Rot Resistance and Fungal Decay

What Fungal Decay Actually Is

Fungal decay is a biological process in which fungi digest the structural polymers of wood — cellulose, hemicellulose, and lignin — as a food source.

The fungi that cause wood decay are not moulds or staining fungi (though those often appear alongside decay). True decay fungi are wood-degrading basidiomycetes (and some ascomycetes) that produce enzymes capable of breaking down the cell wall.

The result is progressive loss of mass, strength, and structural integrity. A decayed board may look intact on the surface while being severely compromised internally. By the time decay is visible, the timber has often already lost a significant proportion of its strength.

The Four Conditions for Decay

As introduced in Guide 4, fungi need four things simultaneously. Remove any one and decay cannot proceed.

Diagram: the four conditions for decay — moisture, oxygen, warmth, and food. Remove any one to stop decay.

The four conditions decay fungi need at the same time

1. Moisture — above ~20% MC

This is the most important factor and the one you have most control over.

Fungi cannot grow in dry wood. The critical threshold is approximately 20% moisture content. Below this, decay fungi cannot extract enough water from the wood to sustain metabolic activity.

In practice:

• Indoor timber at 8–12% MC is safe from decay indefinitely

• Timber consistently below 18% MC is at negligible risk

• Timber regularly cycling above 20% MC is at increasing risk

• Timber persistently above 25–30% MC is at high risk

The fibre saturation point (~28–30% MC) is where free water begins to fill cell cavities. Decay is most active when the wood is wet but not saturated — because the fungi also need oxygen.

2. Oxygen

Decay fungi are aerobic organisms. They need air to metabolise wood.

This is why permanently submerged timber does not rot. Archaeological timbers recovered from waterlogged sites — bog oaks, shipwrecks, lake dwellings — can survive for thousands of years because oxygen is excluded.

However, "permanently submerged" is the key phrase. Timber in the splash zone (alternating wet and dry) is at extremely high risk because it gets both moisture and oxygen.

3. Temperature — 10–35°C

Most decay fungi are active in a temperature band roughly matching temperate climates:

• Below 5°C: Dormant. No significant decay.

• 10–25°C: Increasingly active.

• 25–30°C: Optimum for most species.

• 35–40°C: Most species slow down or stop.

• Above 60°C: Lethal to most decay fungi (this is the basis of kiln sterilisation).

Freezing does not kill fungi — it just pauses them. When temperatures rise again, decay resumes.

4. Food — the wood itself

The cellulose, hemicellulose, and lignin in wood cell walls are the food source. All species provide this food, but naturally durable heartwoods contain extractives that are toxic to the fungi, effectively poisoning the meal.

Types of Fungal Decay

Not all rot is the same. Different fungi attack different components of the cell wall, producing distinctly different types of damage.

How the three main rot types differ

Brown Rot

Brown rot fungi primarily break down cellulose and hemicellulose, leaving the lignin largely intact.

Appearance:

• Wood darkens to a brown colour

• As it dries, it shrinks and cracks into cubical fragments (sometimes called "cubical fracture")

• Advanced brown rot crumbles to a dry, powdery texture when rubbed

Where it occurs:

• Most common in softwoods (though it can also attack hardwoods)

• Typical in structural timber: floor joists, roof timbers, fence posts

• Common in ground-contact situations

Why it matters:

• Brown rot causes rapid strength loss. Because cellulose is the primary load-bearing polymer in the cell wall, destroying it can reduce bending strength by 70% or more with surprisingly little visible damage.

• The cubical cracking pattern is a reliable diagnostic sign.

Notable species:

• Serpula lacrymans — the dry rot fungus (the most destructive brown rot in buildings)

• Coniophora puteana — cellar fungus or wet rot (the most common brown rot in buildings)

• Postia placenta — commonly used in laboratory durability testing

White Rot

White rot fungi break down all three major cell wall components — cellulose, hemicellulose, and lignin — though some preferentially attack lignin first.

Appearance:

• Wood becomes paler (lighter in colour) as lignin is removed

• Texture becomes fibrous and stringy — like a bundle of loose fibres

• In advanced stages, the wood feels spongy and soft

• Does not exhibit the cubical cracking pattern of brown rot

Where it occurs:

• More common in hardwoods than softwoods

• Common on fallen logs and dead standing trees in forests

• Found in outdoor timber that remains damp

Why it matters:

• White rot degrades wood more evenly than brown rot and is sometimes less immediately catastrophic to strength

• However, advanced white rot can reduce timber to a soft, spongy mass with no structural value

Notable species:

• Trametes versicolor (turkey tail) — very common on fallen hardwood logs

• Phanerochaete chrysosporium — used extensively in laboratory research

Soft Rot

Soft rot fungi (primarily ascomycetes and deuteromycetes) attack the cell wall from within, creating characteristic cavities in the secondary wall layers.

Appearance:

• Surface becomes soft and degraded while deeper wood may remain sound

• Darkened, softened surface layer — often described as "mushy"

• Less dramatic visually than brown or white rot but progressively destructive

Where it occurs:

• Very wet conditions — soil contact, cooling towers, water features, marine environments

• Often where conditions are too extreme for basidiomycete decay fungi (very wet, very acidic, or chemically treated)

• Can attack timber that has been preservative-treated, where brown and white rot fungi have been killed

Why it matters:

• Soft rot progresses slowly but can be very damaging in ground-contact applications over long periods

• It is the primary decay type in Use Class 4 and 5 situations (ground contact and marine)

• Resistance to soft rot does not always correlate with resistance to brown/white rot

Dry Rot: The Special Case

Among all wood-decay fungi, dry rot (Serpula lacrymans) deserves special attention because of its unique behaviour and its devastating impact on buildings.

What makes dry rot different

• It can transport water. Dry rot produces specialised structures called rhizomorphs — root-like strands that can carry moisture across dry surfaces (masonry, concrete, plaster) to reach timber that would otherwise be too dry to decay. This is how it earns the misleading name "dry" rot — the timber it attacks may appear dry, but the fungus has transported water to it.

• It spreads aggressively. Dry rot can grow through masonry joints, behind plaster, and across inert surfaces to colonise timber in adjacent rooms.

• It produces distinctive fruiting bodies. The mature sporophore (fruiting body) is a large, fleshy, rust-red pancake shape that releases clouds of red-brown spores.

• It has specific temperature preferences. Dry rot prefers temperatures around 20–22°C and is relatively intolerant of high temperatures — it dies above about 40°C.

Identifying dry rot

• Cubical cracking (brown rot pattern) in affected timber

• White, cotton-wool-like mycelium in humid, still air

• Grey or white rhizomorph strands (sometimes mistaken for roots) growing across surfaces

• A distinctive mushroom-like smell

• Rust-red spore dust on surfaces near fruiting bodies

Why it's so feared

Dry rot outbreaks in buildings can require extensive remediation: removal of affected timber, treatment of masonry, and sometimes structural rebuilding. The cost and disruption can be enormous. Insurance claims for dry rot are among the most expensive for residential properties.

Wet Rot: The Common One

Wet rot is far more common than dry rot and is caused by a range of fungi, most commonly Coniophora puteana (cellar fungus).

Key differences from dry rot

• Wet rot cannot transport water — it only grows where timber is already wet

• It does not spread through masonry or across dry surfaces

• Damage is typically localised to the area where the moisture source exists

• It is usually easier and cheaper to remedy — fix the moisture source, replace the affected timber

Common locations

• Window sills and frames where paint has failed and water is entering

• Timber in contact with wet masonry (e.g. joist ends embedded in damp walls)

• Sub-floor timbers in poorly ventilated spaces

• Exterior timber without adequate maintenance

Identification

• Darkened, softened timber (may be brown rot or white rot depending on species)

• Often localised around an obvious moisture source (leaking gutter, failed flashing, rising damp)

• No extensive mycelial growth across non-timber surfaces (distinguishing it from dry rot)

• Timber may feel soft when probed with a screwdriver or knife

Staining Fungi and Moulds: Not Decay, But Not Harmless

Not every fungus that grows on wood causes decay. Two other categories are common and worth understanding:

Blue stain (sap stain)

Caused by fungi such as Ophiostoma and Ceratocystis species that grow through the sapwood, discolouring it blue-grey to black. These fungi:

• Feed on cell contents (sugars, starches), not the cell wall

• Do not significantly reduce strength

• Are primarily a cosmetic defect

• Occur mostly in freshly sawn softwood sapwood with high MC

• Can increase permeability, potentially making the wood more susceptible to subsequent decay

Blue stain is common in pine and spruce and can develop rapidly in warm, humid conditions if timber is not dried promptly after sawing.

Surface moulds

Moulds grow on the surface of damp wood (and many other materials), producing coloured spore masses — black, green, pink, or white.

• They do not penetrate or degrade the cell wall

• They can usually be cleaned off without permanent damage

• However, they indicate excess moisture — which means conditions may also favour decay fungi

• Some moulds produce allergenic or toxic spores that pose health risks in indoor environments

The presence of mould on timber is a warning sign: the moisture conditions are wrong, and if they persist, decay may follow.

How Extractives Protect Against Decay

We introduced extractives in Guide 4 as the chemical basis of natural durability. Here's a closer look at how they work.

Mechanisms of protection

Extractives defend against fungi through several mechanisms:

• Direct toxicity: Some extractives are directly poisonous to fungal cells, killing or inhibiting growth on contact. Thujaplicin in Western Red Cedar and tectoquinone in teak are examples.

• Enzyme inhibition: Decay fungi produce enzymes (cellulases, laccases, peroxidases) to break down wood. Some extractives inhibit these enzymes.

• Antioxidant activity: Many wood extractives are phenolic compounds with antioxidant properties. Since some decay mechanisms involve oxidative reactions, antioxidant extractives can slow the process.

• Reduced moisture uptake: Some extractives (particularly oily ones like those in teak) reduce the wood's ability to absorb water, keeping it below the decay threshold more often.

Why extractive protection fades

Extractive concentrations are not permanent in all species:

• Leaching: Water-soluble extractives can be slowly washed out by rain, especially in end-grain-exposed situations. This is one reason why the durability of some species decreases over very long periods of outdoor exposure.

• Degradation: Some extractives break down under UV light or chemical reaction over time.

• Concentration gradients: As noted in Guide 4, extractive concentration is typically highest in the outer heartwood and lower near the pith. Inner heartwood may be significantly less durable than outer heartwood.

This is why even naturally durable species benefit from good design and maintenance — the extractives are not an infinite shield.

Practical Prevention: Keeping Timber Sound

Prevention is always cheaper and easier than remediation. The strategy is simple in principle: control moisture.

1. Keep timber dry

The single most effective strategy. If wood stays below 20% MC, decay fungi cannot grow — regardless of species, temperature, or exposure.

• Use adequate roof overhangs to protect wall cladding

• Ensure good drainage away from timber structures

• Ventilate sub-floor spaces, roof voids, and behind cladding

• Fix leaks promptly — a small leak today becomes a rot outbreak in a year

2. Use durable species (or treat)

Where timber will get wet — ground contact, exposed decking, marine environments — choose a species with the appropriate durability class (Guide 4) or use preservative-treated timber.

3. Design for drainage

Every detail should shed water, not hold it:

• Slope horizontal surfaces (even a slight fall makes a big difference)

• Avoid flat, horizontal end-grain exposure

• Use drip grooves on the underside of sills and exposed horizontals

• Raise timber off ground contact with metal brackets, DPC membranes, or stone pads

• Leave ventilation gaps between timber and solid surfaces

4. Protect end grain

End grain is the Achilles heel of outdoor timber. Water enters through end grain 10–15 times faster than through face grain. Seal it with:

• End-grain sealant (wax-based or purpose-made)

• Metal caps or flashings on exposed post tops

• Design details that minimise end-grain exposure to weather

5. Maintain finishes

Coatings, oils, and stains slow moisture cycling. They don't prevent decay on their own, but they reduce the frequency and duration of high-MC episodes.

Re-apply finishes on a regular schedule — before they fail completely, not after.

6. Inspect regularly

Timber in exposed conditions should be inspected annually:

• Probe suspect areas with a pointed tool (screwdriver, bradawl). Sound timber resists; decayed timber yields.

• Check for discolouration, softening, cracking, or fungal growth

• Pay attention to high-risk areas: joints, end grain, ground contact zones, behind flashings

Detection: How to Spot Decay Early

Early detection prevents small problems from becoming structural failures.

Visual signs

• Discolouration: Darkening (brown rot) or lightening (white rot) compared to surrounding timber

• Surface texture changes: Cubical cracking, fibrous texture, or soft surface layer

• Fungal growth: Mycelium (white, cotton-like strands), fruiting bodies (bracket fungi, mushrooms), or rhizomorphs

• Paint or finish failure: Localised paint peeling or blistering over timber can indicate moisture problems beneath

Physical probing

The simplest and most reliable field test:

• Push a screwdriver or bradawl into the timber. Sound wood resists firmly. Decayed wood yields easily and may crumble.

• Focus on high-risk zones: joist ends, window sills, post bases, areas near plumbing

Moisture metering

A pin-type or capacitance moisture meter can identify timber at risk before decay starts:

• Readings above 20% indicate decay risk

• Readings above 28% indicate conditions where decay is likely already active

• Regular monitoring of critical timber can catch moisture problems early

Professional assessment

For structural timber in buildings, specialist surveyors can use:

• Resistance drilling (micro-drilling): A thin drill measures resistance through the cross-section, detecting internal decay that isn't visible from the surface

• Ultrasonic testing: Sound waves pass through the timber; decayed zones slow the wave speed

• Core sampling: Small-diameter cores removed for visual and laboratory analysis

Remediation: When Decay Has Already Started

If decay is detected, the response depends on the extent and the cause.

Step 1: Identify and fix the moisture source

This is non-negotiable. If the moisture source remains, any repair will eventually fail. Find the leak, fix the drainage, improve the ventilation, or redesign the detail.

Step 2: Assess the extent

Probe and inspect beyond the visible damage. Decay often extends further than it appears, especially with dry rot.

Step 3: Remove or repair

• Localised wet rot: Cut back to sound timber (at least 300 mm beyond visible decay), replace with durable or treated timber, and address the moisture source.

• Dry rot: Requires more aggressive treatment. Remove all affected timber plus at least 600 mm of apparently sound timber beyond the last visible decay. Treat surrounding masonry with fungicidal solution. Replace with treated timber. Ensure ventilation.

• Structural timber: If decay has compromised structural members (joists, beams, rafters), engage a structural engineer. Partial repair, sistering (bolting new timber alongside), or full replacement may be needed.

Step 4: Monitor

After remediation, monitor the area. Check moisture levels periodically. Inspect for any recurrence.

Species Resistance: A Closer Look

Different species resist different types of decay differently.

Species resistance to brown rot, white rot and soft rot, and ground-contact suitability

Notice that soft rot resistance does not always match brown/white rot resistance. Western Red Cedar performs well against brown and white rot but less well against soft rot in ground contact. This is why Use Class 4 (ground contact) demands the highest-performing species or treated timber.

The Bigger Picture: Decay in the Built Environment

Fungal decay is not just a timber problem — it's a building performance problem.

Most decay in buildings results from building defects: failed flashings, blocked gutters, inadequate ventilation, rising damp, plumbing leaks, and condensation. The fungus is the symptom. The moisture is the disease.

A well-designed, well-maintained building can keep non-durable timber sound for centuries. A poorly maintained building can destroy even naturally durable timber in a decade.

This is why timber performance in service depends as much on architecture and maintenance as on the species chosen.