Shou Sugi Ban Wood Longevity Vs. Color Longevity

This is a long and technical one because this subject is important, complex, nuanced and fun.

Owners Want Low Maintenance and Beauty

Architects and owners come to us for siding that is low maintenance, beautiful, and has color longevity. This is usually because they have experienced the disappointment of:

Unoiled Gendai a couple of weeks after installation.
  • Cedar that has silvered out
  • Redwood to which paint did not adhere
  • Expensive tropical hardwoods that went from rich reds or browns to gray within a year
  • Traditional oil finishes that require laborious annual re-oiling to keep the wood color fresh and vibrant 

Our customers want the beauty and wholesomeness of natural wood material, but without the maintenance of standard commodity lumber. There is also the background of a painting culture in North America and Europe, so good color longevity and the need for at least some repainting maintenance in the future are assumptions in the West. Shou sugi ban however is a little different from commodity lumber.

Custom Home Siding vs Tract Home Siding

Here is the same unoiled Gendai-clad house after about 8 months of weathering.

With this background, and due to low material cost, great paint adhesion, insurance discounts, and frankly, simply intense marketing in parallel to supplier consolidation, the default siding solution today is cement board. 

Tract Homes Use Cement Board

For tract homes, cement board is the best solution (not taking cosmetics or carbon footprint into consideration), but for custom homes people want beauty and wholesomeness for such an important line item on their build.

Custom Homeowners Want Shou Sugi Ban

People building custom homes invest a lot of money, often have sophisticated design taste and longer-term considerations than with tract homes. Statistically they will live with their choices for longer. They don’t want a maintenance chain around their necks for the foreseeable future.

With so much investment, people building custom homes do more research, consult with their designer, and eventually learn about shou sugi ban and get excited. And then they find us and get really excited.

Shou Sugi Ban Color Longevity

And then we have to burst their bubble and serve a dose of reality, since the heat treatment will improve wood longevity and not necessarily color longevity. To be fair it is also an opportunity to explain what exactly no maintenance siding entails, and to tell a story of intertwined traditional culture and technology development. 

The Loss of Shou Sugi Ban

The tradition is there for a reason, and to a large extent has been lost post-war. Traditional millwork has been developed over hundreds of years by many generations of incremental improvement. Roofing and siding product technology has not really improved at all post industrial revolution, though production precision and efficiency has jumped to light speed.

Despite compelling sales pitches developed in a context of corporate economies of scale, most modern engineered products are completely inferior to what was available pre-WWII in terms of design and quality. (If you read my other blog posts or call me up you’ll see that I touch on this subject a lot since it is an ethical issue I’m concerned with. 

Shou Sugi Ban is Sustainable

Natural materials used correctly encourage sustainability in terms of both human health and ecology–not to mention surrounding ourselves with beauty. As an example the post-war trend of tongue & groove being specified on exterior applications is almost as egregious as cement board siding.) 

Modern oil finishes available today, however, have been developed to improve color longevity and color variety over what was available in the past. Still, wood color will change and it must be re-oiled to stay fresh.

Fresh Suyaki right side, 10-20 years old on the left side.

Shou Sugi Ban Durability

Shou sugi ban is heat-treated wood in which the hemi-cellulose has been burned off, neutralizing food content for fungi to metabolize and therefore exhibiting improved wood longevity. True, Suyaki has a 100% UV-blocking and hydrophobic black soot layer. From what I’ve seen in Japan, when made correctly, it will hold the consistently black soot layer for 40-50 years. But Suyaki is a very exotic and foreign material, easily blemished. Additionally, it can be more expensive to install since western carpenters don’t know what to think of it.

Therefore unless a customer wants the flamboyant appearance of Suyaki, they request one of our brushed products. However, once the soot layer is brushed off, it will change color from weathering just like any other wood siding. One of the most important subjects we bring to the attention of our customers at first point of contact is that wood longevity is a different subject than color longevity.

This Suyaki must be at least 20 years old and was installed by hand. The hammer face half-moons don’t show until decades after install.

Background on Oil Finish Use

Oil finishes are applied to slow down the color weathering process in order to keep the original color for longer. Up until about 20 years ago 75% of shou sugi ban installations in Japan were unoiled, and only a quarter prefinished with an oil stain. Fast forward to today and it is the converse, with 75% of projects factory-finished with an oil stain and only a quarter left unoiled. 

There have been dramatic changes in oil formulations over this time period, as traditional monomer oils have been replaced with high urethane content formulations, solvent carriers replaced with water carriers, and various longevity enhancers such as silicone doping have come into use. 

Popular Shou Sugi Ban Pigments

Also, different colored pigments have gone in and out of fashion, such as lots of red pigments being used in the 1990’s, black pigments in the aughts, and brown pigments being popular today. Plus though gray pigments are one of our best sellers here, they have never been used in Japan and we simply do not know how gray shou sugi ban will weather over time.

What this means is that the market has changed so much since prefinishes came into common use that we do not have a good volume of photos of weathered projects that originally had an oil finish. Almost all of the historical photos in our reference photo gallery are of old projects that have never had oil applied, and therefore have dramatically different appearance than the projects we are delivering today will have after decades of weathering. Customers often request weathering reference photos from us but these are not so easy to come up with for the above reasons.

This is a great photo to show how traditional clear oil allows wood to change color. The left side was installed one year before the right side. It is a carpenter’s home so will never be finished. This is a north-facing elevation without roof overhang, so the color change is primarily from moisture.

Shou Sugi Ban Wood Longevity Constraints

Note that whether wood is heat treated shou sugi ban or regular siding, installation over a screen wall substrate is the most important determining factor in structural longevity. This is because without an air gap modern vapor barriers in direct contact with the siding will actually cause the wood to rot from the backside due to moisture build-up and slow dry time. A vented air gap will allow the siding to dry out, and the more quickly the wood dries out the longer it will last due to lower propensity for fungal growth and resultant wood rot.

How Long Does Shou Sugi Ban Last?

Another factor in wood longevity is UV protection because correctly manufactured and installed shou sugi ban will “wear out” between 80-150 years due to surface wood fiber UV degradation and plank thickness erosion. 

The planks get thinner and thinner as they erode until eventually they split out and allow water and sun into the more sensitive substrate (either mud set, insulation or water resistant barrier). All oils offer at least 95% UV protection simply from dried oil solids residue on the wood surface. Exterior grade oil finishes also have visible and invisible pigments that bring this up to near 100% UV protection. This of course results in delayed weathering of the wood fibers, and therefore if the siding is re-oiled periodically it will last much longer than unoiled or un-maintained siding.

Additionally, re-oiling the wood means that the oil-bound pigments are reapplied periodically, and the color is freshened up at that time. So re-oiling is necessary with any kind of wood siding if the owner wants the walls to stay fresh in color.

The upper section is likely Pika-Pika and 60+ years old. Bottom section is also Pika-Pika but much younger and lighter.
Both of these walls look to be 10-20 years old. Left side is black Gendai and right side is unoiled Pika-Pika.

More on Oil Finishes

Another important discussion is the different types of oil formulations commonly used and how they weather over time. For example traditional oils generally derived from tung or linseed oils will have medium-length color longevity, and modern alkyds with modified safflower and soybean oil bases will often have better longevity. This is complicated by improved color longevity from urethane or other polymer additives or molecular modifications, or degraded color longevity due to mineral oil content added to improve penetration and make application easier. We spend a lot of time educating our customers on finishes since they ask a lot of questions. However, our responses always center around guiding them into whatever sample swatch or product in photos they personally like the best. All of the finishes we use have improved color longevity over what was available 20 years ago and definitely over unoiled wood.

Gendai with black oil, about 20 years old. Note the weathering difference between the two different levels.
This house is an interesting combination of Gendai, together with Pika-Pika soffit. My guess is that it is located near water due to discoloration in the soffit. Soffit generally does not change color.

Shou Sugi Ban Color Refinishing is Optional

The takeaway is that unless Suyaki is chosen, shou sugi ban will weather in color within around 10 years. The owner needs to decide whether to re-oil periodically or to accept the traditional Japanese patina aesthetic of a no-maintenance siding. Therefore, shou sugi ban is a “maintenance-optional” siding. 

The kura left side has Suyaki and the garage right side is clad with Gendai, both unoiled and around 20 years old. Note the rule in Japan is for stucco to be above wood, whereas in the West wood is generally above stucco as standard.

Color longevity is generally dictated by oil finish maintenance, and this also affects wood integrity longevity over the long term. Though the two subjects are interrelated, customers need to understand that Japanese siding heat treatment improves wood longevity and not necessarily color longevity. Also keep an open mind. Unoiled shou sugi ban will weather into an opulent, textured, varied, organic, rich patina. After traveling to many historical places in Japan researching siding, unoiled wood is hands-down my favorite.


Does Shou Sugi Ban Rub Off?

Yes, shou sugi ban will rub off on your hands if you touch it. That is the natural soot layer.

Do You Have To Seal Wood After Shou Sugi Ban?

No, you do not ‘have to’ seal wood after shou sugi ban. Traditionally, shou sugi ban was not oiled in Japan. It is a rare practice now, but can leave a beautiful natural coloring. To maintain the shou sugi ban color you want, you should oil the wood.

Does Shou Sugi Ban Make Wood Waterproof?

Yes, to an extent, shou sugi ban makes wood waterproof.


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