Hardwood - The Hard Facts

A Crash Course in Wood Stability
February 1, 2019

How species and cut will impact the performance of your hardwood floor

It's likely that "performance" isn't the first word you associate with wood. But if you're in the market for a new wood floor, it should be.

Every species of wood--hardwood or softwood--has a specific performance profile. The factors most people consider of the available metrics in wood flooring are almost universally hardness, grade and color. But it's rare that we have a client ask us about dimensional stability.

Hardness, coloration and grade can be manipulated by a homeowner. Every species of wood has a different hardness rating, and for most species of flooring that rating stays about the same regardless of the way the flooring is milled. The color palette of your floor will vary greatly depending on finish selection, not to mention the myriad of traditional stain, active stain, wood bleach, iron acetate, penetrating oil and other color processes. Similarly, you can select from up to five different grades in most flooring species depending on the amount of character you want your floor to illustrate.
You might be surprised to learn that you also can pay a little more to get a floor that is less likely to develop gaps between rows over time. Most domestic hardwood species are available via two different families of milling: plain-sawn and radial-sawn. Comparing these two cuts typically shows a dramatic differences in stability (and sometimes in hardness, too).

Understanding Dimensional Stability

As an organic, hygroscopic material, wood will absorb and release moisture to stay "in balance" with the ambient humidity of the surrounding air. Dimensional stability is a measurement of shrinking and swelling as a result of moisture changes in the wood--a high dimensional change coefficient ("DCC") means a floor will move a lot with humidity changes and permanent gaps will open between planks over time, and a low DCC means limited expansion/contraction and a tight floor for years or decades.

(If your house stays at a perfect 40% humidity level twenty-four hours a day, fifty-two weeks a year, then you can skip this section of the article.)

Assuming you live on the planet earth, the interior humidity level of your home is fluctuating constantly. Outside ambient humidity levels peak in the early morning hours and wane through the afternoon. In low-desert climates, the outside humidity might only change appreciably during rainstorms. In high desert climates (like that in northern Utah), the ambient outside humidity can range from 80% on a winter morning to 10% on a summer evening.

Compound those external factors with the moisture released from your fixtures and appliances like your shower and dishwasher, and the enormous humidity impact of your HVAC system, and it's easy to see how a floor with a high DCC is a wise investment.

Hickory, while very hard, has a regrettable DCC of 0.00411. That seems like a tiny number, but math don't lie: a 3% change in moisture content of a 5" hickory floor translates to a potential gap of 1.5mm per row. That's about the same thickness as a penny--a huge gap by flooring standards.

American cherry is one of the most stable domestic species of hardwood. While not very hard, it has a DCC of only 0.00248, meaning the same 3% change in moisture on a 5" cherry floor will only yield gaps of 0.94mm.

Most online resources list dimensional stability figures for plain-sawn wood. The measurement of plain-sawn stability is called "tangential stability."

Milling and Stability

There's a reason that plain-sawn milling is the standard for both stability and hardness measurements. Plain-sawn milling yields a higher output, which is to say that you'll get more plain-sawn planks out of a tree than you would radial-sawn planks. Plain-sawn wood is, therefore, more cost-effective and more environmentally responsible on first glance.

However, for nearly every domestic species, a radial-cut floor is more stable than its plain-Jane counterpart. Better stability means smaller gaps and less stress on the both tongue-and-groove millwork and the mechanical fasteners (usually staples) in your floor. So while you may pay a little more up front, the floor will last longer. That improved service life means less frequent repairs and replacement, saving you money (and saving trees) in the long run.

The two most popular radial cuts of wood are rift-sawn and quarter-sawn. Those two cuts usually come bundled together--wood nerds enthusiasts commonly call this milling "R&Q." (You can get all-rift or all-quartered flooring, but be prepared to pay a hefty upgrade fee--someone has to go through a batch of R&Q and hand-select the rift from the quartered, then bundle them both back up.)

Besides the unique aesthetic variations between plain-sawn and radial-sawn wood, there's almost always a huge stability advantage to choosing the radial cut. Radial shrinkage is typically about half that of the plain-sawn rate. So where your 6" plain-sawn oak floor might shrink 2.2mm (yikes!) with a 4% drop in moisture content--admittedly, that's a huge drop--a 6" R&Q white oak floor would only be at risk of developing 1.2mm gaps.

Radial Cuts, Plank Width and Species

For some species, then, picking a radical-cut product will mean the difference in a floor that develops gaps in a year or less versus a floor that never really develops gaps at all. But not all species are readily available in rift or quarter-sawn. So, when considering your flooring species choice, you might want to weigh your plank width against available cuts.

White oak might be labeled the "default" species for hardwood flooring in North America. It's readily available for sustainable harvesting in the US and Canada, it comes in a bunch of different grades, and it's hard enough for most people to live on without having to walk tip-toe in cushioned socks.

The downside to white oak? It's not very stable: the tangential DCC is on the bad side of the scale at 0.00365 (and red oak is slightly worse). An example of how that math works out is provided above.

Luckily, white and red oak are both readily available in R&Q milling. That's why we recommend using the R&Q cut in wider-plank floors, or in homes without a humidifier in the HVAC system.

Some domestic species, however, aren't generally offered in radial cuts. Hickory only comes in plain-sawn from nearly every mill we're aware of. You could special-order R&Q hickory, but be prepared to pay through the nose for it. This low stability is a reason why hickory isn't recommended for use over radiant heat systems, and why it's highly encouraged to camouflage the inevitable gaps of a hickory floor with beveled edges.

R&Q maple is available from some mills, but it's expensive and hard to order in large quantities. The reason? Most lusted-after maple features, like birds-eye and tiger stripe, are only visible in the radial cut, and nearly all of that material goes to cabinetry shops or luthiers of stringed instruments. Because maple has a low stability rating, we recommend that you limit your maple floor to 3.25" or smaller to keep big gaps from opening up (or buy engineered flooring).

American cherry and walnut are both available in radial cuts, though it's unusual to see either in R&Q. As a result, the price tag for a radial cherry or walnut floor is usually quite a bit higher than their plain-sawn counterparts. And the R&Q cut of either species is typically limited to 6" or narrower.

Fir flooring is almost exclusively milled as "vertical grain," another term for radially-cut. That's why the original fir floors in a 1915 home are still pretty tight--a hundred-year old 3.25" oak floor would have enormous gaps in it.

Wrapping Up

Our go-to recommendation for wide-plank flooring is radially-cut white oak. The price is competitive, the stability is great and R&Q oak provides a timeless, elegant and organic look. Narrow maple flooring can be either a nod to a historic home or can be very contemporary when finished with a raw-look polyurethane. Textured hickory lends a classic-rustic vibe and can take a pounding, but you can expect gaps to open over time with wider planks. If you have a little extra room in your budget, an R&Q walnut floor is luxuriously, strikingly beautiful and will stay tight for generations.

Remember where you live when you're picking a species--the continued beauty of your hardwood floor depends on an educated product selection. Signature Floors can help you understand and choose the right species for the unique environmental requirements of your home. That's the kind of performance you'll want to live with!

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