EN 350 — the material side
EN 350 places heartwood in one of five durability classes based on standardised resistance to wood-destroying fungi. Two caveats matter more than the class itself:
- Sapwood is never durable. A DC 1 species carries DC 5 sapwood. If the specification does not exclude sapwood, the class on the datasheet is not what arrives.
- Within-species variation is real. Growth rate, provenance and extractive content shift performance inside a class, which is why field results scatter around the nominal rating.
| Class | Description | Specification note |
|---|---|---|
| DC 1 | Very durable | Highest natural resistance in standardised tests |
| DC 2 | Durable | Suitable for many above-ground exposed uses untreated |
| DC 3 | Moderately durable | Workable above ground with good detailing |
| DC 4 | Slightly durable | Needs protection or treatment in exposed use |
| DC 5 | Not durable | All sapwood; requires treatment or a dry use class |
EN 335 — the exposure side
Use class describes the service situation and the biological agents present. The step from UC 3.1 to UC 3.2 is the one most often assessed too optimistically: it is decided by whether water can accumulate and how quickly the element dries, not by whether it looks sheltered.
| Use class | Exposure | Moisture regime | Typical element |
|---|---|---|---|
| UC 1 | Interior, dry, covered | Consistently below 20 % MC | Internal joinery, structural frame in heated space |
| UC 2 | Interior or covered, occasional condensation | Occasionally above 20 % MC | Roof structure, unheated store, ventilated crawlspace |
| UC 3.1 | Exterior above ground, water does not accumulate | Frequently above 20 % MC, rapid drying | Ventilated vertical cladding under an overhang |
| UC 3.2 | Exterior above ground, water accumulates | Frequently above 20 % MC, slow drying | Decking, exposed horizontal rails, unventilated junctions |
| UC 4 | Ground or fresh-water contact | Predominantly above 20 % MC | Posts, sleepers, landscaping, jetty substructure |
| UC 5 | Permanent salt-water contact | Permanently wet, marine borers present | Marine piling, harbour structures |
Four ways to close the gap
When the required use class exceeds the species’ durability class, there are four legitimate routes. They are not equivalent in cost, carbon or reliability.
Natural durability
Specify a heartwood with a durability class that already meets the use class. No treatment, but sapwood exclusion, sourcing and cost become the constraints.
Preservative treatment
Raise an otherwise non-durable species to the required use class. Penetration and retention must be specified to the use class — a UC 3 treatment does not serve a UC 4 detail.
Modification
Acetylation, furfurylation and thermal modification reduce the wood's ability to hold water in the cell wall. Performance is measured against the same use classes.
Design out the exposure
Overhangs, ventilated cavities and drainage can move a detail from UC 3.2 to UC 3.1. Cheaper and more reliable than upgrading the material.
Why a class is not a service life
Durability classes rank relative resistance under standardised test conditions. They cannot tell you how long a specific board on a specific façade in a specific climate will last, because that depends on accumulated wet-warm hours — a function of local climate, orientation, overhang, cavity and dimension.
WoodIntel keeps the two separate: the class is the input, the modelled service-life distribution is the output, and the uncertainty is reported rather than hidden. All outputs are modelled estimates, not measured performance.