Durability Classes of Wood
The most important indicator for this is the wood’s durability class. In this guide, we explain the systems used to determine these classes, what the various specifications entail, and which solution is best to offer your customers.
2 testing systems that determine the wood quality grade
To measure the durability and resistance of wood, refer to the European standard NEN-EN 30. Within the wood industry, we primarily use two testing systems to determine how well the wood resists biological damage such as wood rot, mold, and insects.
Ground Contact Test
This is the most commonly used test. In this test, untreated wooden stakes measuring 50 x 50 mm are placed in the ground. Then, measurements are taken how long it takes for fungi to attack the wood. This measurement serves as the official basis for the five durability classes. Depending on the type of wood, this test can last anywhere from less than 5 years to more than 25 years.
Usage Classes
In addition to durability, we at Stevens Hout consider the usage classes. These classes (formerly known as risk classes) determine how humid the environment surrounding the wood may be. The system ranges from Class 1 for dry interior applications to Class 5 for wood that is constantly submerged in saltwater. This allows us to always select the perfect type of wood for every structure.
Discover the 5 durability classes of wood
The most commonly used classification system divides wood into 5 durability classes, based on its expected lifespan when in direct contact with the ground. Class 1 indicates very durable, and Class 5 indicates non-durable. Thanks to this classification, you can always choose the most durable wood for outdoor use.
Class 1
- Highly sustainable
- Average service life in the ground of more than 25 years
- Wood species such as thermally modified spruce, Thermo Fraké wood
- Suitable for patios, facades, fences, patio furniture…
Class 2
- Sustainable
- Average lifespan in the ground: 15 to 25 years
- Wood types such as block-profile lumber, Western Red Cedar
- Suitable for siding, decking, and luxury patios
Grade 3
- Moderately sustainable
- Average lifespan in the ground of 10 to 15 years
- Wood species such as Douglas fir, treated pine, treated spruce, and Meranti
- Suitable for fencing, canopies, and hedges
Grade 4
- Not very sustainable
- Average lifespan in the ground: 5 to 10 years
- Types of wood such as untreated pine, untreated spruce
- Suitable for indoor structures
Grade 5
- Not sustainable
- Average lifespan in the ground of less than 5 years
- Wood species such as birch and poplar
- Suitable for indoor floors and indoor furniture
A sustainability class for every application: how to make an informed and appropriate choice
Wood for a deck
Deck boards are installed horizontally, are exposed to a lot of UV light, and stay wet a little longer after a rain shower. For this reason, choose Class 1 or 2. Hardwoods such as Thermo Es also protect the deck from scratches caused by heavy use.
Cladding
If you choose sustainable siding, it’s best to install wood or ThermoWood siding vertically so that it dries faster. That’s why you should opt for Class 2 as your quality standard, using wood species such as Thermo Ayous or Thermo Spruce. Untreated softwood in Class 3, such as Douglas fir or larch, is also an option, provided you use proper ventilation and install it above ground.
Wood-frame construction and lumber
For the interior structure of the home—such as interior walls or roof structures—that is fully protected against moisture, use Softwoods such as spruce and pine. This wood is naturally classified as Class 4 or 5. Nevertheless, it excels in structural strength, making it suitable for use as structural lumber and carpentry wood.
Stevens Hout supplies wood in all durability classes
Stevens Hout’s product range covers virtually all durability classes. Our greatest strength is the ThermoWood line. Through thermal modification, wood species that are naturally less durable receive a significant upgrade. One example is Thermo Tulipwood, which moves up from Class 4 or 5 to Class 2.
We offer a wide range of untreated softwood, such as Oregon Pine and Southern Yellow Pine, which are classified as Class 2. You’ll find everything you need in durability classes 1 through 4.
Wood in durability class 5 is not durable. As soon as it comes into contact with moisture or the ground, mold will start to damage your structure within five years. That is why we at Stevens Hout have made a conscious decision not to supply this class of wood. This way, we guarantee a result that will last a long time.
Stevens Hout, your partner in sustainable wood
With 90 years of experience as a lumber importer in Antwerp, Stevens Hout works proactively with you on your projects. We supply certified lumber with the exact durability class your customers need.
Are you curious about our current inventory, or would you prefer to receive a quote right away? Please contact us. We’d be happy to assist you.
FAQ
What durability class does my wood need for outdoor use?
That depends on the exposure. For applications in contact with soil or water, such as posts, it’s best to choose Class 1 or 2. For a deck, facade, or garden structure without soil contact, Class 1 through 3 is sufficient. Not sure? Our specialists will be happy to advise you on the right type of wood for your project.
What is the difference between durability class and risk class?
The durability class indicates the nature of the wood itself: how naturally resistant it is to fungi and insects, ranging from Class 1 (highly durable) to Class 5 (not durable). The risk class describes the application: how harsh the conditions are in which the wood will be used, ranging from dry indoor environments to constant contact with water. By combining both, you can choose the right wood for every situation.
Is wood with a lower durability class still suitable for outdoor use?
Yes. Wood species such as pine and spruce are naturally less durable. By treating them with preservatives or thermally modifying them, they are upgraded to a higher durability class. This gives you an affordable alternative to tropical hardwood, with a lifespan suitable for decks, siding, and garden structures.