Techniques: Shou-sugi-ban (aka "Yakisugi")

What is Shou-sugi-ban (SSB)?
An environmentally-friendly and highly effective traditional Japanese method of wood preservation by means of heating & charring.
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How is SSB (Shou-sugi-ban) applied?

For on-site applications, modern SSB is performed using a propane torch to directly heat the wood. Pre-prepared SSB is also available from some manufactures who use electric or gas-fired conveyor-belt kilns for automated production. Traditional Japanese SSB is done by arranging the boards to be treated in such a way that they can be ignited, charred to desired depth, and extinguished in a controlled fashion.

How does SSB preserve wood?

Heating (charring) the upper layers of a piece of wood changes its chemical and physical structure, making it far more resistant to sunlight, insects, and to the fungi that cause wood rot. Browned or blackened wood surfaces are converted from a cellulose/lignin matrix to one made mostly of charcoal. Unlike most natural wood surfaces, charcoal is almost impervious to the ultraviolet (UV) component of sunlight. In Japan, where naturally-decay-resistant species such as cedar are used, fully-exposed SSB siding lasts 80 years or more.

What are the environmental and maintenance benefits of SSB?

Unlike many other modern finishes (polyurethane varnishes, latex- and oil-based paints,) SSB has requires no chemicals. SSB'ed bare wood can be installed without additional coatings, though frequently linseed or other natural oils are used to enhance its appearance. Unlike paints and varnishes, SSB does not require maintenance. Optional periodic re-application of linseed oil every 5 to 7 years will help to prevent a "dry" look but it isn't necessary for the durability of the SSB treatment itself.

How well does it work?

Under moderate exposure conditions such as those found in most of northern California, SSB technique can increase the service life of exposed, plain (i.e. non-pressure-treated) softwoods like common Douglas Fir and Pine
by 300% or more.

Plain Douglas Fir wood (the most common type of lumber in Northern CA,) left exposed to sun and rain starts to turn grey within a year and begin physically breaking down (powdering and cracking) within 3 to 5 years. SSB'ed wood installed in identical structures and locations can last 15 years or more.

Is SSB technique safe to use?

Modern site-applied SSB is performed using a manually-operated hand propane torch, similar to a roofer's torch. The operator controls the torch at all times. Typically SSB is performed in the early to mid mornings when wind conditions are calm. A charged (fully pressurized) 50' garden hose with a hand-activated spray nozzle is kept at the ready to control any incidental sparks or embers.

Where appropriate, workers will pre-soak a 10 to 15 foot radius around the SSB working area to further reduce spark and ember hazards.

Site-applied SSB does generate a fair amount of short-lived wood smoke. The base wood is plain, untreated, and uncoated, so the smoke is identical to that generated by an ordinary camp fire. To mitigate smoke impacts simply close doors and windows during the SSB firing process.

How long does it take?

SSB application rates vary considerably according to wood water content and desired depth/darkness of finish. A kiln-dry board receiving a light finish takes around 2 seconds per board-foot, while a freshly-milled undried board receiving a dark "alligator char" finish might take 15 to 20 seconds per board foot.

Small projects where only a few boards are treated may need around 10 to 15 minutes of active SSB firing, while larger projects (like fences and siding for buildings) typically involve a number of hours of active firing divided up over several work days.