Proper subfloor preparation includes making sure the subfloor material is level.  This concrete subfloor has been filled to repair low spots.


A student at the National Wood Flooring Association’s Subfloor Preparation School performs a moisture test on a concrete subfloor.

Everyone in the building, construction and remodeling industry is familiar with the old adage “Measure twice and cut once.” This common sense approach reminds all of us to take the steps necessary to avoid mistakes up front and do the job right the first time.

The same philosophy can be applied when installing wood flooring. Before any wood flooring installation work begins, the flooring contractor’s number one priority should be to see that the job site conditions are acceptable for proper installation of the wood flooring to take place. Knowing what to look for before any work begins can save the flooring contractor big headaches down the road, and significantly impact his or her profits.

The first thing a contractor needs to understand is that installing wood flooring is often one of the last jobs to take place on any building, construction or remodeling project. In many cases, this means that the flooring contractor may be pressured to get the floor installed quickly to make up for previously missed deadlines from other trades. This means that the flooring contractor often is encouraged to begin installation without taking the necessary precautions and steps to ensure a quality flooring installation. In the long run, it is always in the contractor’s best interest to insist that certain job site conditions be met, before wood flooring installation can begin.

Prior to the wood flooring being delivered to the job site, the building should be completely enclosed to the outside environment. This means that all doors and windows should be installed properly with working locks and latches. Having the construction site open to the outdoor elements could expose the wood flooring to excess moisture or humidity, which could potentially damage the wood and jeopardize a successful long-term installation.

A student at the NWFA’s Subfloor Preparation School repairs a high spot on a concrete subfloor.



Prior to the wood flooring being delivered to the job site, the building should be completely enclosed to the outside environment. This means that all doors and windows should be installed properly with working locks and latches. Having the construction site open to the outdoor elements could expose the wood flooring to excess moisture or humidity, which could potentially damage the wood and jeopardize a successful long-term installation.

Students at the NWFA’s Subfloor Preparation School measure the levelness of a plywood subfloor and make necessary corrections.



All final grading around the building should occur before the wood flooring is delivered to the job site as well. Any surface drainage on the construction site should direct water away from the building to minimize moisture issues that could negatively impact the wood flooring. In addition, all concrete, masonry, plastering, drywall and other wet trade construction work should be completed and thoroughly dried before the wood is delivered. This includes any painting and texturing that will take place on the job site, both inside and outside the building. Again, exposing the wood floors to these kinds of elements could introduce excess moisture to the materials and potentially damage them.

Flooring contractors also should make sure all basement areas and crawl space areas are dry before the wood flooring is delivered as well, with proper vapor barriers and ventilation securely in place and working. In general, crawl spaces should be a minimum of 18 inches from the ground to the underside of the floor joists. Crawl space vapor barriers should be a minimum of 6 mil black polyethylene. Perimeter venting should be equal to a minimum of .1600 of the crawl space square footage, with vents properly positioned to foster adequate cross ventilation for the entire crawl space area.

A student at the NWFA’s Subfloor Preparation School secures a plywood subfloor to the flooring joists to prevent squeaks.



Another important job site condition that must be met before the flooring can be delivered to the building is for all of the interior heating and cooling systems to be installed and working properly. These systems should be used to bring the building as close to normal living conditions as possible before the wood arrives. In addition, the moisture content of the subfloor material should be checked so that it is acceptable for the geographic area as this will impact the long-term performance of the wood floors as well.

Once all these conditions are met, the wood flooring can be delivered to the job site. After the wood is delivered, it is important to allow enough time for the flooring to acclimate to the current job site conditions. The time required to accomplish this will vary depending on the type of wood flooring used, the species of wood being installed, and the flooring manufacturer’s acclimation specifications. It is important to perform a moisture test on all wood flooring, as well as the subfloor, before the installation process begins. There should be no more than a 4% moisture content difference between properly acclimated wood flooring and subflooring materials, taking into consideration normal living conditions and equilibrium moisture content.

Finally, flooring contractors should remember that wood flooring will reflect the shape of the subfloor underneath it, so the contractor must verify that the subfloor is both solid and flat. If there are high peaks in the subfloor, the flooring contractor will need to sand them down so that they are level. If there are low valleys in the subfloor, the flooring contractor will need to fill them in so that they are level as well. While checking for the flatness of the subfloor, the contractor also should make sure that the subfloor is securely fastened to the flooring joists, which will prevent the floors from squeaking down the road.

Following these basic guidelines to ensure proper job site conditions will help prevent any unforeseen problems, resulting in quality wood flooring installations that will last a lifetime with proper care and maintenance.