vapor retarder membrane. Various vapor retarders, underlayments, and sealers have been introduced, which are engineered to enhance the longevity and performance of hardwood that has been glued down over plywood or OSB subfloors. While traditional adhesives provide only a bond, system adhesives can provide four or five additional solutions in one pail, including sound control, moisture control, crack suppression, and cutback residue isolation. Most importantly, these premium-grade products enhance the stability of the installation and actively combat unwanted sound and moisture transmissions, while reducing the likelihood of physical irregularities in the hardwood. They also are available in a variety of technology groups, including urethanes and polymers, allowing installers to find a solution for nearly any hardwood project.

When properly planned and applied, a full-spread system adhesive utilized over a wood subfloor may offer a greater defense for hardwood flooring from unwanted moisture and sound.

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With so many adhesive options on the market, installers targeting a glue-down installation over a wood subfloor must consider several factors in order to select the right fit and application for the job. The compatibility of the adhesive with both the plywood or OSB subfloor and the hardwood flooring must be ensured at the onset of the installation, as the type, length, and width of any wood flooring product could dictate which adhesive technologies are well-matched. Further, the vapor permeability of the adhesive must fall within the Class II requirement as dictated by NWFA Installation Guidelines, so as not to trap any unwanted moisture within the subfloor. Proper trowel and spread rates also must be utilized in accordance with manufacturer specifications. Adhesives that allow for different application options (sound and moisture control or bond only) may differentiate between unique trowels for each approach. In short, the guidelines of both the flooring and adhesive manufacturers should be followed diligently as they relate to overall compatibility, adhesive coverage, open time, shelf life, storage, and more.

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When properly planned and applied, a full-spread system adhesive utilized over a wood subfloor may offer a greater defense for hardwood flooring from unwanted moisture and sound. Separately, flooring adhesives can deliver numerous benefits, but aggregating these layers in one comprehensive system could present the ultimate value. There are now systems available on the market that can be acquired courtesy of one supplier. Every layer of the installation can be obtained from one manufacturer, with guaranteed compatibility and optimized performance. Installing hardwood over wood subfloors imposes unique challenges, but also unique rewards. When the proper precautions are taken, a wood subfloor can be a long-lasting, fruitful foundation.

John Brown is the technical sales manager for Sika-DriTac. He can be reached at

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