Creating a stable subfloor

A stable subfloor (or substrate) is critical to a room’s structural integrity, and it is key to long-lasting hardwood floors. Choosing the best quality moisture barriers, primers and adhesives will help maximize cohesive strength, protect against damage, and guarantee a solid surface. Understanding how moisture affects wood flooring is key to ensuring the subfloor is sufficiently fit to support a hardwood floor, and that it is well-prepared for the hardwood floor installation.

Expert flooring contractors know the value of moisture mitigation and moisture testing, as there is a lot of liability in moisture. The average cost of a tear-out and reinstall from a moisture claim (a.k.a. moisture failure) is roughly seven times the cost of the original floor. Failing to do a proper moisture test can be a painful and expensive business lesson; not to mention, quite a difficult situation for the customer.

There is simply no real way to know what the moisture levels are and the risks they pose without testing.

Know your subfloor 

A concrete floor slab is one component of the floor system. Other parts of the system that help determine how a finished floor performs over the long term include curing compounds, primers, vapor retarders, patching and leveling compounds, adhesives, the flooring, sealers and finishes, floor cleaning and polishing materials as well as the building envelope.

Check and prepare your subfloor

Since many flooring failures are caused when the subfloor isn’t properly checked and prepared, we’ve provided guidelines for staying on course.

• A concrete slab should be clean, dry and flat. When a slab is dirty, the adhesive will form a bond with debris instead of the slab. Contaminants like curing compounds, paint overspray, plaster or adhesive from an old floor covering can interfere with adhesion, so the slab may need to be shot blasted, abraded with a heavy-grit sandpaper or ground with a grinding disc.
• When concrete floors are kept relatively dry while in use, many problems can be avoided. Nevertheless, water is a necessary ingredient in concrete, and at times, floors are accidentally exposed to water during construction, and then later in the floor’s life.
• Wood behaves like a living thing that changes in relation to moisture and humidity; it shrinks when exposed to low humidity and expands when exposed to high humidity. Never install a hardwood floor over an unresolved moisture condition. It could damage the flooring and severely impact the life cycle of the floor.
• Follow best practices when testing for moisture conditions so they can be properly addressed, and an avoidable dilemma can be averted.

Tools and materials needed to test and prepare the subfloor

Be sure to follow the National Wood Flooring Association (NWFA) guidelines as well as manufacturers’ guidelines on how to properly acclimate the site.

1. Ready the building for the flooring, then get the flooring ready for the building Also known as “conditioning the space,” it gets the relative humidity aligned to expected occupancy comfort levels. The climate of the region where the flooring will be installed is also an important consideration. Different regions have different requirements for floor preparation since the outdoor climate will affect indoor conditions.
2. Test the subfloor to ensure there are no moisture issues Contractors have two options to test: a calcium chloride test or a relative humidity test. Both tests meet NWFA guidelines. ASTM F2170 recognizes results from an on-site relative humidity (RH) moisture test performed in concrete floor slabs in 24 hours, versus 72 hours for calcium chloride testing.
3. Use a moisture barrier or primer that is trusted and tested: Bona R540
   Bona R450 is a moisture barrier/primer that mitigates vapor transmission on absorbent and non-absorbent subfloors, including Gypsum-based underlayments. Once Bona R540 has completely dried and the subfloor is prepared, follow best practices for applying adhesives. Bona R540 is compatible with the entire Bona Adhesives System, and Bona’s silane-based adhesives are GREENGUARD Gold certified, for indoor air quality.
4. Every species of wood has a Dimensional Change Coefficient (DCC). Know the moisture content (MC) of each of the following to determine the DCC:

• The substrate
• The wood
• The temperature of the room
• The relative humidity (RH)

Over the years, Bona products and technology have helped to expand the use of wood floor construction both commercially and residentially throughout the U.S. Today’s primers and adhesives enable contractors to use wood floors products in applications that would have been prohibitive in the past.