Black locust is naturally rot-resistant due to its high density and the presence of specific compounds like robinetin, a flavonoid that repels insects and prevents decay-causing fungi. This natural resilience allows black locust wood to withstand harsh outdoor conditions without the need for chemical treatments or thermal modification. Essentially, black locust’s rot resistance is built-in, evolving over centuries to protect it against environmental threats.
In contrast, thermally modified wood undergoes a high-temperature heating process between 180°C and 230°C ((350F - 450F) in a low-oxygen environment to improve its moisture resistance and decay resistance. While this process can help prevent rot, it comes at a cost: the intense heat not only uses significant energy, increasing its carbon footprint, but it also changes the wood’s structural integrity. The high heat breaks down the cellulose and hemicellulose fibers in the wood, which reduces its natural flexibility and strength.
As a result, thermally modified wood becomes more brittle and prone to cracking and splitting, particularly when exposed to temperature fluctuations, heavy loads, or high-traffic areas. This weakening of the wood’s internal structure may limit its longevity compared to naturally resilient options like black locust. Additionally, the brittleness introduced by thermal modification can make it more challenging to work with during installation, potentially requiring additional handling or precautions to prevent breakage.
Black Locust Decking vs. Thermally Modified Wood: Key Comparisons
To help you make an informed decision, here’s a comparison between black locust decking and thermally modified wood across essential factors like durability, maintenance, lifespan, and more.
1. Durability and Longevity
- Black Locust Decking: Black locust is inherently durable and naturally resistant to rot, decay, and insects, lasting 50+ years outdoors without the need for preservatives or maintenance.
- Thermally Modified Wood: Thermally modified wood gains durability through high-temperature treatment, enhancing rot resistance but not to the same extent as naturally durable hardwoods like black locust. Its lifespan is generally shorter, and it may require regular maintenance to sustain performance over time.
2. Sustainability
- Black Locust: Black locust is a highly sustainable wood choice, grown in fast-growing, managed forests across North America. Unlike many hardwoods, black locust trees reach maturity relatively quickly—within 30-40 years—and thrive in various soil types without needing fertilizers or pesticides. This growth rate makes black locust an efficient, renewable resource, easily replenished in sustainably managed forests.
Because black locust requires no chemical treatments, heat processing, or artificial enhancements to achieve its durability, it has a significantly lower carbon footprint compared to thermally modified wood. The energy-intensive processing required for thermal modification is entirely unnecessary for black locust, as its natural composition provides the durability, rot resistance, and stability that outdoor applications demand. By choosing black locust decking, builders and homeowners can reduce their reliance on resource-heavy treatments, minimizing overall environmental impact while opting for a long-lasting, low-maintenance material.
- Thermally Modified Wood: Thermally modified wood is often sourced domestically, which can help reduce the carbon emissions associated with importing exotic hardwoods. However, the heat-treatment process itself demands extensive energy, usually powered by electricity or natural gas. This treatment involves heating the wood to temperatures between 180°C and 230°C (350F - 450F) for several hours in a controlled environment, which not only consumes significant energy but also increases the environmental footprint of the final product.
During this high-temperature treatment, wood must be kept in a low-oxygen environment to prevent combustion, and the equipment used for this process—such as large kilns—operates at a high energy cost. This energy-intensive step directly impacts thermally modified wood’s carbon footprint, contributing greenhouse gasses and adding to its overall resource demands. When thermally modified wood is produced on a large scale, the cumulative energy use and associated emissions can be considerable, especially compared to woods like black locust that require no modification to enhance their durability and stability.
While thermally modified wood does avoid the use of chemical preservatives, the carbon costs of its heat treatment can outweigh these benefits, especially in climates where the treated wood may need additional maintenance or sealing to withstand exposure. For builders and homeowners looking to prioritize sustainability, black locust provides a more environmentally friendly choice that combines natural durability with a lower carbon impact.
3. Natural Stability
- Black Locust: Black locust’s density and structural integrity provide excellent stability with minimal warping or splitting, even in extreme weather.
- Thermally Modified Wood: Thermally modified wood undergoes a heat treatment process that alters its cellular structure, reducing its ability to absorb and release moisture, which makes it more dimensionally stable. This process improves resistance to warping, swelling, and shrinking in variable climates because the wood’s sugars and moisture-absorbing compounds are partially broken down by the heat, making it less responsive to humidity changes. However, these improvements are typically not as long-lasting as those found in naturally stable woods like black locust.
The issue lies in the durability of the changes introduced by thermal modification. Because the process removes or weakens some of the wood’s natural polymers, the modified wood can become brittle and lose its natural elasticity. This brittleness can lead to structural issues over time, especially in outdoor environments where exposure to fluctuating temperatures and moisture levels is constant. Studies have shown that thermally modified wood can begin to degrade faster than naturally stable hardwoods when exposed to repeated cycles of moisture and temperature variation, such as freezing and thawing or wetting and drying.
4. Historical Context
Thermally modified wood has been commercially available since the early 1990s, with its development spearheaded by industry leaders and the VTT Technical Research Center of Finland. This process was designed to enhance the properties of timber through heat and steam treatment, resulting in improved durability and dimensional stability.
In contrast, black locust (Robinia pseudoacacia) has a rich history of utilization spanning several centuries. Native to North America, it was extensively used by Native Americans for crafting bows and tools due to its exceptional strength and durability.
European settlers in the 17th century recognized its value, employing black locust wood for building structures, fence posts, and shipbuilding components. Notably, the first permanent English settlement in Jamestown, Virginia, utilized black locust for constructing buildings, with reports indicating that many of these structures remained sound a century later. This historical context highlights that while thermally modified wood is a relatively recent innovation, black locust has been a trusted material for durable applications for over 400 years.
5. Maintenance Needs
- Black Locust Decking: Black locust requires little to no maintenance and can be left untreated, developing a silver-gray patina over time. Only periodic cleaning is needed to retain its structural integrity.
- Thermally Modified Wood: To maintain its appearance, thermally modified wood benefits from regular sealing and care, especially if installed in areas with high moisture or intense UV exposure.
6. Weathering and Appearance
- Black Locust Decking: Known for its beautiful aging, black locust naturally weathers to a sophisticated silver-gray, adding to its aesthetic appeal in outdoor settings.
- Thermally Modified Wood: Thermally modified wood initially offers a rich, uniform color but may weather unevenly without proper maintenance. Regular treatment may be necessary to sustain its appearance over time.
Why Black Locust Decking is the Superior Choice
Opting for black locust decking gives you a high-performing, eco-friendly decking solution that doesn’t require energy-intensive processing to achieve durability. With its natural resilience, black locust provides a low-maintenance alternative that minimizes environmental impact, making it an excellent choice for both residential and commercial projects. Its ability to perform well in outdoor settings without added treatment makes black locust a standout option in sustainable building materials.
For more information or to request black locust decking samples, please contact us at Black Locust Lumber. Our team is here to support you in selecting a beautiful, durable, and environmentally conscious decking solution.
