By Tony Morgan
Part 1: Best Practices for Glue-Down
Don’t let the term “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 recommended for quite a few locations where you would not expect to see wood flooring due to the strength of its construction, minimizing natural movement with 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.
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. 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 and floating floor installations will be covered in parts 2 and 3 of this article series.
Acclimate Engineered Flooring
Regardless of whether or not the flooring is glued, nailed, or floated, the environment needs to be aligned with the engineered wood flooring requirements before installation. It’s important to follow the manufacturer’s instructions 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 and the surrounding environment are properly aligned according to the manufacturer’s specifications. There are two types of wood moisture meters available: pin meters and pinless meters, the latter having nondestructive electromagnetic measurement capability.
Concrete Moisture Measurement Is Critical
Another thing to check is the moisture condition of the concrete slab. Excessive moisture in a slab can damage any wood floor. 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 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 and adhesive manufacturers’ instructions, remember this mnemonic: “If you wanna know, go below.” Scientific testing has determined that measuring the relative humidity (RH) conditions within a concrete slab yields more valuable data than only measuring the moisture evaporation emission rate (MVER) at the surface of the slab. To measure the moisture inside the concrete, you will want to conduct a test known as the ASTM F2170 in-situ RH test.
The RH test uses sensors or probes to measure the RH at a specific depth within the concrete – 40 percent of the slab’s thickness for a slab drying from one side, or 20 percent 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 within the slab 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, according to the NWFA Installation Guidelines, don’t allow more than a 3/16” difference in height within a 10′ radius or 1/8” within a 6’ radius.
Before applying adhesives, you must fill in any voids or deflections in the slab with a cementitious patch or a self-leveling underlayment that is compatible with the wood flooring adhesives. An adhesive is not intended to fill voids or deflections.
Use elastomeric 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 break its bond. 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. 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 to be properly troweled over the concrete slab. Then the engineered wood planks can be laid into the adhesive and locked together at their tongue-and-groove joints. The adhesive manufacturer should have instructions regarding specific trowel requirements. Use a notched trowel to spread the adhesive.
NOTE: Wood adhesives recommended today are much more environmentally friendly than in the past, but they cost more.
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.
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.
Lay the flooring into the adhesive with the tongue side of the board facing the center of the room (or as otherwise recommended by the flooring manufacturer). 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 check the adhesive transfer. The percentage of transfer is dictated by the adhesive manufacturer for the product being used.
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-to-150 lb. roller after installation to ensure proper adhesive transfer.
Traffic also should 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 transition moldings
- May require rigorous subfloor prep
- Labor costs are higher
- Adhesives can add costs
Coming Soon: Nail-Down Installations
Next, we’ll present the second article in our series on engineered flooring installation on concrete – best practices for nail-down installations.
Tony Morgan is a senior technician for Wagner Meters, where he serves on a team for product testing, development, and customer service and training for moisture measurement products. Along with 19 years of field experience for several electronics companies, Tony holds a BA in Management and has an AAS in Electronics Technology. He can be reached at 800.505.1283 or visit wagnermeters.com.