Understanding wood durability: Durability classes of wood

What is wood durability and why it matters? 

Durability refers to a wood’s resistance to biological degradation, such as rot, fungi, and insect damage.  

For outdoor applications, these factors are critical because it affects the lifespan, safety, and maintenance needs of the structure. Choosing durable wood type lowers long-term maintenance costs and ensures your building stands for decades.  

Wood durability classes 

Wood durability is influenced by the species, its density, moisture resistance, and any treatments it has undergone.   

Wood durability is commonly measured using the EN 350 standard, which ranks materials from Class 1 (very durable) to Class 5 (not durable). For example, teak and ipe fall into Class 1, while species like larch and cedar are considered Class 4.   

Naturally durable hardwoods such as teak, ipe, and oak are well-known for their longevity, but they come with a hefty price tag and often environmental concerns. On the other hand, softwoods like pine and spruce are more affordable and easily available but degrade quickly without treatment.  

Pressure-treated wood introduces chemical preservatives to enhance durability, but these treatments may leach into the environment and still don’t prevent warping or cracking over time.  

What is thermally modified wood and how durable is it? 

Thermal modification takes natural wood and enhances it with only high heat and steam. It changes the structure of the wood at a cellular level, enhancing its durability and stability without the use of chemicals.  

The process removes sugars and resins that typically attract fungi and insects, significantly increasing resistance to biological degradation. As a result, thermally modified wood can reach durability class 1.   

How does thermally modified wood compare to other wood products? 

• Untreated pine and spruce often degrade within a few years in outdoor conditions, while thermally modified pine and spruce can last upwards of 15 years.  

• Compared to tropical hardwoods, thermally modified ash and oak offer similar performance but are lighter, easier to work with, and sourced from certified, sustainably managed forests.  

• Unlike pressure-treated wood, thermally modified wood contains no leaching chemicals and maintains its performance throughout its structure, not just on the surface.  

• Wood-plastic composites, while resistant to rot, can crack, warp, or fade under UV exposure. They are difficult to repair once damaged and may retain heat, making them uncomfortable underfoot in sunny conditions.  

When choosing a material for outdoor construction, thermally modified wood stands out as a compelling alternative to both traditional softwoods and more expensive hardwoods or composites. The thermal modification process enhances durability, stability, and resistance to environmental elements, delivering long-lasting performance without the use of chemicals.