Hardwood floors are beautiful, durable, and can last for decades when properly maintained. When issues do arise, they are typically caused by moisture, either from below or above. Therefore, a majority of hardwood issues can be avoided before the first piece is installed by properly preparing the substrate to receive the flooring. In this article, we will be examining glue-down installation methods over concrete. Other methods, such as pneumatic nailing and click-together planks, bring their own challenges which will not be addressed here. This article also does not address laminate or bamboo, although many of the same considerations apply, especially regarding moisture.

Wood flooring adhesives are typically formulated for either dry environments or damp environments. Be aware that there is a difference between a high-moisture slab and one that exhibits hydrostatic pressure—moisture being actively forced up through a concrete slab caused by a nearby pond or water source. No adhesives recommend use in areas with hydrostatic pressure. Asking the right questions before you start is important to the finished product:

  • Question One: Is my site ready to receive hardwood? Visit the National Wood Flooring Association (nwfa.org) for guidelines and a free site evaluation form that addresses things like slope, drainage, gutters, and sprinkler systems that can contribute to moisture in the slab and can be properly taken care of by landscaping. 
  • Question Two: Does my slab have a vapor barrier? By building code, on-grade slabs should have a plastic moisture barrier and a drainage path beneath them to stop moisture intrusion. If your slab does and there are no obvious signs of seasonal moisture (white mineral deposits on the surface or dark coloration near walls, for example) then you can move on to the next step. If a moisture vapor barrier is not in place or if it is unknown, it’s worth considering applying an epoxy or polyurethane barrier to the concrete surface. Typically, this involves shot-blasting the concrete, mixing and applying the moisture barrier, then using a self-leveling underlayment to make the substrate flat. This approach can also be used over newly poured concrete seven days old or older.
  • Question Three:  How much moisture is in my slab? There are two common tests for moisture that are conducted with pros and cons to each. Which one you should use is typically dictated by the flooring you will be applying and the adhesive you will be using. A moisture emission test is conducted by the procedure in ASTM F1869 and produces a number like 3 pounds in 1,000 square feet. A relative humidity test is conducted per ASTM F2170 and is listed as a percentage. Both are useful and recommended to help you determine which adhesive to use. They are typically conducted in 3 locations for the first 1,000 square feet to be covered and 1 for every 1,000 feet after to give a roadmap of the slab. Adhesives for dry areas typically are in the 3 lbs./75% RH range while adhesives for damp areas can go higher. When the moisture exceeds the recommendations of the adhesive and the flooring being installed, a moisture barrier as described above will need to be installed prior to installation.
  • Question Four: In what condition is my slab? Hardwood installations require flat, dense concrete that is level to a tolerance in plane of 3/16” in 10 feet or 1/8” in a 6-foot radius. If that doesn’t describe your slab, you will need to do some surface prep to prepare it. The most common approach is to cover what is there with a self-leveling underlayment. For high moisture situations the self-leveling underlayment or patch must be an exterior rated if a high-moisture adhesive is being used. However, care must be taken not to go over any active movement joints with the self-leveler. The self-leveling underlayment will flow to a degree but must be tooled to be uniform across the entire area. In an ideal situation, there should be at least three people present when installing a self-leveler; one person mixing, one person carrying the buckets and pouring out the mix and one person tooling and smoothing the self-leveler. It also helps to mark the desired height of the leveler on adjacent walls. HVAC and direct air-flow should to turned off while the self-leveling underlayment cures.

If you are specifying hardwood, the answer to the questions about moisture barriers and moisture levels will help you determine what to specify. You may need an epoxy or polyurethane moisture barrier, a self-leveling underlayment (including a primer) or both. It is rare that a floor is ready for installation with just a bucket of glue and a trowel. You will also need to determine which glue to use. This is typically determined by knowing what type of wood will be installed and the moisture level of the slab.

For all types of hardwood installations, it is important to allow the wood to acclimate. Most installers have a moisture testing device so they can determine what the moisture content of the wood is prior to installation. Acclamation occurs when the wood and the substrate are within 2 to 4% of each other. When the slab has high moisture, the base boards and the casings can be checked to determine acclimation. Over time, the moisture content of the wood will match that of the room. Note that exotic species of solid wood flooring may require longer acclimation periods. Installing very damp wood can lead to separation and installing very dry wood can lead to tenting or cracking of the wood. Once the floor is flat and the wood flooring is acclimated, it’s time to get installing:

  • Layout: Laying out a floor requires assessing which direction the wood will run, the presence of obstructions and doorways, and the length and width of the wood versus the length of the room. A few minutes spent looking at these factors can save frustration later. 
  • Application: Wood glues are typically trowel applied. Those designed as moisture barriers often require a special trowel clip which ensures that the application does not cause any voids which would allow moisture through. Achieving a flat substrate is key to getting the 100% coverage needed to encapsulate the moisture.  Use the trowel suggested by the adhesive container suitable for the wood flooring you have selected. The adhesive must be wet for it to bond with the wood, apply only as much as you can cover within the window allowed by the adhesive (30-45 minutes typically). Push wood into the adhesive taking care not to get excessive adhesive between the planks or on the surface. Follow manufacture recommended installation and direction of application. Any adhesive on the surface of the wood should be removed immediately with the appropriate cleaner. Refer to the adhesive manufacturer for recommended removal procedures. If a special type of cleaner is needed, have this on hand during the installation. Always work backwards towards the room exit and avoid walking on the newly installed flooring.
  • Allow the adhesive to cure: Follow the instructions for how long to stay off of the floor before allowing foot traffic. Normally this is 24 hours but can vary depending on temperature and humidity.
  • Finish: While it is not within the scope of this article to explain the entire finishing process, unfinished hardwood will need a finish applied. Typically, this involves sealing, staining, and several layers of finish with more sanding between coats.

By understanding the requirements of a glue-down hardwood installation, the common failures associated with this flooring can be avoided. Understanding what you are covering and the process for installation is the key for specifying and installing hardwood successfully.