The Science Behind Shou Sugi Ban’s Fire Resistance

Over the past ten years, yakisugi “shou sugi ban” has been rapidly gaining popularity in the West. It’s becoming known not only for its beauty, but also for its reputation of being resistant to fire. But is shou sugi ban’s fire resistance really true?

The Japanese have maintained shou sugi ban’s fire resistance for hundreds of years. They claim that houses clad with it do not catch fire as readily as non-heat-treated wood cladding. Our recent lab tests of shou sugi ban corroborate these findings for the first time in the western hemisphere. 

How exactly does shou sugi ban effectively fight fire with fire? What is the science behind cypress testing to a Flame Spread of ASTM/UL Class C, but our heat-treated cypress testing to Class A? We did some research to find the answers.

Graphs showing the typical flame spread lab test result of Nakamoto Forestry's Suyaki. These graphs show shou sugi ban's fire resistance.
Typical flame spread laboratory test result of Nakamoto Forestry’s Suyaki™

What Causes Wood To Combust?

The two main components in softwood lumber are carbohydrates (like cellulose and hemicellulose) and lignin. Carbohydrates make up the vast majority, at around 65-90%, while lignin makes up the other 10-35%. A linked chain of glucose molecules or sugar makes cellulose. Hemicellulose is also made of sugar, but in addition to glucose, it’s made with a range of different polysaccharides. Lignin doesn’t contain any sugars and is the main structural component of wood. The idea behind heat treating wood to increase longevity is that the treatment burns off carbohydrates in the wood. This leaves behind only the structural component lignin. Without the carbohydrates, fungi and wood-eating insects are unable to survive, which increases the wood’s lifespan.

In addition to being insect and microbe food, those carbohydrates are also fuel for fire to consume. In order for wood to ignite, its temperature must be high enough that pyrolysis takes place and the chemical reactions of combustion start. Ignitability of wood is dependent on thermal properties, moisture content, and the way you apply heat to the material.

It’s general knowledge that wet wood is more difficult to ignite than dry wood. This is a factor that affects the ignition of wood. Furthermore, thin kindling ignites more easily than thick logs, and softwoods ignite at a lower temperature than hardwoods. Less common knowledge is that wood burns in stages. Cellulose is the first component of wood to ignite. It burns away quickly, leaving mostly lignin and other sugars behind, which become charcoal. Charcoal is the last component of wood to burn, as it requires higher temperatures than cellulose to ignite. This is the key to shou sugi ban’s fire resistance: the cellulose has already been burned away. This leaves a surface that requires much more extreme heat than non-heat-treated cypress in order to ignite.

When wood burns, flames spread across the surface, and if the heat released is high enough, it ignites more material. The heat released by a burning area has an effect on flame spread rate. This directly raises the temperature of the surrounding material. The factors governing the rate of heat release are essential for the flame spread.

Fire retardants used on wood generally have a borate base. One applies the fire retardant to the wood in a solution with a water carrier for penetration. Borate embedded in the wood swells dramatically from heat during a fire. This expansion of the wood constricts the flow of oxygen. This prevents flame spread and is the mechanism behind most lumber fire retardants. One theory is that case hardening from the Japanese heat treatment also constricts the flow of oxygen into the wood during combustion, slowing down flame spread. While we are not sure of this, it might also be a factor.

Testing the fire resistance of our shou sugi ban charred wood before sending off to the lab.
Testing the flammability of our shou sugi ban charred wood before sending off to the lab.

Trial By Fire

Last summer we experienced the flame retardancy of our product when the Tubbs Fire burned around a house that was clad with our siding. The fire came right up the side of the house but did not catch fire. The homeowner believes that the heat-treated wood prevented their house from igniting and burning to the ground. The only noticeable damage was the resin we use on the backsides of the planks melted and bled down the face of the exposed wall. Cypress characteristically has some loose knots as a species as opposed to cedar which has tight knots. This is due to how slowly or quickly the lower branches die and rot as the trees mature.

Unfortunately a lot of recent demand for our siding comes from projects within the wildland urban interface zones in California due to concern about increased wildfire exposure. It has been interesting to get laboratory proof of shou sugi ban’s fire resistance.


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