Fungal Decay in Timber: How to Identify It and What to Do

What Fungal Decay Is and Is Not

Fungal decay — commonly called wood rot — is the degradation of timber by fungal organisms that use the cellulose and lignin in wood cell walls as a food source. It is not the same as surface mould or staining fungi, which affect appearance but do not structurally degrade the wood. True decay fungus attacks the structural components of the cell wall, reducing the wood to a fraction of its original strength before any visible sign appears on the surface.

Decay fungi require four conditions simultaneously: moisture above approximately 20% MC, oxygen, a temperature between 5°C and 40°C, and an organic food source. Removing any one of these conditions stops fungal activity. The most practical lever is moisture: dry timber at 18–20% MC or below will not decay regardless of temperature or the presence of spores.

Types of Decay

Brown rot breaks down the cellulose and hemicellulose in cell walls, leaving the lignin behind. The residue is brown, brittle, and cracks into characteristic cuboid blocks. Brown rot removes the structural cellulose that gives wood its strength, leaving a hollow-feeling lightweight residue. Affected timber may retain its shape and surface appearance while being almost completely structureless internally.

White rot attacks both cellulose and lignin simultaneously, leaving a white, stringy, fibrous residue. It is most common in hardwoods and produces a bleached, spongy surface. Soft rot occurs in very wet conditions — timber in ground contact or consistently saturated — producing a softened surface layer that can be scraped away while the interior remains sound.

• Brown rot: cuboid cracking, brown residue — attacks cellulose; very common in conifers
• White rot: white, stringy fibrous residue — attacks cellulose and lignin; common in hardwoods
• Soft rot: surface erosion in very wet conditions — scrapes away from still-sound interior
• All types cause catastrophic strength loss — wood may look intact while structurally failed

How to Identify Decay in Structural Timber

Early-stage decay may be invisible on the surface. The most reliable field test is probing with a pointed instrument — a screwdriver, awl, or knife. Sound timber resists penetration; decayed timber offers little or no resistance. Probe in areas of highest moisture risk: timber ends embedded in masonry, areas of known moisture ingress, timber near gutters or roof penetrations, and any timber showing surface discolouration.

Brown rot at an early stage produces a slightly darker colour and wrinkled surface texture — the wood has shrunk internally but the surface has not yet cracked. Late-stage brown rot is unmistakable: the wood crumbles under finger pressure. White rot produces a bleached, spongy surface with a stringy texture when surface fibres are pulled.

How to Stop Active Decay and Prevent Recurrence

The first step is to address the moisture source. Decay cannot continue below 20% MC — removing moisture kills the fungus, though it does not reverse damage already done. Fix the leak, improve drainage, or increase ventilation.

Once the moisture source is addressed, structurally compromised timber must be replaced. Do not attempt to reinforce visibly decayed structural members. For adjacent sound timber that has been exposed to damp conditions, in-situ borate treatment — drilling and injection with concentrated borate solution — provides protection while the moisture issue is resolved. Specify VPI-treated replacement timber to prevent recurrence.