When installing the boards, don’t set in the adhesive and slide into place. Memory return often results in gaps.

Some manufacturers may specify working with the tongue leading the way; others may prefer it with the groove out. Check with the manufacturer on the proper orientation.

In January at the Surfaces show, NOFMA released a new Technical Publication, “Installing NOFMA-Certified Solid Wood Flooring Directly to a Slab Using a Wood Flooring Adhesive.” This is a completely new NOFMA position on adhesive installation for NOFMA certified solid wood products. Until this time, direct glue-down of solid wood was an approved method of installation only for parquet flooring products.

As NOFMA reviewed its position on this topic, the new developments in adhesive technology showed promise for solid wood flooring to have greater success in the rapidly growing slab construction market. Previously, NOFMA installation guidelines for a concrete slab required a plywood subfloor or sleepers be attached to the slab for a nailing base. The house had to be constructed specifically for the particular subfloor system. This was frequently not done and the height change associated with these systems prohibited using solid wood flooring in many cases. Engineered flooring was typically the only product available for a unidirectional wood flooring installation.

In addition to tapping into place some manufacturers may require taping the floor to maintain position. Confirm the type of tape and how often to apply.

The primary issue with a unidirectional solid wood installation and slab construction is wood expansion related to the inherent moisture associated with the slab. As adhesive technology has advanced, issues associated with moisture and slabs were addressed. The new adhesives became more moisture resistant, elastic and stronger. Also the formulations have gotten away from the chlorinated solvents and asphalt cutback mastics that are more heavily regulated due to VOCs. Chlorinated solvents are virtually gone from the wood flooring market and the cutbacks are becoming harder and harder to find.

Among the most important issues for a successful direct adhesive solid wood flooring application is to properly set up customer expectations before the installation even begins. The flooring should be expected to perform similarly to a nail down system with respect to expansion and shrinkage and related cupping and gaps. Also, the occasional hollow sound is acceptable as long as the flooring is properly adhered. Slight seasonal cupping can also be a normal and acceptable condition particularly with wider widths. Gluing down solid wood that is wider than 5” is not recommended.

To spread at the recommended rate use the proper trowel notch and spread at the recommended angle.

Another issue is that additional flooring will likely have to be ordered since the adhesives will not hold bowed or crooked boards in place when initially installed. That means some boards that are perfectly fine for a nail-down floor cannot be used in a glue-down application. Some of the slightly bowed or crooked boards can be installed and stay in place until the glue sets and thus some slight gaps may occur at installation that are considered acceptable. These gaps should be filled later during finishing.

For successful glue down installation, attention to detail is critical. Start by checking to see if the manufacturer of the flooring recommends this installation procedure. Some NOFMA manufacturers do not recommend that their solid products be glued down. Also check to be sure the adhesive is recommended by the manufacturer for use in this type of installation. NOFMA requires that solid wood flooring should only be placed on “above grade” or “on-grade” slabs. Not even when only one side of the slab is below grade with the majority of the perimeter above the grade level, is solid considered an acceptable product for installation. Engineered flooring continues to be the only wood flooring product recommend for use in below grade applications.

Since the gluing process does not initially force or hold the boards in place the slab must meet the specific conditions and tolerances directed by the adhesive manufacturer. Use the adhesive manufacturer’s recommended procedures to test for slab moisture. Most slabs will have to be at least 60 days old before testing. Use a straightedge to check the slab for flatness. A 2” to 3” diameter metal pipe 10 feet long is a good tool for this. Typically, the slab should have no more variation than 3/16” in 10 feet and any variation should be gradual. The slab must be clean in that all materials that will interfere with the adhesive bond should be removed. This includes both foreign debris and a dusty or scaly slab surface.

Remember, once installation begins the contractor is essentially accepting all slab conditions as correct. If problems occur, you will then have to demonstrate and have documentation that all the appropriate tests and checks were completed. If deficiencies were found, they should have been remedied. Documentation can include a short written note sent to the builder/general/owner describing the conditions and possible results if deficiencies are not fixed.

Lay out is critical, square the working lines with the room walls. Position the lines so the focal areas are balanced. Spread adhesive to the lines so precise alignment of the first course will be correct.

We all know that concrete slabs emit water vapor continually. As long as the conditions are not excessive and wood flooring is protected, good performance is expected. The adhesive or other components of the flooring system should provide the moisture protection necessary for normal vapor emissions and the occasional temporarily elevated moisture condition, the temporary spill or abnormal rainy weather. An excessive moisture condition, such as a broken pipe or continuing water intrusion from a sprinkling system, is beyond the function of normal protection.

The new NOFMA publication states that a compatible vapor retarder should be placed on the slab where potential moisture conditions are high. For the extra protection this is good insurance for all applications, not just where the potential is elevated. Also, check with the adhesive manufacturer for the vapor retarding performance of the adhesive. It’s a good idea to use an adhesive for which the vapor retardance performance is warranted. To ensure adequate vapor retardance, the adhesive should also be full spread. Just a bead of adhesive does not provide adequate moisture protection where adhesive is not present. Also, spreading the adhesive with a notched trowel may expose the slab and not provide adequate protection where the trowel scrapes the concrete. For instance, with a notched trowel, up to one half of the surface can be exposed by the “trowel teeth” and when the flooring is installed the compressed ridge may not cover the exposed space completely, particularly if the spread rate is skimpy.

With some adhesives you can see the working lines through the adhesive. In most cases, just spread it to the line.

The publication explains the critical details for handling and storage as related to acclimation; proper jobsite conditions, slab requirements, and installation procedures.

The very critical topics include “Proper Acclimation,” which can only be determined by using a moisture meter and “Job Site Conditions,” which must be near occupancy conditions. The “Installation Procedures” section directs that pre-racking four or five runs and selecting those boards that will allow this rack to lay flat and together is the criterion for determining acceptable boards.

The publication is available on the NOFMA web site www.nofma.org under Publications. Again, check with the manufacturers and follow their recommendations, take no short cuts, do not use adhesives with a water component, apply adhesive as a full spread adhesive, and use a compatible vapor retarder to protect the flooring from the unusual condition.