Yakisugi is a process in which wood is burned or charred on one side to make it more durable as a siding material.
BY BILL HENSLEY
AS IT WARMS AND DRIES THROUGH MULTIPLE CYCLES OF DROUGHT, the Western United States is increasingly prone to wildfire, with every year being worse than the previous. But fire has been used by indigenous people as a forest management tool for thousands of years, keeping travel routes open, managing food supplies and encouraging foliage growth to attract game animals. In very real ways, fire has been used to protect and nourish the forest as part
of human stewardship of the land.
Until the mid-1800s, when Federal and state governments began suppressing prescribed fires and cultural burns, the Indigenous peoples of California and other Western states set fires intentionally during favorable conditions. When the forest is cleared of undergrowth, a greater variety of trees is encouraged, leading to a healthier more fire-resistant forest. This also creates more room for wildlife to thrive which enables hunting for game animals. When fire suppression became the norm, we allowed forests to fill with fuel. Now, with catastrophic fires becoming the norm, the effectiveness of the traditional fire management ways is being reconsidered and the traditional ecological knowledge is even being incorporated into some management plans.
As the right application of fire can add to the health and longevity of the forest, so too can it add to the health and durability of timber removed from the forest to be used in building.
Originated in Japan, Yakisugi is wood treated by burning or charring the surface of one sideto make it more durable as a siding material. Wood is mostly structural lignin and hemi-cellulose carbohydrates. Charring the surface burns off the cellulose layer and results in a wood siding that is water-resistant, resistant to many insects, and far slower to decay. The soot also creates a more fire-retardant outer layer by increasing the temperature required for combustion. The process to create Yakisugi was first used centuries ago in Japan to make Japanese Cypress cladding more weatherproof. In recent decades it has become more popular throughout the world for sustainable features…and for its astounding beauty.
Yakisugi is written with two Chinese kanji 焼杉 with the Japanese phonetic pronunciation. Yaki ( 焼 ) means fried, grilled, burnt or otherwise heat-treated. Think of the menu at your local Japanese restaurant: Yakisoba (焼きそば = fried noodles) or yakitori (焼き鳥 = grilled chicken). Sugi ( 杉 ) is the Japanese Cypress (Cryptomeria Japonica) tree, also known as Japanese Cedar in the West.
More commonly in the West, wood that has been treated in this manner is known as Shou Sugi Ban. How did Yakisugi (焼杉) become shou sugi ban? Sometimes a third kanji ( 板 = plank) was added to the compound word. Kanji often take on multiple pronunciations depending on the use and at some point the three kanji ( 焼杉 板 ) were mispronounced as shou sugi ban – likely from a non-Japanese speaker looking up the Kanji characters in a dictionary and assuming the incorrect pronunciation. Shou Sugi Ban has become the more common term in the Western hemisphere, but for this article we are going to stick with Yakisugi.
Charring the surface burns off the cellulose layer and results in a wood siding that is water-resistant, resistant to many insects, and far slower to decay.
heat without the burn
The high temperature burn that creates beautiful Yakisugi is not the only heat treatment for wood. Heat treatment of wood — thermal modification — has been used for millennia in various cultures and countries around the world. Another type of thermally modified wood is wood heated to above 180°C/350°F in absence of oxygen to induce changes primarily to the hemicellulose and lignin structures. Again, the result is increased durability. The absence of oxygen is important to these processes as it prevents the wood from burning at these high temperatures. Several methods introduce either steam, nitrogen gas or hot oil into the process.
Heat treatment can be done on all wood species but a main advantage in thermally modified wood is that softwood can be used in applications requiring high durability, and as this durability is achieved without chemicals and additives, it is more environmentally friendly. Other methods of thermally modified wood share some positive characteristics with Yakisugi: Shrinking and swelling is reduced substantially, again helping durability. Resistance to insects and micro-organisms is improved as well.
The story of how we get to our passion in life is often interesting. In William Beleck’s case it started as a high school student in Oregon when his family hosted exchange students from Japan. One of these students invited him to his home country and William spent a year as an exchange student in the Southern Japan city of Saga. Later, he attended the prestigious Keio University in Tokyo and then worked at Marubeni, a major Japanese integrated trading and investment business. After returning to the U.S. he started a venture buying and selling reclaimed lumber, a challenging niche market.
It was a TV show about building a traditional Japanese house with traditional materials that introduced Beleck to the magic and allure of Yakisugi. The house was beautiful, clad with Yakisugi siding. He asked a friend to introduce him to a traditional Yakisugi manufacturer in Japan and the reply was “you are already selling reclaimed lumber to the market leader.” Indeed, Nakamoto Forestry, the largest producer of Yakisugi charred siding in the world, was a customer for Beleck’s reclaimed lumber. He reached out to Mr. Nakamoto about building the Yakisugi market in the U.S. And less than a decade later Nakamoto Forestry is the leader in Yakisugi siding in the U.S., with a 40+ percent market share.
We recently spoke with William to learn more about this ancient and interesting material.
BILL: Tell us a little about heat-treating wood.
WILLIAM: Heat treatment of wood is a universal technology; it’s been used for eons and world-wide there are all sorts of technologies that have been developed. One example is the thermally modified wood in Finland that uses a two to four-day steam heat/dry heat cyclical process that penetrates deeply into the wood. Another of course is the Japanese technology employing a flash heat treatment.
The flash heat treatment is challenging for the lumber. We’re dealing with 2,000 degrees in a typical siding application. Siding must be long and straight, must last a long time and be cost effective. And it must be beautiful, so we have a lot of requirements to fulfill with this application. Every species has its own nuances, but a heat treatment will have a positive effect on durability because it burns off the hemi-cellular layer of the wood, which is what bugs and fungus metabolize.
Most species of wood have some sort of a handicap which is why we stick with Sugi Cypress for our Yakisugi. Some producers might apply a char to Red Cedar or White Cedar, but the latewood growth ring of those is paper-thin. The closest species we have found is the Bald Cypress out of the South. You need a porous wood so that it dries quickly, but for our purposes it must also be rigid and of structural grade in a lateral position — it has to be able to bear weight and Sugi checks that box. It’s porous like the Red and Yellow Cedars or Redwoods, with sufficient tannic acid content to have better durability as siding in general.
With Red Cedar and its paper-thin growth ring, the soot layer tends to fall off. If you burn it, it just turns to powder, and you can’t get a substantial wear surface that will last 50 years. The Pines and the Larches and Douglas Fir work well as exterior siding when they’re painted. Their mineral content is a little different, they’re pitch heavy. Pitch is great for durability in exterior. But these species are not as good for stain-grade finish as the Cypress.
For Yakisugi, what the Japanese say — and what we at Nakamoto say — is that it’s all about the color. One of the main selling points of some species of wood is the rich color variation, not just a monolithic white or pure yellow, but it’s a cosmetically pleasing grain pattern with color and tone variety within the board that makes it visually interesting. And what it will look like in thirty years is an important part of the Yakisugi experience; it’s not just about rot prevention.
“We’ve found that when we send a sample mock-up, we always get the order. It really is just raw blatant beauty. Part of it is the wood, part is the heat treatment and part is the traditional oil finishes we use. They showcase the wood and enhance the depth and beauty.”
BILL: From the designer’s perspective, they’re also thinking “What does this look like tomorrow when I turn it over to my client?” What are designers looking for when considering options for siding and walls?
WILLIAM: They want the ‘whole package’ including cost performance and sustainability. But the immediate visual impact is essential. We’ve found that when we send a sample mock-up, we always get the order. It really is just raw blatant beauty. Part of it is the wood, part is the heat treatment and part is the traditional oil finishes we use. They showcase the wood and enhance the depth and beauty.
BILL: Is that a linseed oil?
WILLIAM: Yes, it’s tried and true. The linseed oil finishes will fade more than the modern waterborne finishes, but fading and color change is part of the allure.
BILL: Tell us about the color options.
WILLIAM: Designers have a vision. They are of course trying to satisfy the client but also themselves. Most are looking for color combinations. The pigment in the finish is part of what helps them achieve that. The pigment in the oil finish also adds UV protection. A clear oil finish showcases the wood, the most popular color we ship is black. It’s a really interesting color, very high contrast in the day, but at night it almost disappears. It’s an aesthetic that people often don’t think about till they’re living in the house.
BILL: Let’s talk about sustainability and environmental impact. Walk me through what’s happening in Japan and the product lifecycle within Nakamoto Forestry that is lowering the environmental impact.
WILLIAM: This is really important. Timber management science and ethics have improved dramatically in the last thirty years — in North America and Japan. Oregon State University has a fantastic program and there are similar timber management programs that the government and universities sponsor in Japan. There are still clear cuts going on around the world which is part of timberland management and harvest. But there needs to be science-based management and it has to include watershed management, plus planning (and planting) three generations in advance of what the market will want.
BILL: And texture is a part of the allure?
WILLIAM: Texture is especially important for designers, and we offer three different Yakisugi surface options. The most easily recognizable is our Suyaki™ texture with its thick soot layer. We employ a light brushing technique to knock down the soot layer in our Gendai™ texture. This creates a smooth, silky texture that is the most specified for exterior applications. A second pass through the brushing process accentuates the contrast between the late wood layer and the burnt ridges. We call this texture Pika Pika™.
BILL: What should the builder or general contractor know about Yakisugi?
WILLIAM: The first important thing to know is screen wall assembly — exterior cladding, air space, drainage plane and weep holes. North American best building practices have not caught up with Japan or Western Europe in terms of using a breathable wall. Some of the common practices using a resin-based vapor barrier do not permit the wall to breathe at all. This can trap moisture and even cause the siding to rot. A vented wall assembly might cost a little more to assemble, but it delivers a great, long-lasting result. The second point to know is that Yakisugi is just wood and is as easy to install as other siding. In fact, it installs 30-50 percent faster with less labor than typical tongue and groove. Part of this is the wide 6 to 8-inch plank compared to the typical 3-inch T&G.
BILL: Does the char side require any special handling to avoid scratching?
WILLIAM: You need to be careful, but no more so than with a pre-painted siding. And many GCs working on projects built on hillside lots want a pre-finished siding anyway to reduce on-site labor. And be sure not to touch your face when handling it.
BILL: How about the impact of very wet or very dry climates?
WILLIAM: Wet climates are easy as long as you have a screen wall assembly with good ventilation. Yakisugi works great in a dry climate as well, but it’s imperative to face-nail it with headed nails to avoid cupping that could result from the drying out and shrinking of the exposed char side. But if it’s face-nailed and vented, it’s safe and long lasting in an arid climate.
BILL: Tell us about the increased fire resistance enabled by the exterior char side. It sounds almost counter-intuitive.
WILLIAM: I really didn’t believe it until we sent samples to the lab for fire testing, but the improved fire resistance is dramatic. Regular cladding species have a surface flame spread rating of Class B (measuring 30-75 on the ASTM e84 test) or Class C (80 and above on the ASTM e84 test) while Yakisugi siding has a surface flame spread rating of 20-35 depending on the product. That’s because the soot layer raises the combustion temperature by about 200 degrees. Also, we suspect, but have not confirmed that the case-hardening of the surface layer prevents oxygen from penetrating as fast to feed the flame.
BILL: Three human generations?
WILLIAM: Yes, the most important thing is that we’re planning far ahead and not cutting down old growth. Unfortunately, there is no old growth left in Japan — they cut the old growth forests down hundreds of years ago, but we stopped before we cut down the last two or three percent here in the Pacific Northwest. British Columbia, Alaska and Siberia are still cutting some old growth. We need to get away from that. So if an architect specifies Clear Heart Cedar and it’s not second generation, that’s unethical, same for tropical hardwood decking. We must be considerate of what we leave for future generations.
Proper timberland management is a big driver in Japan. The science behind it and planning three generations in advance is critical. The harvest cycle needs to be long enough to generate a high grade of wood, at least 70 or so years. We use cultivars that are straight and fast growing to deliver a foot and a half or two-foot diameter logs within 80 years. There are lots of sugi sub-cultivars that have been developed over a couple hundred years and we use the ones that are most appropriate.
BILL: What is that one thing that really gets you excited about the business you are in?
WILLIAM: It’s probably creating opportunities for biophilic design, that intersection of nature and its beauty with humans. You just can’t beat natural materials. If you are building a million-dollar house, why not choose something that’s healthy and beautiful?