The Importance Of Substrate Prep -- This wood floor installation developed a loss of bond and failed because old asphalt adhesive residue was not thoroughly removed from the concrete substrate.

The use of proper and correctly prepared substrates is of critical importance to the successful installation of any floor covering material.

For the benefit of those who don't have a good understanding of substrates and their properties, I'll begin with the basics. A substrate is any smooth surface over which the flooring product is installed. Substrates may be any of various underlayment types, existing resilient floors, Portland cement concrete, poured gypsum underlayment, and wood panels.

Improper or incorrectly installed substrates can lead to a number of job-related problems. In the case of vinyl floor installations, such problems include tunneling over underlayment joints, wood show-through, discoloration from staining elements in the wood, joint telegraphing, and more.

With wood flooring, if is to be installed over on-grade concrete, one of the most critical areas of substrate preparation is the need for a dry slab. If the slab is not adequately dry, any number of problems can result. Among them are cupping of the wood's face, loss of bond due to expansion and, in some cases, mold growth and subsequent discoloration (darkening) of the wood flooring.

Another serious substrate problem frequently found in wood installations occurs when the concrete substrate has an asphalt adhesive on the surface. Even with minimal moisture exposure, a loss of bond may result due to the moisture/alkali affect on the asphalt adhesive. In the current era of urethane adhesives, application of a concrete curing compound can also result in adhesive bond loss if the product is not thoroughly removed before the floor is installed.

Important substrate concepts

The following are some basic definitions of common substrates, their components and related terms:

The subfloor is the structural part of your installation "sandwich," while the underlayment is the "smooth finish" product situated directly atop the subfloor and upon which your flooring product will be installed. Suitable underlayments -- depending upon the particular application -- include wood paneling, gypsum poured concrete and cementitious backerboards.

To prevent moisture damage in the finished installation, a sleeper-constructed subfloor using 2-by-4s is often employed to isolate wood flooring from an on-grade concrete slab.

Sturdi-I-Floor is an American Plywood Association-rated wood substrate that performs as a combination subfloor and underlayment. It's worth noting that some vinyl floor manufacturers recommend that a ?-inch underlayment be placed over the Sturdi-I-Floor product to prevent potential staining resulting from nails, construction adhesives and so forth.

Cementitious Backerboard Units (CBUs) are manufactured in various sizes. Traditionally, their primary use has been for shower walls as an alternative to water-resistant (WR) gypsum board. CBUs are also used in conjunction with sound-deadening products for certain ceramic tile and stone floor installations.

Lauan Plywood is a popular underlayment but, for flooring applications, only Type I exterior-grade BB, CC or OVL should be used. Otherwise, problems such as vinyl discoloration, indentation, etc. may develop in the finished installation.

Fire-rated plywood is also seen in some specifications. This material can create bonding difficulties when water-based adhesives are used. Staining, as well as expansion due to moisture exposure, can also come into play when fire-rated plywood is used.

Gypsum poured underlayments have presented problems for installers who are inexperienced or lack sufficient knowledge of the preparation required when these products are used in a flooring installation. Many adhesive manufacturers do recommend their products for this type of installation provided an acrylic sealer is first applied. The sealer slows adhesive soak-in and also binds to the surface to produce a better troweling surface. If a urethane adhesive is used to install ceramic tile, an acrylic sealer is not required.

A Stain That Can't Be Washed Away -- Discoloration in vinyl flooring can often be traced to staining elements in the wood underlayment and/or the fasteners that anchor it to the subfloor.

New guidelines in the offing

An ASTM committee is currently working to establish related guidelines which should emerge under the detail heading ofStandard Practice for Installation of Poured Gypsum Concrete Underlayments and Preparation of the Surface to Receive Resilient Flooring.

This proposed standard would cover the installation of poured gypsum concrete underlayments on a wood structural-panel subfloor. In addition, the proposed standard is expected to set standards for compressive strength (expressed in pounds per square inch), set time, burning characteristics, and density. Field cube samples will be sent to the appropriate manufacturers for testing. Standards will also be set for compressive strengths based on live load deflection.

So in summary, a floor covering installation is a three-layer sandwich composed of an underlayment, adhesive and the finished flooring material. And also remember the old saw: Your bond is only as good as what it is stuck to.

In the final analysis, it all comes back to the "Standard of Care" and your interpretation of same.