Natural durability and decay: fungi, extractives and protection of timber

Why this matters in the sawmill

A sawmill handles wood at the moment of greatest biological vulnerability. A freshly felled log is full of water and of the sugars and starch in the sapwood that fungi and insects feed on. Between felling and the dry, stable board that leaves the mill there is a window, often a few weeks in warm weather, in which discoloration can set in and decay can begin. Stained boards are downgraded, decayed pieces are rejected, and shipments with mould can be refused.

This article explains what fungi need, how the kinds of damage differ, why some heartwood resists attack, how European durability and use classes are structured, and what that means for log yards, stacking and drying. It is an orientation and not a preservation specification: follow current standards and the preservative supplier's instructions.

What decay fungi need

Decay fungi are living organisms and need four things together: food (the wood itself), moisture, oxygen and a suitable temperature. Remove any one and decay stops.

Moisture is the limiting factor. A Forintek Canada technical review by P. I. Morris states that decay by wood-rotting basidiomycetes proceeds at roughly 22–24 % moisture content (MC) once the fungus is established, that infection by spores probably does not occur below about 29 % MC, that 20 % is frequently quoted as a maximum safe MC, and that the optimum for decay is between 40 and 80 %. Drying below about 15 % stops decay but may not kill the fungus, which can revive when the wood is wetted again. The older US Wood Handbook chapter on log and lumber storage gives the same practical threshold: untreated lumber uniformly below 20 % MC is described as immune to attack by fungi. (This applies to untreated lumber in service-like conditions; our wording that stain and decay 'start again' above it is an interpretation.) The European use-class system uses the same figure: ATIBT's summary of EN 335 says that for outdoor use above ground with fast drying (class 3.1) a fungal risk exists above 20 % moisture.

Oxygen. Normal air contains about 21 % oxygen. Morris reports that growth of wood-rotting fungi is not reduced until oxygen falls below roughly 1–2 %.

Temperature. Most wood-rotting fungi thrive between 20 and 30 °C, with some growing fastest at 34–36 °C. Growth stops above about 46 °C and the fungi may not be killed until about 60 °C; each 10 °C drop roughly halves growth rate, and growth continues until temperatures very close to 0 °C (Morris). The Wood Handbook chapter gives a compatible picture in Fahrenheit (converted here): stain and decay fungi grow most rapidly at about 21–32 °C, no more than one fifth as rapidly at 10–16 °C, and cease growth at about 0 °C. These figures are for fungi in general. The blue-stain fungus described below is reported to be killed at a lower temperature (a trade source, NELMA, gives above about 54 °C, converted from 130 °F).

Kinds of damage: rot, stain and mould

Not all fungal damage is equal. Distinguishing the types helps you decide how urgent a problem is.

Brown rot degrades mainly the cellulose and hemicelluloses of the cell wall, leaving a brittle, brown, cube-cracked residue. According to Morris, it mostly attacks softwoods and is the most economically important decay type in temperate building timber. Morris describes the cellulose being chopped into short lengths, resulting in rapid early strength loss.

White rot degrades both lignin and cellulose, leaving bleached, stringy or fibrous wood. It is typical of hardwoods, and strength loss is comparatively slow at first. The split "brown rot in softwoods, white rot in hardwoods" is a tendency, not a law.

Soft rot is caused by fungi that, according to Morris, typically attack wood in permanently moist conditions such as soil or cooling towers.

Blue stain (sapstain). Stain fungi, such as species of Ophiostoma (a genus name from general knowledge, not checked in the pages cited here), live on the sugars and other contents of the sapwood cells and colonise freshly felled logs and green timber quickly. A trade source (NELMA) gives optimal growth at about 70–90 °F (21–32 °C) in wood above 22 % MC, dormancy below about 50 °F (10 °C) and, as noted, death above about 130 °F (54 °C). The Morris review says the main effect of staining fungi is disfigurement, that they go dormant if the wood dries out, and describes impact-strength reduction as an unusual case. A common yard test (general practice, not taken from the cited pages): rub the surface; mould smears or brushes off, blue stain is in the wood and does not.

Mould is surface growth of spores on damp wood. The Morris review says moulds normally require a high relative humidity to produce spores, that most need an MC above about 20 % for growth and sporulation, and that the discolouration is often confined to the surface (our addition: it can usually be brushed or planed off). Some moulds are known to cause respiratory problems in sensitive individuals.

Stain and mould cost grade; only rot destroys structure. The Wood Handbook chapter also describes chemical stains (gray, yellow, pinkish to deep brown) that form in moist sapwood during log storage, especially in moist, shaded conditions; it does not tie them to blue stain or to a temperature range.

Insects in general terms

Insects are a separate hazard with a different timeline. The older Wood Handbook chapter on storage notes that insects are inactive at or below 50 °F (10 °C), that activity roughly doubles with each 10 °F (about 5-6 °C) rise above that, reaching a maximum near 80 °F (27 °C). It calls ambrosia beetles usually only a minor hazard in lumber stored away from forested areas or sawmills, and says powder-post beetles are particularly damaging to large-pored hardwoods such as oak, ash, hickory, walnut and pecan. Subterranean termites are described as by far the most prevalent type in the United States; that they need ground contact is general knowledge, not checked in the pages cited. For Europe, the ATIBT durability sheets classify timber against two beetles, the house longhorn beetle (Hylotrupes) and the furniture beetle (Anobium), as simply susceptible (S) or durable (D). A one-minute borate dip followed by seven days of covered storage is one chemical option described in the Wood Handbook chapter.

Heartwood, extractives and natural durability

In a living tree, the inner wood is gradually converted to heartwood. In many species the tree deposits extractives in the inner wood as it turns to heartwood. A review of the literature on heartwood formation and natural durability (Taylor, Gartner and Morrell, Wood and Fiber Science, 2002) covers the function of heartwood, the process of formation and the variation in both the quantity and the quality of heartwood; its abstract notes that heartwood may differ from sapwood in many properties, including natural decay resistance.

A 2024 critical review in the journal Insects (Kirker and co-authors) calls heartwood extractives the principal source of resistance to wood-destroying organisms in naturally durable woods, with specific gravity, density and water exclusion contributing. Compounds named include terpenes, stilbenes, resin acids, tropolones and antioxidants (ellagitannins appear in one example). They work alone or in combination, as a mixture of toxic and non-toxic components. Four points matter for practice:

1. Sapwood should not be counted on. ATIBT's sheets state that durability characteristics concern only the heartwood of mature wood unless sapwood is specifically mentioned; they do not say in so many words that sapwood is not durable, and Taylor and co-authors say only that heartwood may differ from sapwood in decay resistance. Treating sapwood as non-durable is the standard practical reading.
2. Durability varies. The Kirker review notes that extractive distribution, composition and effectiveness vary within and between species, and with region, growing conditions, individual tree, silviculture and age. Old-growth wood tends to contain a greater proportion of extractives than second-growth or plantation wood.
3. Extractives are not permanent. Water-soluble ones can leach out and UV exposure degrades them.
4. Extractives are not a detachable preservative. The review reports that when stilbenes from pine, Osage orange and eucalypt were impregnated into non-durable wood, their effect against decay fungi fell to roughly one hundredth of that in the original heartwood.

Morris gives examples of how this plays out in a North American context (and notes that durability varies with tree age and position in the tree): western red cedar and yellow cypress are listed as durable; Douglas-fir and southern pine are listed as moderate; most spruces, hemlocks, true firs and lodgepole, jack and ponderosa pines are non-durable. For European timber, I did not verify species-by-species classes in the sources opened here, so I do not list them; consult the current standard or a supplier's datasheet.

Durability classes and use classes in Europe

Two systems are used together.

Natural durability classes (EN 350). ATIBT's technical sheets, which cite EN 350:2016, describe five classes for resistance of heartwood to wood-destroying fungi, from 1 (most durable) to 5 (not durable). The usual wording is very durable, durable, moderately durable, slightly durable and not durable, but ATIBT's legend labels class 4 'not very durable', so I could not confirm the official class names from an authoritative open copy of EN 350. They also list classes for impregnability from 1 (impregnable) to 4 (non-impregnable, 'refractory'), the beetle classification S (susceptible) and D (durable) for Hylotrupes and Anobium, and termite classes S, M and D. The older EN 350-1:1999 was a guide to the principles of testing and classification of natural durability against fungi, insects (beetles and termites) and marine organisms; the catalogue entry shows it withdrawn from 5 September 2016 with EN 350:2016 as successor. Other systems use different scales (the Kirker review uses a four-level scale in its table), so be careful when comparing figures across sources.

Use classes (EN 335). These describe the exposure of a component and therefore the biological risk. In ATIBT's summary (which cites NF EN 335, AFNOR, 2013): class 1, fully sheltered and never wetted, with risk limited to insects and termites; class 2, sheltered but with occasional non-persistent wetting, with fungal risk limited to surfaces; class 3.1, outdoors above ground with short wetting because it dries quickly or the design sheds water; class 3.2, outdoors above ground with frequent long but non-continuous wetting; class 4, in ground or fresh-water contact; and class 5, permanent contact with seawater. The idea is to match material durability or treatment to the use class.

Storing logs and sawn timber

Logs. The risk depends on temperature and wetness. According to the Wood Handbook chapter on storage, blue stain can develop in the sapwood of exposed log ends within 2 weeks after felling in warm weather, and discoloration may penetrate into the log as rapidly as 1 ft (about 30 cm) per month, which is a maximum rather than a typical rate. Options listed there include dry storage with bark, end coating, wet storage, pond storage, water sprinkling, freezing, and transpiration drying (leaving felled trees with leaves or needles on, which lose water). Losses from transpiration drying vary with species: the chapter reports that oak with narrow sapwood will not lose much more than 10 % moisture content, while sweetgum and yellow-poplar can lose over 30 %, which tells you the effect is large but species-dependent. The chapter describes low oxygen levels as inhibiting fungi and insects in wet or sprinkled storage; linking this to the 1–2 % oxygen threshold from Morris is our inference. Practical priorities are first-in, first-out and shorter storage in the warm season.

Green sawn timber. The same chapter reports that wood at 25–30 % MC, solid-piled in warm weather, can show stain within a week. Its guidance on stacking is to use dry stickers spaced correctly to minimise warp; stickering is advised for green lumber and for partly dried lumber above 20 % MC, whereas below 20 % it can be solid-piled unless further drying is wanted. The points on foundations, gaps between piles and a roof are common practice added by us, not checked in the chapter. Antistain dip treatments have limited safe times before the board must dry, from 1 day at 80 °F (27 °C) or above, 2 days at 70 °F, 1 week at 60 °F to 1 month at 50 °F (10 °C).

Dried timber. Per the Wood Handbook chapter, kiln-dried lumber at or below 12 % can be stored outdoors only in dry weather and for a short time (extended storage causes excessive moisture regain); lumber as low as 12–14 % can be kept in open sheds without significant regain; and lumber at or below 10 % regains moisture if stored for extended periods in high relative humidity. NELMA recommends kiln-dried lumber (dried to 19 % or less at the mill) for wood packaging, says kiln drying alone is not a full stopgap, and calls storage in protected areas or under wrap critical, because stain-free packaging can stain later from wet outdoor storage or condensation in transit.

Basic protection options at overview level

1. Drying. Getting and keeping wood below about 20 % is the cheapest and most reliable protection, and it is also the one the mill controls.
2. Design and detailing. Keep wood out of ground and standing water, let water drain, allow ventilation, protect end grain, and shelter components, in line with the use-class logic above.
3. Natural durability. Choose heartwood of durable species for exposed uses, remembering that sapwood in the same piece is not durable and that durability varies.
4. Preservation. Chemical treatment, from short-term antistain or borate dips in the mill to pressure treatment for ground contact. The Kirker review states that biocides are approved for 5 or 10 years (in its EU context), so approved products change; use only current approved products.
5. Thermal modification. Heating wood to roughly 150–240 °C changes its chemistry (the review places thermal modification at 150–240 °C and describes processes of 180–250 °C in very low oxygen). A 2025 review (Jančíková and Jablonský, Sustainable Chemistry) reports that effects depend strongly on temperature: in one study under nitrogen on Corymbia citriodora and Pinus taeda, 160 and 180 °C reduced the decay resistance of Corymbia while 200–240 °C improved it in both species; in Gmelina arborea heated for 3 h, weight loss rose from 5.4 % at 180 °C to 15.1 % at 220 °C; the review cites the view that termite resistance needs treatment above about 215 °C with weight loss above 20 %, though one cited study found Cryptomeria heated at 212 °C still highly susceptible. Equilibrium moisture content fell by 37 % in a poplar study (180–220 °C, 2–10 h; the review does not say which condition gave the 37 %). The price is mechanical: in a study of two African species (Brachystegia spiciformis, Julbernadia globiflora) treated at 215–245 °C, bending strength of one fell by about 53 %; the review's property labels there look unreliable. Properties, including strength, vary widely with species and process, so ask for the supplier's test data.

Practical rules of thumb for sawmills

• Treat 20 % MC as the line (the Wood Handbook calls untreated lumber uniformly below 20 % immune to fungi): above it, the clock is running, faster when warm.
• In warm weather, expect blue stain within days to two weeks on exposed sapwood; saw, dip or sprinkle accordingly.
• Move logs first-in, first-out and keep sawn timber off the ground.
• Sticker green boards promptly, protect from rain, and do not close-pile wood above 20 %; after drying, wrap or shelter.
• Never treat sapwood as durable, whatever the species.

What is uncertain or varies

• Thresholds are approximate. The 20 %, 22–24 % and 15 % figures come from specific reviews and applied to building timber; fungal species and wood species vary.
• Durability is not fixed. It varies with tree, site, age and heartwood position; the classes describe laboratory results on heartwood.
• Time-to-damage figures from the older Wood Handbook chapter come from North American conditions; European climates and species will differ.
• Thermal modification figures come from a review of varied studies, and some values in it are ambiguously labelled (the poplar and African-species figures lack the temperature that produced them); use them only as an indication of direction.
• I did not verify species-by-species EN 350 classes, the official EN 350 class names, or the formal treatment requirements per use class; no authoritative open copy of EN 350 or EN 335 was read, only ATIBT's summaries and a catalogue entry.

Key points

• Decay needs moisture above roughly 20–24 % in the wood (spore infection probably above about 29 %, per Morris), oxygen above about 1–2 % and moderate temperature; remove any and it stops.
• Brown rot is the most damaging in temperate construction timber; blue stain and mould are mainly cosmetic but cost grade.
• Heartwood extractives give some species natural durability; sapwood is always perishable, and durability varies between trees.
• EN 350 classes heartwood durability from 1 (most durable) to 5 (not durable); EN 335 use classes describe exposure from fully sheltered to seawater contact.
• Fast, uniform drying, good stacking and keeping wood under 20 % MC are the sawmill's main protection.
• Thermal modification can raise decay resistance at higher temperatures but may reduce strength; check supplier data.