Durability ClassifierNatural Decay Resistance Classifier
Classify the natural durability of wood against fungal decay and insect attack using the EN 350 standard. Get the durability class, expected service life and suitable use-class applications.
Why this calculation matters
Durability class decides whether a species belongs in the exposure at all. It applies to heartwood only — sapwood of every species is non-durable.
Durability Classifier
Natural Decay Resistance Classifier
Sets baseline durability.
Sapwood is Class 5 for nearly all species.
Auto-set by species; adjust if known.
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About Durability Classifier
Natural durability is a wood's inherent resistance to decay fungi and wood-boring insects, without any treatment. The European standard EN 350 grades heartwood durability from Class 1 (Very Durable, like Teak and Ipe) to Class 5 (Not Durable, like sapwood and most softwoods). Matching durability class to the EN 335 Use Class of the application is the foundation of specifying wood that lasts. It works as a searchable wood durability class chart, mapping species to the EN 350 durability classes that buyers specify.
Where Is This Used?
Standards and Formulas
EN 350: Durability Class 1 (Very Durable) to Class 5 (Not Durable)EN 335: Use Class UC1 (dry interior) to UC5 (marine)Service life ~ base_life(class) x use_factor x climate_factorSapwood is rated Class 5 for almost all speciesTreatment can upgrade effective class by 1-3 levelsHow the calculation works
Sapwood is forced to the lowest class because sapwood of every species is non-durable, whatever the heartwood figure says. Everything after that is a comparison of what the situation demands against what the timber offers.
Show the formula
Effective class = the EN 350 heartwood class, or forced to Class 5 for sapwood, then improved by treatment (thermal 2, preservative 3, acetylation 4 steps, floored at Class 1). Indicative service life = base life for the class x use-class factor x climate factor (tropical 0.55, arid 1.6).The EN 350 classes and the rule that sapwood is non-durable are published. The service life in YEARS is not, and deliberately so: EN 335 defines the use classes but attaches no service life to them, because the outcome depends on detailing, ventilation, ground contact and maintenance far more than on species. Treat the years as a comparison between options, never as a warranty.
References and what each one provides
- EN 350CENHeartwood durability classes and the rule that sapwood is treated as non-durable.
- ThermoWood HandbookInternational ThermoWood AssociationThe durability improvement published for thermally modified timber.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Reports the published EN 350 class for heartwood. It does not account for sapwood, which is non-durable in every species, and does not describe weathering, UV degradation, insect attack, or marine borer resistance.
Do not use this for Specification sign-off, warranty claims, or substituting for a treatment certificate. It classifies heartwood only.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- EN 350 natural durability classes 1–5
Formula checked against the sources above by an automated regression test (tests/verify-phase2.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter your project details below, then press Calculate.
- Pick your wood portion and use class from the dropdown.
- Press Calculate.
Worked example
Common mistakes to avoid
- Reading durability class as a guarantee. Class ratings assume ground contact test conditions — design still matters.
- Applying heartwood durability to sapwood. Sapwood of almost every species is perishable.
- Choosing by durability alone for pool surrounds. Slip resistance and splinter behaviour matter as much.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers
Full listing in the durability and fire behaviour reference.