Timber Material Durability Class EN 350: How to Read and Specify the Right Class
A practical guide to EN 350 durability classification — what the classes mean, how modified timber performs, and how to specify correctly
If you're specifying timber material for outdoor or high-humidity applications, you've probably seen references to "durability class 1" or "class 2 per EN 350." But what do those classes actually represent — and how do you verify them when your supplier quotes a number?
At Chambroad, we manufacture biomass-modified timber products that are tested and specified against EN 350 and EN 252. Here's what the standard says, how the testing works, and what specifiers need to watch for.
What EN 350 Actually Covers
EN 350:2016 (Durability of wood and wood-based products) replaced the previous 1994 version and applies to both solid timber and wood-based products. It defines two types of durability:
- Natural durability: The innate resistance of the timber species to wood-destroying fungi (basidiomycetes), assessed in laboratory tests on untreated wood.
- Service life (performance-based): The expected lifespan of a timber product in a defined use class and environment, often combining natural durability with treatment/modification performance.
It's important to be clear: EN 350 durability classes for natural durability apply to the timber species, not necessarily to the modified product. For modified timber, performance is typically assessed via EN 252 (field test) or EN 330 (accelerated laboratory test), then mapped to an expected service life class.
Keep in mind: A durability class quoted without a test method or standard reference (e.g., "EN 252 field test, 60 months, <25% mass loss") isn't verifiable. Always ask for the specific test standard and duration.
EN 350 Durability Classes (Natural Durability)
| Class | Description | Typical Service Life (ground contact) |
|---|---|---|
| Class 1 | Highly durable | > 25 years |
| Class 2 | Durable | 15–25 years |
| Class 3 | Moderately durable | 10–15 years |
| Class 4 | Slightly durable | 5–10 years |
| Class 5 | Not durable | < 5 years |
How Modified Timber Is Assessed
Modified timber isn't a natural species, so "natural durability class" doesn't directly apply. Instead, its performance is evaluated through field stake tests (EN 252) or laboratory soil bed tests (EN 330). The mass loss after a defined exposure period determines the durability rating.
Our biomass-modified timber products, when tested under EN 252 conditions, consistently show mass loss values corresponding to durability class 1–2 performance — meaning expected service life of 15–25+ years in high-decay-risk environments (Use Class 3–4 per EN 335).
What to Ask When Reviewing Durability Claims
- Which test standard? EN 252 (field), EN 330 (lab), or EN 350 natural durability? They measure different things. For modified timber, EN 252 is the most relevant.
- Test duration: A 12-month field test isn't enough. Credible data comes from 24–60 month exposures. Our test data covers 60-month field exposures.
- Mass loss percentage: Ask for the actual number. Class 1 typically means <15% mass loss after the test period. "Passes EN 252" without a number is vague.
- Use class context: Durability is always specified relative to a use class (1–5 per EN 335). A product that's class 1 in Use Class 3 may only be class 3 in Use Class 4 (ground contact).
Real-World Performance: Modified vs. Untreated
The difference shows up most clearly in outdoor applications without overhead protection:
Untreated softwood decking
Typical service life 5–8 years in Use Class 3 (outdoor, above ground). Often shows decay starting at year 3–4 in humid climates.
Modified timber decking
Expected service life 15–25 years in same conditions. Our marine-grade flooring products are specified with this performance envelope.
Modified timber in ground contact
Use Class 4 performance varies by species and modification depth. Always request EN 252 data before specifying for ground-contact applications.
For outdoor wall cladding and facade applications (Use Class 3.1), the service life advantage of modified timber is most significant — the facade is protected from direct wetting but still experiences humidity cycling.
Need EN 350 / EN 252 Durability Data?
We provide test reports, mass-loss data, and service-life projections for all our modified timber product lines to support specification and submittals.
Or contact our technical team for test-data review and specification support.
