Engineered Flooring Installation on Concrete: Best Practices for Glue-Down
Previously published by Hardwood Floors Magazine
Don’t let the “engineered” give you the wrong impression. Engineered wood is real wood. Although it’s not made of solid hardwood, it doesn’t mean that engineered flooring does not provide all of the warmth, longevity, and elegance of a hardwood floor.
Engineered wood flooring is chosen for quite a few locations where you would not expect to see wood flooring due to the strength of its construction, offering additional resiliency against temperature and humidity changes. Built with various types of “core” material, engineered flooring provides a wide range of aesthetic benefits as well with the ability to choose from a wide variety of colors, textures, and wood species for the surface layer. Once installed, engineered wood planks look the same as solid wood planks. Installing Engineered wood flooring is a great project for the DIYer who desires the look of a professional wood floor with the strength and longevity offered by its sturdy design.
Some engineered wood flooring products are designed for a glue-down application when installing flooring over a concrete slab. Other products are better-suited for a nail-down installation where they are secured to a wood subfloor with nails or staples. And for do-it-yourselfers, the preferred method is the floating floor installation which uses no glue or nails. Instead, the engineered wood planks are joined together with interlocking joints, creating a single, continuous layer that “floats” over the subfloor.
This article, however, focuses only on the glue-down application. Nail-down installations are covered in this article.
Acclimate Engineered Flooring
Regardless of whether or not the flooring is glued, nailed, or floated, engineered wood flooring needs to be acclimated to its environment before installation – just like solid wood flooring. It’s important to follow the manufacturers’ guidelines for the proper acclimation procedure and timeline as these may differ from one engineered flooring brand to the next.
Get yourself an accurate wood moisture meter and use it to ensure the wood planks have properly acclimated according to manufacturer’s specifications and have achieved equilibrium moisture content (EMC) with the surrounding ambient conditions. This can be done by periodically measuring the moisture content of the wood until it meets the guideline specs for installation. There are two types of wood moisture meters available; pin meters and pinless meters. The latter being chosen more often for their non-destructive electromagnetic measurement capability.
Concrete Moisture Measurement Is Critical
Another thing to check is the moisture within the concrete slab. Excessive moisture in a slab can damage an engineered wood floor and cause the plies to separate. Most flooring manufacturers require moisture tests for concrete moisture to validate their warranty requirements.
- Every concrete slab has moisture and will always have moisture. What you want is a slab with an acceptable level of moisture for the flooring finish you’re installing.
- Besides the moisture already in the concrete, additional moisture can enter a slab from rainwater, poor plumbing or drainage below or at the sides of the slab, and even humidity in the air.
- To find out if your concrete floor meets the dryness criteria set by your flooring manufacturers guidelines, remember this mnemonic: “If you wanna know, go below.” Scientific testing has determined that measuring moisture conditions within a concrete slab yields more valuable data than only measuring the moisture evaporation emission rate (MVER) at the surface[BM1] [LL2] [LL3] of the slab. To measure the moisture inside the concrete, you will want to conduct a test known as the ASTM F2170 in-situ relative humidity (RH) test.
The RH test uses sensors or probes to measure the RH at a specific depth within the concrete – 40% of the slab’s thickness for a slab drying from one side, or 20% for a slab drying from two sides.
Scientific research from leading academic and industrial institutions confirms the reliability and accuracy of the ASTM F2170 RH test at these depths. The readings will most accurately predict the slab’s point of equilibrium, and therefore the true moisture condition that will exist after the flooring installation.
How to Install Glue-Down Engineered Wood Flooring on Concrete
Slab preparation is critical for glue-down and includes sanding, scraping, leveling, and filling low spots because the slab must be flat so the planks can fit correctly.
When “leveling” (actually flattening) a slab, don’t allow more than 3/16” difference in height within a 10-foot radius or 1/8” within a 6-foot radius.
Before applying adhesives, you must fill in any voids or deflections in the slab with a cementitious patch or a self-leveling underlayment. An adhesive is not intended to fill voids or deflections.
Use adhesives specially formulated for wood flooring so they conform to the natural characteristics of wood – expanding and contracting. That is, their elasticity ensures that the engineered wood has an ample amount of space to expand and contract without causing the glue to crack or separate. Be advised: using the wrong adhesive or applying incorrect amounts can lead to a failed flooring installation.
For example, use an adhesive that contains no water — like a moisture cure urethane or modified silane adhesive. These products are a good choice for these installations because they increase the strength of the bonding agent and offer a degree of structural flexibility.
A glue-down installation requires premium wood adhesives be properly troweled over the concrete slab and the engineered wood planks laid onto the adhesive and locked together at their tongue and groove joints.
NOTE: Wood adhesives recommended today are much more environmentally friendly than in the past, but they cost more. They also require considerably more time to trowel, which adds to the overall labor costs. For more information, visit
Expansion gaps should be left between the flooring and wall. The manufacturer should have recommendations for how wide the expansion gaps need to be. Installing engineered hardwood flooring over concrete too tight against a stationary object will not allow room for normal expansion, and may cause a failure.
The adhesive manufacturer should have instructions regarding specific trowel requirements. Use a notched trowel to spread the adhesive. Pass the trowel through the glue at a 45-degree angle and install engineered flooring immediately after the adhesive is spread.
Important: Only spread the adhesive over small areas ahead of you at any given time. You don’t want the adhesive drying before you can get to that area. If you’re using a urethane adhesive, this advice is even more vital since the adhesive is moisture-cured and sets up fairly quickly.
Lay the flooring into the adhesive with the tongue side of the board facing the center of the room. The hardwood may slip and move at first, so be sure to secure one row entirely before moving on to other rows. The initial row will limit the movement of subsequent rows.
Occasionally lift a piece of flooring to make sure there’s a 100% adhesive transfer, necessary especially for adhesives that have moisture mitigating characteristics. If the board is not entirely covered, remove dried adhesive and apply more.
Be sure to follow the manufacturer’s instructions after installing engineered flooring over concrete. For instance, you might be required to roll and cross the floor with a 100-150 lb. roller after installation to ensure proper adhesive transfer.
Traffic should also be restricted for up to 24 hours after completing the installation to allow drying.
If you are still debating whether to install your engineered wood flooring using the glue-down method, consider these points:
- Floor has a solid feel
- Less chance of squeaking or “clicking” due to boards coming into contact with the subfloor with every step
- Permanent installation
- Enables use of flush mount moldings to create dynamic height transitions and allow expansion space
- May require rigorous subfloor prep
- Labor costs are higher, typically $3-5 per square foot
- Adhesives can add up to $1.00 per square foot
Tony Morgan is a senior technician for Wagner Meters, where he serves on a team for product testing, development, and also customer service and training for moisture measurement products. Along with 19 years field experience for a number of electronics companies, Tony holds a B.A. in Management and his AAS in Electronics Technology.