Yakisugi (Shou Sugi Ban) 80-Year Siding Longevity: Truth or Rubbish?

Often people ask us if yakisugi (shou sugi ban) really will last 80 years without maintenance.  When we ask them where they got that number they always have read it on the internet one place or another.  But is it true?  I’ve travelled a lot all over Japan paying attention to how siding is used and how it weathers over time.  I also used to be a remodel carpenter so can age buildings with some accuracy.  Time to clarify this 80-year longevity piece of trivia since it is a nuanced and important topic.

First of all it is difficult to give a blanket longevity statistic for any kind of building material, especially siding which is affected so dramatically by install spec, maintenance, geographic location, and each specific wall’s orientation.  Also, how do you define siding longevity, especially when rot is not a factor? (Correctly installed siding doesn’t rot.) At what point does siding need replacement?  When houses are rebuilt on average every 34 years in Japan and every 72 years in North America, what does it matter if yakisugi lasts 80 years or 120 years without maintenance? Note we are discussing wood longevity in this post, and it is important to differentiate between wood and color longevity.  Color is more dependent on oil stain maintenance–check back for a future post on that.

Let’s start with installation since this is hands-down the most important factor in siding longevity.  If siding is not installed correctly–for example not flashed correctly or installed directly over a resin vapour barrier–then it will start to rot immediately.  Whether it’s cement board, cedar, or yakisugi cypress, if the siding is not allowed to dry out quickly then it is going to rot. Cement board that stays wet will start to delaminate after about 6 months.  Any kind of softwood will start to rot within a couple of years if it stays wet.  So the better the installation, the longer the siding will last.  This is not the longevity limiting factor that I’ve seen in Japan, except on structures where the bottom of the wall is in contact with a stone or cement foundation.  Wood wicks water from masonry and direct contact will cause wood to rot.

Age estimate 40~80 years old; note rot from direct masonry contact, also color difference between high-UV load center of wall and silvering at the bottom from moisture. This is most likely Nakamoto wood. Also note the flat-sawn resaw pattern and the boards are upside-down (see my other post about this here.

Then maintenance.  Wood is degraded over time from UV radiation, the freeze-thaw cycle, abrasive coastal weather, and if it’s not allowed to dry out quickly enough.  Oil stains or paint slow down the weathering process, so regular re-oiling will slow down wood degradation over time.  Yakisugi is never painted, whether due to tradition, the species of wood (tannin-rich porous softwood), or because it is more beautiful stained than painted.  (Wood is categorized as either stain grade or paint grade depending on quality, application, and species.)  Oil stains are hydrophobic causing water to bead and roll off, and pigment in the oil acts as a UV blocker.  UV radiation breaks down the wood fibers and then wind and rain wash the degraded wood off of the surface.

Yakisugi is rarely maintained in Japan, and what I’ve seen is that 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.  Once the boards get too thin, the planks start to split.  This takes 80~150 years depending on plank thickness and wall orientation.  Siding in Japan is standard 10~15mm thick, and a 10mm plank installed facing south or southwest orientation will wear through and start to split, exposing the substrate, in about 80 years.  So what I’ve seen in Japan is that UV degradation is the defining factor in yakisugi longevity since they traditionally rarely do re-oiling maintenance.

historic yakisugi example 100-150yrs
Age estimate 100~150 years; note wood thickness erosion and checking, masonry contact, also 30~40 year old Suyaki patches
historic yakisugi
Age estimate 30~60 years; note quarter-sawing, uncoated traditional cut nails, ferrous streaking, nails backing out, wood erosion making the nails proud

If siding is re-oiled periodically as is standard in North America and Europe, it will simply last longer due to better UV protection.  If reoiled every 5 or 10 years then it should last the lifetime of the structure.  This is where the 80-year guidance breaks down.  Also note that our Suyaki has a soot layer that is hydrophobic, blocks UV, and is aseptic.  This soot layer seems to last about 40 years before eroding off if the correct species and it is heat treated correctly, and no re-oiling maintenance is done during that time.

Age estimate 80~120 years; note intact soot layer directly under the water table (between stucco above and wooden wainscotting), ferrous streaking, 4 nails per board, Suyaki erodes to a Gendai appearance over time, wall is likely north-facing, knots indicate rootstock down and crown up, flatsawn from young trees

In terms of geographic location, the most frequent pre-purchase customer concern we get is from the arid, mountainous southwest, and also from the frigid north-central regions of North America Wisconsin/Minnesota region and east.  For whatever reason we get less questions about climate suitability of our product from the western and eastern seaboards than from inland.  However, in our experience, coastal marine and humid subtropical climates are the most challenging in terms of wood siding longevity.  Coastal climates are abrasive with salty air so wear through wood more quickly, and humid climates encourage the growth of fungi in the wood that can cause rot.  Yakisugi lasts longer than un-heat-treated wood in these applications since the surface is case-hardened for abrasion-resistance, and also the cellulose/hemicellulose that fungi grow on is burned off.  Arid or cold climates cause wood to dimensionally move (cupping especially) and change color, but they do not cause as much abrasion or rot as coastal or humid climates.  This is why yakisugi is more common along the southern coast in Japan than inland or up north.

Finally there is specific site or wall orientation.  What I’ve seen in Japan and North America is that wood will weather differently depending on which direction the wall is facing.  In general east and north facing walls will evenly bleach out until they are silver in color, 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 so will remain the original color.  The center of the wall will turn yellow-orange wood color from high UV and since it dries out quickly after rain.  Then the bottom of the wall will silver out similar to the east and north elevations since it remains wet longer.  Keep in mind this only happens if the siding is not re-oiled.

historic yakisugi
Age estimate 5~15 years; note silvering left side from daily exposure to an adjacent river’s morning dew, right side warm wood color from high UV load

In conclusion, here is where the 80-year guidance comes from: Japanese standard 10mm thick planks that started out as our Gendai or Pika-Pika surface (ie brushed), were installed correctly, were never oiled or re-oiled, were installed on a south or west-facing orientation, and near the coast. Geographic location and wall orientation can’t really be controlled to increase siding longevity.  However, if the siding starts out thicker (our standard is 15mm since yield drops off for any thicker than that), is installed correctly (rain screen & headed ring-shank nails), is finished with an oil stain, and re-oiled periodically, it will last even longer.  How long?  Longer than you need to worry about!

historic shou sugi ban
Age estimate 80~120 years; note short, thin, low-grade planks indicating low budget, square butt joints, also whitewashed mud wall with straw reinforcement and bamboo lathe
historic yakisugi
Age estimate 80~120 years; note overlap bond line between upper and lower stories, cupping and missing board due to using headless nails, traditional mud wall and horizontal nailers between posts

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