Customers often ask us how long shou sugi ban will last. Eighty years is commonly seen on the internet but I am not sure where that specific number came from. I’ve traveled throughout Japan studying traditional siding design and weathering. Shou sugi ban siding will last at least that long, longer if regularly maintained. But it’s important to define what it means for siding to last. Does it mean to keep weather out or does it mean to stay the original color? Also it’s super important to understand that design, installation, and maintenance all affect how long shou sugi ban will last.
Keeping Weather Out
Functionally, siding simply needs to keep water and sun off of the underlayment. To ensure durability, it's beneficial if it doesn't attract bugs, doesn't allow fungus growth, and is easy to maintain. Siding will end its lifespan when it splits into pieces, twists dramatically, gets bug-eaten, or rots. After reaching that point, it will no longer keep the weather out.
What I’ve seen in Japan is that shou sugi ban generally reaches its lifespan when the planks erode so thin from weather that they eventually fall apart. The gradual process takes around 100 years but this depends on conditions.
The protective soot washes off after a few decades and it’s rarely recoated with oil to slow down weathering. Sun exposure breaks down the wood fibers on the surface, and then rain and wind wash them off. The next layer of wood fibers is exposed and the cycle continues. If the building is still standing when the siding reaches its lifespan, the owners will generally replace the shou sugi ban siding.
Changing in Appearance
Ideally siding should also look good while protecting the building from weather. All wood siding including shou sugi ban will change color as it weathers and needs reoiling to stay looking the same. Shou sugi ban certainly will not stay black 100 years without maintenance as many people assume. The soot or oil will erode off eventually and expose the brown wood underneath.
Exposed wood generally turns reddish brown when facing south and gray when facing north. It changes evenly in some areas and patchily in other areas. UV radiation turns the wood reddish brown. It fades tan then gray the wetter it gets, and turns black from acidic rain or mildew. Since it is rarely re-oiled, shou sugi ban is called an organic, living, or patina siding material.
Much modern architecture assumes that wood siding should stay the same color as long as possible. Paint gives 100% UV protection so stays the same color for about 25 years before peeling off. Oils give UV protection 90-99% so the wood color underneath fades more quickly than paint. Also oils wash off gradually over a number of years including the pigment, adding to the fading effect. So oils match the shou sugi ban weathering aesthetic better than paints.
Factors That Affect How Long Shou Sugi Ban Lasts
Design and material
The most important design element in terms of how long the shou sugi ban will last is how big the roof overhangs are. If you go with a northern European roof line without eaves, then the siding will weather quickly. If your design has Craftsman-style 24” or larger overhangs, then the siding will last longer since it’s more protected from sun, wind and rain.
Installation and maintenance
This is the most important factor in how long your siding will last. Siding needs to be installed thoughtfully so it dries out quickly and stays flat on the walls.
The more maintenance you do, the longer your siding will last. "Maintenance" generally means re-coating periodically with a fresh coat of oil. It can also mean using a hammer to tighten up nails that work their way out from temperature and moisture cycles.
You can use shou sugi ban anywhere in the world. But it will weather differently depending on exact site conditions. Customers in northern regions are generally the most concerned with how well shou sugi ban will perform in their area. But in my experience the colder climates simply preserve the wood longer. The most challenging climates for wood siding are the arid desert where the dryness causes the boards to cup, and the humid southeast where humidity causes the boards to rot.
Shou sugi ban is a traditional Japanese siding where the color and texture changes are enjoyed according to the wabi-sabi aesthetic. I think measuring how long it lasts is therefore a functional exercise. So shou sugi ban will last at least 100 years.