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Shou Sugi Ban: The Ancient Art of Wood Preservation

The Wood Guide · Treatment

Treatment

Shou sugi ban

Protecting wood by burning it sounds like a contradiction. Yet in Japan there are façades treated this way that have lasted for more than a century.

  • Yakisugi is the proper name
  • The char layer protects
  • Works best on cedar
yakisugi
Charred surface of yakisugi cladding with a crocodile-skin pattern (Cryptomeria japonica)
Charred surface of yakisugi cladding with a crocodile-skin patternCryptomeria japonica
Burning boards being doused with water after charring
Burning boards being doused with water after charring
Charred boards side by side: from deeply charred to lightly burnt
Charred boards side by side: from deeply charred to lightly burnt
Summary

Why burnt wood lasts longer

The charred layer no longer contains any food for fungi and insects, and it also keeps water out.

When the wood is burnt, its outer layer turns into charcoal. Chemically, that layer is quite different from wood: the sugars and cellulose have broken down, and those are exactly what rot fungi and wood-boring insects live on. What remains is of no interest to them.

The char layer also acts as a physical barrier. It absorbs little water and protects the sound wood beneath from UV light — the two factors that make untreated cladding turn grey and crack. And, counter-intuitively, a charred layer also makes the wood less flammable, because there is nothing left to burn on the outside.

The proper Japanese name is yakisugi: sugi is the Japanese cedar the technique has traditionally been used on, and yaki means burnt. Shou sugi ban is a reading of the same characters that has caught on in the West.

Traditional versus modern

In the traditional method, three boards are tied into a triangular tube and lit from the inside; the fire draws up through the chimney this creates. Today it is usually done with a gas torch or industrially in a kiln — the result is similar, the control greater.

Name
yakisugi; shou sugi ban in the West
Works best on
cedar, and other soft softwoods
Char layer thickness
a few millimetres
Protects against
fungi, insects, UV and water
Service life
in Japan, façades lasting decades or longer
Finish
brush and oil, or leave rough
Maintenance
oil every few years, or let it weather

To be fair: charred wood rubs off. A rough, unbrushed char layer leaves black smudges on clothes, hands and anything that brushes against it — and it stays that way for years. If your cladding runs along a path or next to a door, you are better off brushing off the loose char and finishing the surface with oil. That costs some of the raw effect, but it makes the wall practical.

In order

Making shou sugi ban yourself

  1. Choose the woodSoft softwoods work best: cedar, larch, Douglas fir or spruce. The clear transition between earlywood and latewood gives the finest texture after burning.
  2. Work outdoors and safelyOn a non-combustible surface, well away from buildings and plants, with a bucket of water or a fire extinguisher within reach. Never work alone.
  3. BurningWith a gas torch, evenly along the board, until a black, cracked char layer forms. That typical alligator-skin texture is the sign that the layer is deep enough.
  4. Quench or let it die outLightly spray the board with water or let it die out under control. Then check that nothing is still glowing anywhere.
  5. Brush as you likeNot brushing gives a rough, deep-black board that rubs off. Light brushing removes the loose char. Firm brushing exposes the grain in a grey-brown relief.
  6. FinishingOil fixes the char layer and stops it rubbing off. Without oil the surface weathers faster and keeps rubbing off.
  7. Let it air outLeave the boards outside for a few days before you fit them. The smell of burning fades.
What for

Where it works

Cladding

The classic use, and where the technique comes from. Deep black, striking and very durable with good maintenance.

Garden screens and fences

The contrast with greenery works particularly well. Do brush and oil it where people brush past.

Planters and garden features

The char layer keeps moisture out on the outside. On the inside you still need a liner.

Furniture

For statement pieces and table bases. On seats and tabletops it is less practical: it rubs off.

Interior walls

Possible, provided it is firmly brushed and oiled. Otherwise the char ends up on clothes.

Not in the ground

The char layer protects the surface, but in permanent ground contact you need a class 1 species or pressure treatment.

Quick answer

Frequently asked questions about shou sugi ban

Why does burnt wood last longer?

The charred layer no longer contains sugars and cellulose, and those are exactly what rot fungi and wood-boring insects feed on. It also keeps out water and UV light.

Is charred wood more of a fire hazard?

No, if anything the opposite. There is nothing left to burn on the outside, so fire has a harder time taking hold.

Which wood should I use?

Soft softwoods: cedar is traditional, but larch, Douglas fir and spruce work too. The clear difference between earlywood and latewood gives the finest texture.

Does it rub off on clothes?

A rough, unbrushed char layer does, and for years. Brush off the loose char and finish with oil where people brush past.

Does it need maintenance?

Oiling every few years keeps the char layer intact and the look deep black. If you leave it alone, the surface gradually weathers.

Can I do it myself?

Yes, with a gas torch, outdoors, on a non-combustible surface and with something to put out a fire within reach. Never work alone, and check afterwards for any smouldering.

More in The Wood Guide

Other ways to protect wood

Thermal modification, acetylation and pressure treatment do the same job by other means.

Making wood last longer Pressure-treating wood Historical techniques

Have a question about your project? Just ask — we're happy to help.

Photos via Wikimedia Commons: Cryptomeria japonica — japanese_craft_construction (Flickr), CC BY 2.0 · Burning boards being doused with water after charring — 漱石の猫, CC BY-SA 4.0 · Charred boards side by side: from deeply charred to lightly burnt — Caroistda, CC BY-SA 4.0

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