I have been in the flooring industry for more than 50 years. In that time, both hardwood flooring and their adhesives have evolved. Over the years, traditional narrow-width, 3/4″ thick, long-strip oak flooring has given way to wide-width planks, engineered hardwood, tropical wood species, and bamboo. Cutback and chlorinated solvent adhesives have given way to lower VOC and healthier adhesive alternatives.
NEEDS WILL VARY
The needs of these new hardwood flooring products can vary depending on the width, species, substrate, and environment that the flooring will live in. Wide-width and long planks are less-forgiving to substrate irregularities. These large format planks will tend to bridge substrate irregularities, resulting in hollow spots and even deflection. Flattening the substrate per industry guidelines and using the recommended trowel for wide and long planks will go a long way in preventing hollow spots and deflection. Some adhesive manufacturers will not guarantee some of their adhesives with tropical wood species or certain types of bamboo. Reviewing the technical data sheets for the flooring and adhesive before starting a project is always a good idea. You always should contact technical services if you have questions when working with a product for the first time.
Before you commit to a glue-down application, ensure the selected flooring can be installed as a glue-down. I have spoken to many installers who have been put in a no-win situation with a nail-down-only product sold as glue down over concrete.
KNOW YOUR MARKET
You need to know what environmental challenges the flooring will experience. Some markets can experience significant swings in humidity from summer to winter. Using an elastomeric adhesive is critical since this will allow the flooring to expand or contract with seasonal humidity fluctuations. The most common elastomeric adhesives are modified silane and urethane adhesives. A major advantage of these adhesives is that they do not contain moisture. I prefer modified silane adhesives since they give the benefits of a urethane adhesive without the difficult cleanup. That is because modified silane adhesives do not contain isocyanates like traditional urethanes.
TYPES OF ADHESIVES
Urethane adhesives typically are recommended when gluing down solid tropical hardwood species and solid bamboo, including vertical, horizontal, and strand since they generally are moisture-sensitive. Water-based acrylic and solvent-based adhesives also are available on the market. Water-based adhesives should be used only with engineered hardwood flooring that is not moisture-sensitive in areas where moisture is not a concern. Before you commit to a glue-down application, ensure the selected flooring can be installed as a glue-down. I have spoken to many installers who have been put in a no-win situation with a nail-down-only product sold as glue down over concrete.
For additional information about adhesives, check out the NWFA’s Installation Guidelines at nwfa.org/technical-guidelines/.
If your area is prone to substrate moisture issues or you are doing new construction, using a moisture control adhesive is always a good protocol. You always should do a moisture test before starting a project to ensure that the moisture control adhesive selected can handle the issues that the slab has. Always consider the grade of the slab. On-grade or below-grade slabs are more prone to moisture issues. I always recommend considering the age of the slab when you moisture test. If you have high moisture emitting from a new slab, it is reasonable to assume it will diminish over time.
You also should know the building code in your area regarding under-slab vapor barriers. Most adhesive manufacturers require a slab poured directly on an under-slab vapor barrier per ASTM E1745. There are some areas where the building code does not require an under-slab vapor barrier and others that still allow for a granular (blotter) layer between the slab and the vapor barrier. Extra precautions should be taken in areas with these outdated building codes when selecting a moisture control adhesive or another moisture control system.
USING GLUE WITH NAIL-DOWN
If your market primarily is nailed down, you may not think much about adhesives. As consumer tastes have evolved to wider and longer planks, adhesives have become increasingly important, even for nail-down applications. Using a glue-assist when nailing down wide and long planks can save you time and money by reducing callbacks. NWFA Guidelines now suggest glue-assist on any plank wider than 5”. The primary purpose of glue-assist products is to reduce or prevent friction squeaks between the flooring and the substrate. I know many installers who have gone to full-spread installations for wide and long planks over plywood substrates to reduce squeak complaints.
Gary Scheidker is director of technical services for Taylor Adhesives, Meridian Adhesives Group. He has been in the flooring industry since 1970. In that time, Scheidker has been a flooring installer, estimator, retail salesperson, store owner, distributor territory manager, regional manager, and now director of technical services. He may be reached at firstname.lastname@example.org.