We’re often asked by clients if charred wood will really last 80 years without maintenance. In other words, is yakisugi a ‘lifetime siding’? This 80-year span seems to be a common number people have read on the internet… But is it true?
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The answer is nuanced.

Assigning categorical longevity duration to any building material is a difficult proposition — especially wood cladding, as it is affected greatly by many variables: technical installation, maintenance, geographic location, and the specific orientation of the structure’s walls. Also, how one defines siding longevity is critical, especially considering rot is not a factor, as properly installed siding does not rot. At what point does siding need replacement? When houses in North American are rebuilt on average every 72 years, what does it matter if yakisugi lasts 80 years or 100 years without maintenance? Bear in mind we are discussing wood longevity and it is important to differentiate between wood longevity and colour/finish longevity. Colour/finish is more dependent on oil/stain maintenance.
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Proper installation is unquestionably the most important factor in siding longevity. If siding is not installed properly — that is, not flashed correctly or installed on a strapped/furring/stripped wall with a requisite air gap or rain screen — then it will start to rot quickly. Regardless if it’s cement board, cedar, or charred wood… If the cladding is not allowed to dry quickly, rot will set in. Even cement board that stays wet will start to delaminate after about 6-12 months. All softwoods will start to rot within a couple of years if they stay wet. Quite simply, the better the installation, the greater the longevity of the siding. And this axiom applies to charred wood siding.

Then there are maintenance considerations. Wood degrades over time from the sun’s UV radiation, the freeze/thaw cycle of the seasons, harsh coastal weather, and if it’s not allowed to dry out quickly enough. Oil-based stains and paints slow down the weathering process, so periodic re-oiling will slow down wood degradation over time. Traditionally, yakisugi is never painted due to the tannin-rich porous softwood (or because it is more beautiful stained than painted). Oil stains are hydrophobic and shed water off the surface, while the pigment in the oil acts as a UV blocker (Blackwood uses PPG and Sansin oils). UV radiation will ultimately break down the wood fibres, and in conjunction with wind and rain the degraded wood will be washed off of the board’s surface.
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In Japan, yakisugi is rarely maintained, and the yakisugi planks get thinner and thinner over the decades from UV degradation. Nails will get more proud of the surface as the wood surface erodes, and as the boards get too thin, the cladding starts to split. This process takes 80-150 years depending on initial board thickness and the wall’s orientation to the sun and elements etc. Boards installed facing south or southwest orientation will wear through and start to split, exposing the substrate, in approximately 80 years. Considering that Japanese tradition rarely employs maintenance re-oiling of the charred siding, it’s safe to say that UV degradation is the defining factor in yakisugi longevity.
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Age estimate of un-oiled yakisugi: 100-150 years. Of interest: wood thickness, erosion and checking, masonry contact at bottom of boards which caused moisture to wick into wood, also 30- to 40-year old un-oiled charred boards.  
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If siding is re-oiled periodically as we recommend and as is now the standard procedure in North America and Europe, it will simply last longer because of superior UV protection. If re-oiled every 5 or 10 years, the charred siding should last the lifetime of the structure and beyond. This is where the 80-year guidance breaks down. The char layer comprised of carbon/soot is hydrophobic, blocks UV, and is aseptic. From research of structures in Japan, this char layer seems to last approximately 40 years before eroding off (if the correct species is charred and it is charred deeply enough) and no maintenance re-oiling has occurred in that time.
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Approximate age estimate 80-120 years of un-oiled and non-maintained yakisugi. The char layer directly under the soffit remains intact even after the decades. If maintenance re-oiling of the charred siding isn’t performed, this is the general appearance one could expect. The siding’s char layer slowly erodes to ‘brushed’ appearance with the wood grain exposed. This is why it is critical to re-oil the charred siding periodically to re-fortify the char layer which increases its stability, longevity and maintain its overall aesthetic appearance.  
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Coastal marine and humid subtropical climates are the most challenging environs for wood siding and impact its longevity the most (as opposed to drier and colder environments). Coastal regions are harsh due to the salty air, and this will wear through wood quicker than non-marine environments. Humid climates encourage the growth of fungi in the wood (which can lead to rot). Charred wood will last longer than un-heat-treated wood in these applications since the surface is case-hardened for abrasion resistance as well as the cellulose/hemicellulose that fungi grow on is burned off in the charring process. Arid or cold climates can cause wood to dimensionally move (cupping especially) and change colour, but they do not cause as much abrasion or rot as coastal or humid climates. Proper fastening (i.e. face nailing) is critical to reduce the occurrence of this movement. Charred wood is well-suited for the four season/lakeside locations such as those found throughout Canada’s cottage country.  
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The final factor affecting wood’s longevity is the specific site or wall orientation of the structure. In general, east and north facing walls will evenly bleach out until they are silver in colour, then they will remain an even silver. South and west facing orientation will weather unevenly. The top of the wall is protected from sun and rain by the overhang, and will usually remain the original colour. The centre of the wall will turn a yellow-orange wood colour from high UV since it dries out quickly after rain. The bottom of the wall will silver out similar to the east and north elevations since it remains wet longer. Please bear in mind this only happens on brushed siding or raw wood siding and if the siding is not re-oiled with periodic maintenance.
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Approximate age estimate 5-15 years; note the difference in patina from one wall to the other (silvering/greying left side as compared to right side’s warm wood colour from high UV load).
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There are some considerations and caveats when thinking about charred wood as a cladding for your home, particularly traditional yakisugi where the char layer is not brushed off the planks. Even with proper charring ie. deep, intense heat treating, performed on the suitable substrate ie. cedar, finished and sealed with the correct penetrating UV oil stain, and installed using best practices by skilled contractors, the soot layer can chip and/or flake off from daily wear and tear etc. A deep and stable char layer is still only several millimetres thick. This occurrence should be minimal yet is normal. Pre-finishing the yakisugi with UV oil seals the soot and fortifies the char layer protecting against incidental wear, and periodic maintenance re-oiling re-fortifies the char layer for overall stability and longevity of the material. Incidental chips in the char layer maybe touched up with UV oil stain to minimize their appearance or left as part of the overall organic patina of the charred wood.
 
Yakisugi is a beautifully dynamic material unlike any other and it is the exact reason why so many clients are drawn to it. It’s beauty is born of fire and truly has a magical allure which is undeniable to those who experience it. But it is not immutable and this aspect must be taken into account when considering yakisugi as your cladding and in theory, embraced as part of its charm. Nonetheless, charred wood does have tremendous longevity.  
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The 80-year number one so often hears about regarding charred wood’s longevity would seem to be true for traditional yakisugi in Japan that was installed correctly, was never originally prefinished with oil/maintenance re-oiled, was installed on a south or west-facing orientation, and/or near the sea will last at least this long. When one considers modern yakisugi however, where the siding starts out thicker at 3/4”, is installed correctly over a rain screen and/or a strapped wall that allows the wood to breath and dry out, fastened properly with stainless steel, headed ring-shank nails or screws, is pre-finished with a penetrating UV oil stain, and is periodically maintained via re-oiling, it will last even longer.  
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A lifetime and beyond where future generations will enjoy its unique beauty. How long exactly? Sadly, we’ll be long gone before we ever know!