Growth Rings in Yakisugi “shou sugi ban” Cypress Siding
Growth ring patterns are critical to both yakisugi “shou sugi ban” appearance and, more than most people realize, wood longevity. Anyone considering wood siding is concerned with both of these subjects, along with cost as well. Appearance, longevity, and cost make up the siding selection decision trifecta. In terms of appearance, Westerners typically want their siding to stay the same color forever, while Japanese prefer a rich patina to develop organically over time. Either way, we all desire minimal maintenance and maximum wood longevity.
Growth rings in a log cross-section differ in color and density according to what time of year they grew. They’re either earlywood, with a wider, lighter, and softer growth ring, or latewood, with a narrower, darker, and harder growth ring. A set of earlywood and latewood rings marks each yearly growth cycle, with quick growth during spring, and slow, dense growth later in the summer.
Here is a good reference: https://en.wikipedia.org/wiki/Dendrochronology#Growth_rings
Cypress (we use Cryptomeria japonica, or “sugi”) thoroughly heat-treats without twisting or other dimensional movement even with the high-heat Japanese process when logs are selected for straight, even grain (and the wood is air dried instead of kiln dried). It also has good-looking grain contrast with consistent color variation achievable lot to lot. Its porous earlywood dries out quickly after getting wet from rain and is insulative for the structure when used as siding. Also critical for heat treatment, the latewood growth rings are much thicker and more rigid in proportion to the earlywood growth rings compared with other porous softwood species. With the Japanese surface burning heat treatment this results in a more substantial, protective soot layer that remains for decades after installation instead of eroding off quickly. Soot layer longevity translates directly to wood longevity for siding applications, since the consumable soot layer is hydrophobic, antiseptic, and highly UV resistant.
Generally in lumber grading, the tighter the growth rings, the higher the grade of wood. This is because older, slower-growing trees produce harder, stronger, and more consistent grain. However, we find that younger cypress trees with thick growth rings heat-treat better and dry out more quickly since the grain is more porous. Deeper heat treatment and quicker drying results in better siding longevity. Wood with tight, hard growth rings tends to check and split out more readily when subjected to high heat, and tend to crook and twist from heat treatment. It is very difficult to install crooked and rigid planks, as siding must be tweaked a little during install to get a flat wall plane. Also, siding made from older, denser trees will simply last longer than siding made from younger trees, so there is no reason to heat-treat planks milled from old growth lumber. Therefore for yakisugi, the growth ring density spec is different from that of regular siding.
Younger wood with thicker growth rings shrinks more radially, since each growth ring takes up a higher percentage of the log cross section. Earlywood shrinks more than latewood when the lumber cures due to higher moisture content and less lignin density. Yakisugi is resawn through-and-through for best dimensional stability and weather resistance, so planks from the center of the log close to the pith will cup more than those with tighter grain towards the outside of the log. For cladding applications it is priority to mill a plank that is as long, straight, and flat as possible, and will stay that way when exposed to weather. It is for this reason that slight cupping is accepted in our trade for overall plank straightness in every other axis.
Even within the Hachiro variant of cypress that we use for cosmetic construction lumber, every single tree will have wood with different density, grain, and color characteristics. An old tree growing on an exposed south-facing slope with inclement marine weather and a young tree growing in a wind-protected, moist valley will have very different growth ring patterns due to the climate differences in their immediate environments. Not to mention soil variation location to location or genetic variants region to region.
Why Use Cypress Instead of Other Species?
Our customers often ask why we use cypress instead of cedar, pine, fir, larch, or oak for making yakisugi. Over the past decades we have tested various species available on the international lumber market, and have found that cypress is superior for the Japanese yakisugi heat treatment. Being both the largest siding and flooring mill in Japan, and our affiliates being some of the largest lumber producers and traders in the world, we have access to and mill many different species (our affiliate WoodOne is the largest radiata pine timber producer in New Zealand, and we source larch from Russia). However, we find that sugi cypress simply performs best in aggregate for dimensional stability, thermal performance, surface soot strength, etc., not to mention that we think it has a fabulous grain appearance and develops an opulent patina over time. Due to paper-thin latewood growth rings the soot layer on cedars erodes almost immediately after installation, pine yields inconsistent results, and larch, fir and oak are simply too dense for effective heat treatment by the Japanese method.
It is not a coincidence that the “sugi” in yakisugi or shou sugi ban means Japanese cypress (often colloquially called “cedar” in English for historical reasons). Growth ring patterns are important for species selection, log selection, and millwork, and this is yet another example of why traditional construction and millwork knowledge has persistent value. We find over and over that all kinds of engineering, experiments, and testing can be done to satisfy our intellectual curiosity and economic drive, but that in the end, traditional know-how developed over centuries by those that came before us is usually superior. Engineered products often fail sooner or later due to something unforeseen. Ask any seasoned remodel carpenter. Natural materials combined with holistic and conscious manufacturing will result in products that are beautiful and perform better over the long-term.