Yakisugi or Shou Sugi Ban is a traditional Japanese shiplap or square-milled plank wall cladding that has been charred on the outward face.  In Japan it is commonly used in combination with white stucco in various ratios for exterior wall surfaces, each region having a slightly different aesthetic.  Some regions clad the entire structure with siding, some only wainscoting under a stucco upper wall, and some regions clad up to the shadow line below the roof overhang.  It is installed vertically or horizontally, and is also often used as a cosmetic roof deck on exposed eaves and weathers beautifully as the years go on.

Japan has influenced western design in surges over the past 150 years, the yakisugi aesthetic having been brought into modern global architecture in the form of a black monolithic surface.  Yakisugi is starting to be accepted worldwide as a technically and cosmetically desirable wood cladding, but has often been interpreted as a chic, high-design building material.  In its home market however, it is simply standard, utility wood siding, affordably priced and with improved longevity over untreated wood.

There is a lot of information on the traditional manufacture of yakisugi, namely tying three planks into a triangle tube and lighting the interior on fire before opening and quenching with cold water when ready.  The planks can also be burnished with a propane torch or sent through a kiln for volume production.  The charred surface can be left as-is, or the sooty outer layer brushed off for a more finished look.

The charring is not only cosmetic in effect.  It improves the lifetime of the planks by preventing decay and rot, discourages insect infestation, improves dimensional stability, and improves flame retardant properties.  Wood is made up of fibrous lignin and metabolize-able cellulose, and heat treatment leaves the structural lignin intact while neutralizing the cellulose.  Since cellulose contains the sugars so desirable to insects, bacteria, and fungus, heat-treated wood will last longer than untreated wood.  While the evidence of this on yakisugi siding comes from case studies in Japan, there is technical information on the Thermowood website here.  Note that yakisugi is not the same as thermally modified wood from the Finnish process, though it exhibits similar characteristics due to heat-treatment.