Often people ask us if yakisugi (shou sugi ban) really will last 80 years without maintenance. When asked where they got that number they always have read it on the internet in one place or another. But is this claim about shou sugi ban durability true?
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? Houses are rebuilt on average every 34 years in Japan and every 72 years in North America. What does it matter if shou sugi ban 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.
Installation
Let’s start with installation since this is hands-down the most important factor in shou sugi ban durability and longevity. If the siding is not installed correctly – for example not flashed correctly or installed directly over a resin vapor 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.
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. Shou sugi ban 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. This causes 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.
Shou sugi ban is rarely maintained in Japan, and the planks get progressively 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.
Re-Oiling
If the 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 it’s the correct species and is heat-treated correctly, and no re-oiling maintenance is done during that time.
Climate & Shou Sugi Ban Durability
Coastal marine and humid subtropical climates are the most challenging in terms of wood siding longevity. Coastal climates are abrasive with salty air. These wear through wood more quickly, and humid climates encourage the growth of fungi in wood that can cause rot. Shou sugi ban lasts longer than un-heat-treated wood in these applications since the surface is case-hardened for abrasion resistance. The cellulose/hemicellulose that fungi grow on is also 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 shou sugi ban is more common along the southern coast in Japan than inland or up north.
Site or Wall Orientation
Finally, the last factor that will impact shou sugi ban durability is specific site or wall orientation. I’ve seen in Japan and North America 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 it 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.
Putting it All Together
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!
fantastic analysis, thank you.