Selecting and specifying engineered wood flooring
by Joshua Rubin, AIA
Selecting and specifying finish materials for a project may seem straightforward and limited to the appropriateness of the application and the finish’s appearance. For some materials, that may be the case. However, the process of selecting and specifying finish materials is typically more involved, requiring consideration of multiple aspects of the material or assembly. Wood flooring, specifically engineered wood flooring, requires careful selection, evaluation, and scrutiny.
Description - Hardness
Appearance (color, graining, stain) of the top layer of hardwood, or lamella, of the engineered floor assembly will typically drive the selection of the species of hardwood. While aesthetics is a necessary consideration, the measurement of hardness of the wood species should be evaluated first. Hardness does not necessarily correlate with the classifications of softwood or hardwood. Hardwoods come from deciduous trees (oak, ash, beech, maple) which lose their leaves each year, while softwoods come from coniferous trees (pine, fir, spruce) that stay green throughout the year.
The Janka Wood Hardness Scale, developed in 1906 and standardized by ASTM in the 1920s, is range of measurement of the pounds of force, measured as force pounds (lbf), averaged from radial and tangential penetration, to embed a 0.0444-inch diameter steel ball halfway into the wood test sample (the Janka hardness test).1 The higher the number on the scale, the more resistant that species of wood is to dents, scratches, compression, bending, and overall wear. The measurement is also impacted by the moisture content of the wood at the time of the test. For example, Northern red oak that is green in terms of moisture content yields a measurement of 1,000 lbf.; whereas at 12- percent moisture content, the measurement is 1,290 lbf.2 Northern red oak, due to its wide availability, ease of cutting and nailing, and because it is hard enough not to dent easily, serves as the industry benchmark median against which all other wood species are compared on the Janka scale.
Note, however, the scale does not measure or indicate how likely the selected wood is to crack or to break.
Description – Old growth vs. new growth
The strength, stability, as well as hardness of the wood flooring is also dependent upon whether it is made from old growth wood or new growth wood. Most of the lumber harvested today is from trees that have been cultivated to grow rapidly, leading to a less dense, less stable wood than old growth wood. Old growth wood refers to wood from trees that belonged to forests that grew over hundreds of years. Because of the slower growth, the growth rings are very tight, making the wood quite stable. As a comparison, wood from 1918 has twenty to twenty-five growth rings per inch; whereas wood harvested in 2018 has only seven growth rings per inch.
Description – Construction
With a species selected based on hardness and subsequently, its appearance, the construction of the engineered wood floor assembly is the next most important aspect to consider; it is also the aspect that will determine the floor’s stability and longevity. The layers or plies below the top wear layer are what give the engineered wood flooring its strength and resistance to warping and twisting. The plies make the flooring less susceptible to shrinkage and expansion movement due to fluctuation in humidity, especially when the grain direction in the ply runs at a 90-degree angle to the grain direction of the previous and successive ply; an assembly called crossbanding.
Low quality core materials like oriented strand board (OSB) or fiberboard are often used to cut production costs leading to unstable and prone-to-damage flooring. Baltic Birch plywood or solid hardwood are the only materials that should be considered as acceptable for the core layer. High-density fiberboard (HDF) should only be considered as a core in balanced engineered wood flooring.
There is the potential that formaldehyde was used in the manufacture of the core materials, as it is typically found in the resin binders of the adhesives. Formaldehyde has been identified as a toxic air contaminant, based on public exposure and its potential to cause cancer.3 In 2010, the Formaldehyde Standards for Composite Wood Products Act established emission standards for formaldehyde from composite wood products.4 The Act has since been amended and revised several times to more closely align with the requirements of the California Air Resource Board’s Airborne Toxic Control Measure Act to reduce formaldehyde emissions from composite wood that is manufactured, imported, distributed, or sold in the United States.
Balanced construction is a form of engineered wood floor construction wherein both the lamella and the bottom layer of the plank are of the same species of wood, both typically 4 mm (+/- 5/32”) thick. The balanced construction leads to additional dimensional stability over other engineered wood floor construction, as the top and bottom layers will respond to changes in humidity and moisture at the same rate. This type of floor is typically 3/4-inch thick, approximately the same thickness as solid wood plank flooring.
The engineered wood floor, at a minimum, should consist of at least 3 plies – the wear layer, a core layer, and a base layer. The core layer in a 3-ply engineered board is typically a single board of high-quality plywood or solid hardwood. However, engineered wood flooring that is of multi-ply construction has a core layer comprised of crossbanded, multiple layers (9 to 11). The multi-ply floor will be more resistant to damage from moisture and temperature fluctuations than a 3-ply floor.
The lamella should be as thick as possible as that is the layer which will see all the wear and tear. A 3 mm (+/- 1/8”) thickness wear layer can typically be sanded and refinished up to two-times, depending upon the flooring manufacturer. By comparison, a solid wood floor can be refinished up to seven times, as there is approximately 8 mm of solid wood above the nails securing the floor will be exposed.
The width of the individual plank typically varies from 3-inches to as much as 10-inches. Board as wide as 12” are available, but used primarily for historic renovations, restorations, or reproductions. They are also typically made with Eastern White Pine, which has a Janka scale measurement of 380 lbf.5 A narrower plank will be more resistant to cupping, warping, twisting, and checking (or splitting) than a wider plank. Narrower planks also tend to be shorter in length, whereas the wider planks will be longer.
Wood shrinks or swells in relation to its grain orientation, as wood is an anisotropic material.6 Hence, cupping occurs as a floor board expands in width. A quarter sawn board, producing vertical grain and expanding in thickness, will be more resistant to cupping than a plain or flat sawn board, which expands in width, and which will more easily cup and warp.
The finish of the engineered wood floor requires accounting for factors such as the type and frequency of foot traffic, the potential exposure of the floor to liquids and solid food, and excessive moisture. A surface finish, such as a urethane, is a durable, water-resistant choice that typically requires minimal routine maintenance. Natural oils or waxes are penetrating type finishes that soak into the pores of the wood and harden to form a protective seal. These penetrating finishes require on-going, periodic maintenance consisting of additional application of oil or wax. A commitment from the end user acknowledging the need for this continual maintenance is warranted. A prefinished, acrylic impregnated finish is available from some manufacturers wherein a liquid acrylic monomer is injected into the cellular structure of the wood. The acrylic finish adds a plastic-look to the end product and is best used in highly trafficked commercial areas such as shopping malls or restaurants.
Description – Installation
The control of water, either as liquid or vapor, is paramount to the long-term performance of the engineered wood flooring. The flooring industry reports that approximately 85% of all floor installation failures result from moisture problems, accounting for more than $1 billion damages annually.7 At slabs-on-grade, there is the high potential for water vapor to migrate through the slab and subsequently impact the engineered wood flooring. Accordingly, the moisture content of the substrate must be determined to ensure that it is within acceptable levels per the wood flooring manufacturer. Section 09 64 33 should include both an anhydrous calcium chloride test for vapor emission and a relative humidity test as required for all concrete floor slabs, both at slabs-on-grade and at elevated slabs, whether new or existing. The specification should also require alkalinity and adhesion testing, with the substrate alkalinity within the range of 5 Ph and 9 Ph. There are different requirements depending on the type of substrate present.
Moisture in the air is also of concern; specifically, the air near the floor, as the relative humidity of the air at the floor is typically different from the humidity measured in the center of the room. The relative humidity of the air is typically controlled by the heating, ventilation, and cooling (HVAC) system. As such, prior to installation, the engineered wood flooring needs a chance to adjust to the prevailing moisture in the environment into which it is being installed, so the flooring will be at its most typical size. Most engineered wood flooring manufacturers will indicate in their installation instructions that the floor needs at least 72 hours to acclimate. The instructions will also mandate the required relative humidity conditions for the manufacturer’s warranty to remain valid.
Description – Maintenance
Every engineered wood floor product will require maintenance; no product is maintenance free. Each engineered wood flooring manufacturer will have their own proprietary requirements for maintenance as a condition of their warranty. These requirements need to be discussed with the end-user prior to finalizing the product selection, as the requirements may be rigorous (such as having to apply additional coats of wax or oil). Typically, dry sweeping or dry dust mopping will be required on a regular basis. All spills should be cleaned immediately. Using water to clean wood floors should be avoided. Water can and will break down the finish (except for acrylic impregnated finished products) and damage the wood flooring over time. Excessive water, typical of wet mopping, will lead to the floor cupping.
Conclusion
Engineered wood flooring can be a cost-effective, reasonable alternative to solid wood flooring. And just like solid wood flooring, engineered wood flooring is subject to damage from excessive moisture and thus requires careful selection of the wood species depending upon the intended location and usage. Substrate preparation is vital to ensure the long-term performance of the installation, in addition to advising the end user as to the flooring manufacturer’s maintenance requirements. Research, investigate, and question as you consider the species, construction, and context.
Footnotes
- “Wood Handbook - Wood as an Engineering Material”, General Technical Report FPL-GTR-190, Madison, WI. USDA Forest Service, April 2010.
- See Wood Handbook, Chapter 5, for a tabulation of Janka measurements for commercial woods both imported to and grown in the United States.
- Formaldehyde | California Air Resources Board. (May 2020). California Environmental Protection Agency. https://ww2.arb.ca.gov/resources/fact-sheets/formaldehyde
- Formaldehyde Emission Standards for Composite Wood Products. (March 2021). US EPA. https://www.epa.gov/formaldehyde/formaldehyde-emission-standards-composite-wood-products.
- See Wood Handbook, Chapter 5.
- “Problems, Causes, and Cures” Technical Publication No. C200, St. Louis. MO. National Wood Flooring Association, 3 rd Edition, 2018
- “Why Flooring Professionals Must Test for Moisture and Alkalinity.” Wood Moisture Solutions, January 19, 2021. https://www.moisturemeters.com/why-flooring-professionals-must-always-test-for-excessivemoisture-and-alkalinity/.
Resources/References
- Wood Handbook | Wood as an Engineering Material; General Technical Report FPLGTR-190. United States Department of Agriculture, Forest Service.
- National Wood Flooring Association. https://nwfa.org/technical-guidelines/
- American National Standard for Engineered Wood Flooring (ANSI/HPVA EF 2020).
- Wood Floor Business. https://www.woodfloorbusiness.com/
Disclaimer: The viewpoints expressed in this article are solely those of the author(s) and have not been approved by, reflective of or edited by other individuals, groups, or institutions. This article is an expression by the author(s) to generate discussion and interest in a particular topic. Though the article may cover specific legal and professional practice concepts, it should not be construed as professional advice. Always seek the advice of a professional licensed in your state for questions pertaining to the interpretation of laws and regulations.
About the author
Joshua Rubin, AIA, LEED AP is the technical director for Perkins&Will in their Washington, DC office and is responsible for creating a culture of technical excellence. Joshua sees himself as a teacher – tasked with creating an environment wherein the staff are empowered to ask questions, think critically, to be curious, and to fail successfully.
The CCA knowledge community regularly publishes white papers on topics related to construction contract administration. If you'd like to receive white papers and other related content, please sign up for CCA.