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Wood Longevity Vs. Color Longevity With Shou Sugi Ban

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 with an almost universal request to guide them into a siding material that will have as little future maintenance as possible while keeping the same color as long as possible.  Our customers are focused on color consistency over time, especially after disappointing past experiences with cedar that has silvered out, redwood that paint did not adhere to, expensive tropical hardwoods that went from rich reds or browns to blah gray within a year, or traditional oil finishes that require laborious yearly re-oiling to keep the wood color fresh and vibrant.  They 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.

Unoiled Gendai a couple of weeks after installation. It has already bleached out from initial UV and dew exposure.
Here is the same unoiled Gendai-clad house after about 8 months of weathering. I drove by the house again last week and it has bleached out quite a bit more after over one year. This is my favorite yakisugi.
The reverse board and batten wall is custom shou sugi ban that has been re-oiled within the last two decades. The post to the right is oiled red indicating located in the Kansai region, and it is an art gallery so in a historical neighborhood (I’m not sure of location since someone else took this pic). The lighter wood at the bottom is not heat-treated. It is simply a lathe ko-ushi to hide mechanical equipment such as AC condensor or natural gas meter between the structure and curb.
Knots are more dense so do not burn as deeply as the field. Therefore they change color more quickly, and in this case after several years in northern orientation are starting to silver out.

Custom Homes vs Tract Homes

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.  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.

People building custom homes are also spending a lot of money and often have sophisticated design taste.  They also have longer-term considerations than with tract homes, since statistically they will live with their choices for longer.  They don’t want a maintenance chain around their necks for the foreseeable future. So people having custom homes built do research online, consult with their designer, and after a little research or a tipoff they find out about shou sugi ban and get excited.  And then they find us and get really excited.

Fabulous Vancouver custom home with shou sugi ban Pika-Pika gable facade.

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 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.  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 T&G 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.

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, and from what I’ve seen in Japan, when made correctly will hold the consistently black soot layer for 40~50 years.  But Suyaki is a very exotic and foreign material, easily blemished, and 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 from color longevity.

Fresh Suyaki right side, 10~20 year old on the left side. Note the stained thick-stock boards on the fence and the burned thin-stock Suyaki. We can only heat treat thin stock by the Japanese method per traditional millwork parameters. Trying to burn thick stock material causes it to move too much if enough heat is applied and does not heat treat if not enough heat is applied.
There is nothing more fabulous than fresh Suyaki. Buildings like this in historical neighborhoods are reclad with shou sugi ban every 100 or so years instead of being re-oiled.
This Suyaki B&B wall is likely 10~20 years old since the soot is already starting to erode. Note how the exposed wood is turning silver indicating north elevation or shade from an adjacent structure.
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. The batts were burned separately from the boards and not as deeply, therefore the soot is eroding off them faster than the boards.
This is what Suyaki looks like after 60~100 years without having been oiled or re-oiled ever. Pretty ugly building so please don’t judge Suyaki on this one!
This is what unoiled Suyaki with southern exposure, minimal overhang, and lots of adjacent landscaping abrasion looks like after 100+ years.

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.  Additionally, 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.  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 oughts, 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.

Most likely the original structure to the right side was unoiled Gendai, and is around 70 years old. The left side addition looks to be Gendai with black oil and is much newer, maybe less than 15 years old. It is protected from UV so is difficult to age accurately.
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. Any UV load would come from cloud diffusion.

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 vapour 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.

Another factor in wood longevity is UV protection because correctly manufactured and installed shou sugi ban will “wear out” over 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 mudset or insulation or WRB). All oils offer at least 95% UV protection simply from dried oil solids residue on the wood surface, and 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.

My guess is that this was originally Gendai (brushed) material, and based on construction details the structure is maybe 80~120 years old. The insulation is mud & straw with bamboo lathe, and power is all knob and tube. This was a budget build therefore a lot of bond lines and low grade wood. Elevation is probably NE or NW since it has gray and brown patina.
I had to think about this for a while before figuring out what is going on with the weathering pattern. I think that the upper section is original Pika-Pika 60+ years old, maybe more. The bottom section is also Pika-Pika but is much younger and lighter. I think the upper section was not graded for color and the bottom section was graded for sapwood only (we call it “white”). The bottom section is maybe 5 years old and must have replaced a worn-out lower section. It is on a busy pedestrian road so must have gotten beat up over the decades. The wall plane to the left must be facing east and wall plane to the right must be facing north judging by the color. Not fabulous but minimal maintenance every 2 generations isn’t bad either.
This is what happens with low-grade yakisugi never oiled for 100+ years in southern exposure. Great example of the longevity limitation of wood siding without maintenance. Check out them taters for cost performance and carbon footprint over the long term.

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.

Not sure why there are two types of siding on this house, but both look to be 10~20 years old. Left side is Gendai with black oil and right side is Pika unoiled. The Pika has consistent sapwood without any heartwood. We call this grading “white”.
Gendai with black oil, about 20 years old. Note the weathering difference between the exposed 2nd floor plane above and the shaded 1st floor plane below.
This house is an interesting combination of select grade Gendai oiled and probably re-oiled black, together with an almost clear grade Pika-Pika soffit. The Pika-Pika does have some knots, but was never oiled. My guess is that it is located near water due to discoloration in the soffit. Soffit generally does not change color at all since it gets almost no UV or moisture. Sashes look to be maybe 20~40 years old so that is how I would age this structure.

The Takeaway

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.  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.

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.

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