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.
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.
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.
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.
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 sitewww.nofma.orgunder 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.