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Natural durability: heartwood, extractives and durability classes
Why some heartwood resists attack
As a tree ages, the inner wood becomes heartwood. In many species the tree deposits extractives there. A 2024 review calls heartwood extractives the principal source of resistance to wood-destroying organisms in naturally durable woods, with density and water exclusion also contributing. Compounds named include terpenes, stilbenes, resin acids, tropolones and tannin-type substances. Four practical points follow.
- Sapwood should not be counted on. Durability statements concern only the heartwood of mature wood unless sapwood is mentioned. Swedish Wood states that sapwood generally has no resistance to biological attack. A piece that contains sapwood is not covered by the heartwood figure.
- Durability varies. Extractive amount and effectiveness vary within and between species and with region, growing conditions, individual tree, silviculture and age. Old-growth wood tends to contain more extractives than plantation wood.
- Extractives are not permanent. Water-soluble ones can leach out and UV exposure degrades them.
- Extractives are not a detachable preservative. When stilbenes from pine, Osage orange and eucalypt were impregnated into non-durable wood, their effect against fungi fell to roughly one hundredth of that in the original heartwood.
Durability classes (Europe)
European practice rates heartwood resistance to fungi on a five-step scale from class 1 (most durable) to class 5 (not durable), standardised in EN 350. The classes are laboratory-based and describe heartwood only. There are also impregnability classes (1 easy to 4 refractory) and beetle classes (S susceptible, D durable). Other systems use different scales, so be careful when comparing numbers across sources. Class names and values should be taken from the current standard or a supplier's data sheet.
What the species profiles say
Values are as reported in the portal's species profiles, mainly European sources. Sources sometimes disagree, so treat them as indications.
| Species | Heartwood durability as reported | Notes |
|---|---|---|
| Spruce | Class 4; not naturally durable | Suits dry or ventilated uses; poor impregnation |
| Pine | Class 3-4 (sources differ) | Sapwood highly susceptible; heartwood suitable outdoors without ground contact per one source |
| Larch | Class 3-4, class 4 in ground contact; sapwood class 5 | Forum reports of longer life conflict with the class; specify with 3-4 |
| Oak | Durable under an older edition, but field trials gave poorer results | Exclude or protect sapwood for external use |
| Sweet chestnut | Class 2 | Thin sapwood milled off for ground use; high variability in the Turkish burial study |
| Robinia | Class 1-2; most durable European timber per the source | Juvenile heartwood is much weaker than mature heartwood |
| Beech | Class 5 | Unsuitable for exterior use without treatment |
| Ash | Class 5 | Unsuitable outdoors; impregnation hardly possible |
Robinia illustrates variation within a species. In 16-week laboratory tests, mature heartwood lost on average 0.7 % mass to a brown-rot fungus and 1.7 % to a white-rot fungus, while juvenile heartwood lost 10.1 % and 17.0 %. These are laboratory, not field, data.
Worked example: choosing a species for fence posts
A customer asks for untreated posts set in the ground.
- Ground contact is use class 4 (see next lesson), the most demanding of the above-ground and ground classes.
- Spruce (class 4), pine and larch (class 3-4) are not safe choices without treatment or modification.
- Beech and ash (class 5) are worse.
- Robinia and sweet chestnut heartwood are the species the profiles describe as usable without chemical preservation, provided sapwood and juvenile wood are removed.
Checklist
- Is the piece heartwood only, with no sapwood?
- Is the class from the current standard or a data sheet, not from memory?
- Is it a class for the right exposure (ground contact differs from above ground)?
- Do you know the tree's origin and age, which affect durability?
Key points
- Heartwood extractives give natural durability; sapwood is treated as non-durable.
- Durability varies between and within trees and species, and extractives can leach or degrade.
- EN 350 classes run from 1 (most durable) to 5 (not durable) and describe heartwood only.
- Robinia and sweet chestnut rank highest in the profiles; spruce, pine and larch are middling to poor; beech and ash are class 5.
- Sources can disagree, so quote classes with their source and avoid promising more than the class.