Woodworm vs Dry Rot: How to Tell the Difference

Woodworm and dry rot are both found in the same places — dark, damp, poorly ventilated timber in older British properties — and both cause serious timber damage. But they are completely different problems: one is an insect infestation, the other is a fungal disease. Confusing the two leads to the wrong treatment, money wasted and the underlying problem left to grow.

This guide explains how to distinguish them, what causes each, and what to do about both.

What is woodworm?

Woodworm is the collective name for infestations by wood-boring beetle larvae — most commonly the common furniture beetle (Anobium punctatum), but also deathwatch beetle, house longhorn beetle and powderpost beetle.

The damage is caused by larvae boring tunnels through timber over three to five years. The visible signs are exit holes (1–3mm round), bore dust (pale, powdery frass), and softening of the timber surface over time.

Woodworm is an insect infestation. It requires conditions that attract adult beetles to lay eggs — unfinished timber surfaces, and a moisture content above about 12%.

What is dry rot?

Dry rot is caused by the fungus Serpula lacrymans, which breaks down and digests the cellulose in wood, leaving only the lignin structure behind. This causes the characteristic cuboidal cracking and powdering of severely affected timber.

Despite its name, dry rot requires moisture to grow — it needs timber at above 20% moisture content to become established. The “dry” refers to how the timber looks in the late stages of decay, when the fungus has extracted moisture from the wood and the rotted material crumbles and powders.

Dry rot is a fungal disease. It requires a source of moisture and can spread through masonry, mortar and other non-timber materials — a critical difference from woodworm.

Key differences at a glance

Feature	Woodworm	Dry rot
Cause	Wood-boring beetle larvae	Serpula lacrymans fungus
Visual signs	Round exit holes (1–3mm), bore dust	Cuboidal cracking, orange/brown fruiting bodies, white mycelium
Smell	None	Distinctive musty, mushroom-like odour
Timber feel	Softening in heavy infestations	Dry, crumbly, powders when squeezed
Spreads through masonry?	No	Yes — mycelium passes through brick and mortar
Moisture needed	12%+ for egg-laying	20%+ to establish
Treatment	Insecticide (permethrin, boron)	Fungicide, masonry treatment, source of damp must be fixed

How to identify woodworm

• Round holes: 1–2mm (furniture beetle), 3mm (deathwatch), 6–10mm oval (longhorn)
• Bore dust: Fine, pale, powdery frass beneath and around holes
• Adult beetles: Reddish-brown insects 3–5mm long, found on windowsills May–September
• Tunnels: Visible when timber is cut or broken — winding galleries packed with frass
• Location: Concentrated in sapwood layers

How to identify dry rot

• Cuboidal cracking: Timber splits into rectangular blocks with right-angle cracks — unlike the grain-following tunnels of woodworm damage
• White mycelium: Cotton wool-like white growth on timber surfaces and adjacent masonry — can appear in the absence of visible fruiting bodies
• Fruiting bodies: Distinctive flat, pancake-shaped bodies with an orange-red pore surface and white margin — mushroom-like smell
• Brown rot: Affected timber turns a deep reddish-brown and crumbles easily when dry
• Musty smell: Strong, distinctive mushroom or cellar smell — not present with woodworm
• Spread: Dry rot mycelium penetrates and passes through masonry, potentially appearing in rooms remote from the original moisture source

Can woodworm and dry rot occur together?

Yes — and this is common. Both thrive in the same conditions: cool, damp, poorly-ventilated timber. Roof spaces and sub-floor voids with rising damp, blocked ventilation or leaking plumbing often harbour both problems simultaneously.

If you find woodworm in sub-floor joists, check carefully for signs of fungal decay as well. Areas where joist ends are built into damp masonry are particularly likely to show both woodworm and early-stage rot.

Different causes, different solutions

Woodworm treatment: Professional insecticide treatment (permethrin spray for accessible softwood; boron gel for structural timber). Addressing damp as a secondary measure to remove conditions that favour egg-laying.

Dry rot treatment:

1. Find and fix the moisture source — dry rot cannot survive without it
2. Cut out all affected timber well beyond visible decay (typically 300–500mm into apparently sound wood)
3. Treat all exposed masonry with fungicidal solution
4. Apply masonry biocide to all surfaces in the affected area
5. Replace timber with pre-treated wood
6. Improve ventilation

Dry rot treatment is more invasive and expensive than woodworm treatment — and cannot succeed without identifying and eliminating the source of moisture. A fungicide treatment applied to timber while the damp persists will fail.

What to do if you are unsure

The smells, visual signs and timber feel are usually distinctive enough to tell the two apart — but not always. A professional timber survey is the most reliable way to get an accurate diagnosis. Our specialists treat both woodworm and the damp and rot conditions that often accompany it.

Read more: Signs of woodworm · Is woodworm dangerous? · Dry rot and damp treatment