Technical Troubleshooting: Starting with a Solid Foundation – Evaluating Concrete Substrates

Just a few decades ago, installing wood floors on concrete subfloors was not possible. Over the years, however, technology has changed and improved, and concrete subfloors are now quite common for wood floors.

To install wood successfully, it’s important to have an understanding of how to evaluate concrete substrates, how the conditions of the concrete substrates can affect the outcome of the wood floors, and how to address those conditions to maximize the end results.

Before the successful installation of any wood floor, the conditions of the concrete underneath need to be taken into consideration.

As an installer, you need to be able to identify all the different types of concrete subfloors on which the floors will be installed. The concrete type influences which moisture test will need to be used, how the wood flooring needs to be prepared, the type of installation system needed, and even what type of flooring can be used.

A slab-on-grade, or slab-on-ground, is a concrete slab poured directly on the ground. It is typically four to six inches thick. This slab type should have a vapor retarder installed below it, so there is a barrier between the concrete and the ground below.

A suspended concrete floor is a slab where its perimeter, or at least two of its opposite edges, is supported on walls, beams, or columns that carry its self-weight and imposed loading. The floor spans between supports and will normally deflect under load to a dimension that is limited by the design used.

Pre-stressed concrete is a method for overcoming concrete’s natural weakness under tension. This type of concrete slab is most often used in foundations for commercial and residential construction. Pre-stressed concrete slabs can be on-grade or suspended. Pre-stressing concrete can be accomplished in two different ways:

  • Pre-tensioning is accomplished by stressing wires or cables. These wires or cables are stretched between two anchors before pouring concrete.
  • Post-tensioning is accomplished by stressing tendons. The end of each tendon is anchored to the concrete section after the concrete has hardened.

Before installing wood floors over a concrete subfloor, you need to check and document the following.

  • The slab must be normal weight concrete, with a minimum of 3,000 PSI. Normal weight concrete is durable, has sufficient cement to give it good finishing characteristics, and can be placed fairly wet without sacrificing quality. (Lightweight concrete may require alternative installation methods or systems.)
  • The slab must be free from non-compatible contaminants. These include sealers, waxes, oils, paints, and drywall compounds.
  • The slab must be a minimum of 30 days old before moisture testing can take place. This does not mean any slab that is 30 days old is ready to receive flooring. Complete all moisture tests and preparation before using any moisture mitigation or adhesive systems.
  • The slab must be sound. Check for hollow spots, loose or crumbling areas, and cracks. Many cracks, hollow spots or crumbling areas may be an indication of structural issues that should be addressed by a professional or engineer.
  • The slab must be flat to within 1/8” in a 6’ radius or 3/16” in a 10’ radius. If otherwise specified, follow the minimum tolerance indicated by the flooring manufacturer. Check floor flatness using a laser, straight edge, or string line.
  • The slab must meet minimum Concrete Surface Profile (CSP) requirements set by the adhesive manufacturer. The normal CSP required for most wood flooring adhesive systems is between CSP 2-4.
  • The slab must be dry and meet the moisture content requirements of the flooring and adhesive manufacturer.
  • Identify construction joints within the slab and address flooring installation appropriately by using transition pieces, expansion joints, or another method that allows the flooring to move independently at these junctions.

Construction joints, also known as cold joints, are used where two successive placements of concrete meet. This cold joint becomes a weakened joint that will crack when either slab moves.

Control joints, also known as contraction joints or saw cut joints, are formed, sawed, or tooled grooves in the concrete slab used to induce shrinkage cracking at specific locations.

Isolation joints are a separation between adjoining parts of a concrete structure, usually a vertical plane, at a designated location used to prevent a bond and allow movement between the slab and the adjoining structure.

Learn more about understanding floor flatness tolerances here.

The NWFA provides a Jobsite Checklist that can aid in checking the concrete subfloor before each job. The Jobsite Checklist is available for download by visiting nwfa.org. For more information, contact the NWFA at 800.422.4556.

Brett Miller is VP of Certification and Education at the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.