Wood is a desirable building product for many reasons including aesthetic appeal, sustainability, and being economical. While many different approaches have been used to improve its durability and longevity, particularly in exterior applications, the Japanese process of charring the surface of the wood has gained popularity in the United States in recent years. Referred to as yakisugi in Japan and often as “shou sugi ban” in the West, the material is being specified and used in residential and commercial designs due to its enhanced durability and the aesthetic appeal of the charred wood. This course provides an in-depth exploration of authentic yakisugi/ shou sugi ban and points out the differences with some similar treatment processes which can vary in their performance.

The content covers the history, science, and modern architectural applications of this sustainable material, emphasizing its environmental benefits and ability to protect wood from fire, moisture, and pests. By examining some example projects, readers can gain practical knowledge on incorporating charred wood into modern designs, balancing tradition with contemporary sustainability practices.

OVERVIEW OF YAKISUGI/ SHOU SUGI BAN

The term “shou sugi ban” is used in the Western world (North America, Europe, etc.) but is unknown in Japan. It is believed that the term was created by an American who misread the original Japanese characters related to a difference in the Chinese and Japanese languages. The original Japanese term is “yakisugi,” with “yaki” meaning burnt, charred, or heat treated, and “sugi” referring to Japanese cedar that is indigenous to the country. When speaking specifically of boards made from this charred or burnt species, the term “yakisugiita” is used. In order to be clear and accurate, in this course we use the original Japanese word of yakisugi to refer to the wood products being discussed, even though many may understand it to be what they have been colloquially calling shou sugi ban.

It is important to note at the outset that yakisugi is not a treatment process, rather it is a total product that yields long-term performance benefits and a range of aesthetics. It begins with selecting the proper species and grades of wood and cutting and milling it to the correct thickness. It is then subjected to burning or charring the wood in a controlled manner and allowing it to dry to the desired moisture content. It may also include some further finishing either at the time of manufacture or in the field. The combination of selecting the right wood and subjecting it to carefully controlled steps is what ensures a quality yakisugi product is developed suitable for use in buildings.

Historical Context

Yakisugi is believed to have originated in Japan several centuries ago, although the date of its first appearance has yet to be confirmed. Nonetheless, it has been successfully used to fire-harden homes constructed of wood in densely populated Japanese cities and urban areas that frequently encountered catastrophic structural fires. While the original users may not have fully understood the science, it is now known that during the heat treatment, hemicellulose, which is essentially a carbohydrate, is burned off. What’s left behind is the lignin, which is the structural component of wood. The end product is a hardened wood plank that is resistant to decay, insects, and weathering. Additionally, the soot layer increases the temperature threshold needed for combustion which dramatically reduces flame spread. Historically, yakisugi was made directly at the jobsite by the carpentry crew. However, the process has been refined after transitioning to manufacturing in lumber mills. This transition has led to more consistent increased longevity and dimensional stability for both interior and exterior use.

In the United States, heat treating, or charring wood has been studied since at least 1927 as a method of wood protection. The charring process, for example, has been used commonly on the bottoms of wooden fenceposts to prolong their surface life. Interest in using yakisugi in the United States and Europe for other purposes has grown in the past two decades. For some architects, the appeal has been purely aesthetic as a way to add bold coloring to a facade. For others, the performance and longevity of the product caught their attention. For those interested in sustainability, the fact that it is made out of wood with a very favorable carbon reduction profile brought them to look closely at it. Others are attracted to the cultural and historical understanding including the dramatic transformation of the wood by fire.

Currently, yakisugi is used in the U.S. predominantly for building exteriors in residential and low-rise commercial projects. Most commonly, it is used as exterior cladding on vertical surfaces such as walls, fences, and similar applications. It has also been used for soffits and similar horizontal surfaces which may be less exposed to the elements. In building interiors, its use includes both vertical wall surfaces and horizontal ceiling surfaces. Note that the Japanese heat treatment process is never used for flooring or furniture. Japanese cedar is too soft for flooring, and yakisugi planks are too thin for most furniture applications. The burning of flooring, furniture, or counters has been done for artistic purposes, but it is not the Japanese yakisugi heat treatment.

Wood Species and Treatment

The wood that is used for Japanese yakisugi is “Cryptomeria japonica,” which is commonly known as Japanese cedar or sugi cedar. These trees grow abundantly throughout Japan with planted forests in the southern part of Hokkaido and in Okinawa in the northern part of the main island. Sugi cedar has several characteristics, making it particularly suitable for this product while other species tend to fall short. In particular, sugi cedar is well-suited for the intense heat mineral-rich fiber. Species such as pines, larches, or firs have more pitch and less minerals, so they have a greater tendency to check when charred and are not as durable. Other species of cypress, cedars, or red- woods may have good durability but when subjected to the charring process, they do not retain the beneficial soot layer as long as the sugi cedar does.

Japanese or sugi cedar has a number of additional desirable characteristics, particularly for exterior siding products. It is straight-grained, fast-drying, flexible, tannin-rich, and strong. It features a thick, dense latewood growth ring, which burns to a more substantial, longer lasting soot layer. Compared to other species, its chemical properties respond well to fire, and it becomes incredibly dimensionally stable when milled, dried, burned, and quenched by traditional protocols.

It is important to be clear on the type of wood being used by a manufacturer of yakisugi since different manufacturers may use different types of wood that are less suitable than sugi cedar and can produce disappointing results when used in a project. It is also important to be aware that there are “yakisugi-inspired” products that try to be imitations but don’t have the same aesthetics or 100-plus years of demonstrated durability. A different process known as aburi is a surface burning treatment done using a torch to many species and used on furniture or beams for a darker appearance. Thermally modified wood is a separate low temperature process developed in Finland that penetrates more deeply into the wood and can be used on thick stock lumber for improved durability in exterior applications. Carbonization by the Chinese process is a high-temperature, hardening process used mainly for flooring. There is also chemically treated wood which relies on the chemicals used on various wood species to create its appearance which may be similar to yakisugi at first but will weather very differently over time.

Whenever assessing available heat-treated wood products, the specifics of wood species and the details of how it is manufactured are critical for understanding the true perfor- mance and design characteristics.