Janka Scale Standards for Wood Species

When looking for lumber that will last underfoot, qualities such as durability, dimensional stability, and hardness are highly desirable. Since wood varies in hardness from species to species, a successful flooring selection should be determined by more than just a simple hardwood designation. The Janka hardness scale serves to quantify the hardness of a wood species, acting as an accepted industry standard for determining the suitability of specific wood species for flooring purposes. The information below provides a brief overview of the origins, usage, and formula of the Janka hardness scale, surveying its consequential role in determining the strength and durability of softwood and hardwood floors.

Origins and Usage

As the brainchild of Austrian wood researcher Gabriel Janka, the Janka hardness scale was invented in 1906 and standardized in 1927 by the American Society for Testing and Materials (ASTM). Designed to determine the exact hardness of different types of wood, it remains widely used in world of design and architecture, allowing for informed decisions on which hardwoods will work well in different environments. “To evaluate a particular wood species for project suitability, we look at two physical characteristics of that wood: dimensional stability and hardness,” says Andy Kjellgren, VP of A&D Sales at TerraMai. “The Janka hardness scale is the universally accepted metric used to demonstrate relative wood hardness, as compared across species.”

Photography Courtesy of TerraMai
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Overview and Formula

Depending on where it is installed, different levels of hardness may make one species of wood more desirable than others. “We like to say that it’s TerraMai’s job to help designers and architects stay out of trouble,’” says Kjellgren. “Some of that trouble comes when a well-meaning designer specifies a softwood floor in a more classical, high-end space. A softer species can show dimpling from high heel traffic and will simply show signs of wear and tear more readily than a harder floor. Sometimes the aesthetics of a patina-rich softwood are exactly what a designer is looking for.” A Janka hardness rating reveals the extent of possible damage and denting for the floor, while also showing how a specific wood will react to a saw. Generally speaking, the higher a rating is on the Janka hardness scale, the harder or stronger the wood will be. Likewise, higher levels of hardness indicate that the wood will be more difficult to saw.

The formula for the Janka hardness scale involves measuring the force that is required to drive a 0.444-inch steel ball into a specific plank of wood to a depth that equals half its diameter. The test determines the hardness of the wood by measuring the pounds per square inch (PSI) of force that is needed to push the steel ball halfway into the wood. Starting at zero for the softest wood species, the scale ascends up to 4,000, with hardwoods typically falling between the 800 mark and the upper limit. “TerraMai uses the Janka scale to guide clients through wood selection in projects where the exposed wood surface will be challenged, as is the case with most flooring or exterior decking applications,” says Kjellgren. “We’ll ask the client, ‘how open are you to seeing signs of use in the flooring?’ or ‘do you want the decking to maintain a formal appearance long after the installation?’ If they want a floor or deck that truly resists showing signs of use, then we will guide them toward specifying a harder wood. If they are embracing another aesthetic, we can consider softer woods, as long as they are milled, finished and acclimated properly.”

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Wood Species: Hardwoods and Softwoods

It is often assumed that a wood species that has been classified as hardwood will inherently have a higher level of hardness compared to a species identified as softwood, but this is not always the case. Determining factors for differentiating between hardwoods and softwoods can be boiled down to biological distinctions. “Hardwoods come from flowering trees, like oak and walnut, while softwoods generally come from conifers, like pine and cedar,” says Kjellgren. “In terms of design and building, softwood and hardwood fall along the same continuum of hardness and can be viewed along the same hardness scale. Elm is considered a soft hardwood and Heart Pine would be considered a hard softwood.”

While most hardwoods will inhabit the upper half of the Janka scale, there are many instances in which softwood species are essentially equivalent to some hardwood species on the Janka scale. “When evaluating wood for a project, a designer absolutely wants to consider hardness, as discussed above, and dimensional stability,” says Kjellgren. “Interestingly, softer woods are often more dimensionally stable than hardwoods. This is why a full understanding of environment and application is essential to specifying wood products that exceed the expectations of the end user.” While there is an extensive array of elements that should be considered when specifying wood species for interior installations, the Janka scale serves to gauge the hardness of the wood in question, acting as an indicator of how well it will be able withstand the wear and tear of time and helping to forecast the future of the flooring underfoot.

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