Woodworm: How to Identify and Treat It
Nearly every period property has woodworm holes somewhere. The questions that matter: is it active, which beetle is it, and has it done structural harm? This guide answers all three.
How do you tell if woodworm is active? Look for fresh evidence, not just holes: frass — pale, gritty bore dust like fine sawdust — spilling from holes or dusting surfaces beneath timber is the key activity sign, along with pale-rimmed, clean-edged new flight holes (old ones darken with age and dirt), adult beetles or their bodies on sills between April and September, and new holes appearing in dated or painted surfaces. Most UK infestation is Common Furniture Beetle (Anobium punctatum), making 1–2mm round holes; treatment for active cases is insecticidal (permethrin-based) spray or paste, £400–£1,200 for typical North London treatments. Historic, inactive woodworm needs no treatment at all.
What woodworm actually is
'Woodworm' is the larval stage of several wood-boring beetles. Adults lay eggs in timber crevices; larvae hatch and tunnel through the wood — eating it — for two to five years; then they pupate and the emerging adults chew their way out, leaving the familiar flight holes, and fly off (April–September) to mate and restart the cycle. The holes are exits, not entrances: by the time you see them, that generation's damage is done, and the question is whether the cycle is continuing.
The overwhelming majority of UK cases are Common Furniture Beetle (Anobium punctatum): 1–2mm round holes, sapwood-focused, in floorboards, joists, rafters and furniture. Rarer but more serious: Deathwatch Beetle (Xestobium rufovillosum, ~3mm holes, favours old damp hardwood — oak-framed and historic buildings); and the destructive House Longhorn Beetle (oval holes 6–10mm, mainly a Surrey-belt problem, roof timbers). Species identification changes the response, which is one reason surveys beat sprays-on-spec.
Active or historic? The assessment that saves money
Holes alone prove only that woodworm existed sometime in the last century — a huge share of 'woodworm' in North London's Victorian and Edwardian stock is long-dead history needing nothing but reassurance. Activity evidence: fresh frass (pale, gritty dust from holes or beneath timber — the single best indicator); fresh holes with clean pale edges against older darkened ones; adult beetles alive or dead on sills in flight season; larvae revealed in lifted or cut timber; and new holes in painted, stained or dated surfaces.
Simple monitoring settles borderline cases: vacuum away dust, mark and photograph an area, tape paper beneath suspect timber, and re-inspect through a flight season for new frass or emergence. A professional timber survey (£280–£450, combined with damp) confirms activity, species and — critically — whether any structural sections are weakened.
The moisture connection
Wood-boring beetles prefer damp timber: larvae develop faster and infestations sustain in moisture contents above ~12%, which is why active woodworm clusters where building moisture problems live — under-ventilated subfloors, damp joist ends near failed DPCs, humid roof voids, cellars. Bone-dry modern interiors, by contrast, often starve infestations out naturally.
This makes moisture control genuine woodworm treatment: restoring subfloor ventilation (£150–£900), curing the rising damp wetting joist ends, fixing the roof leak above the rafters. Deathwatch beetle makes the link explicit — it thrives only in damp, part-decayed hardwood, so its presence is always also a damp finding. Any woodworm treatment plan that ignores moisture is treating symptoms.
Treatment: what works and when
For confirmed active Common Furniture Beetle: insecticidal treatment — permethrin-based fluid sprayed onto accessible timber surfaces (floor voids opened, boards lifted as needed) kills adults and larvae as they breach the treated zone; paste and gel formulations penetrate deeper for heavier sections; and structural timbers weakened by decades of tunnelling are repaired or spliced. Typical North London costs: £400–£1,200 for a loft or floor treatment, more where access and repairs extend the scope. Treated timber carries residual protection through subsequent flight seasons; guarantees of 10–20 years are standard from PCA-member firms.
Deathwatch in historic buildings needs a subtler campaign — moisture reduction first, targeted treatment, sometimes heat or monitoring regimes suited to conservation settings. And historic inactive woodworm needs precisely nothing: paying to spray dead holes is the commonest waste of money in timber treatment.
Structure, prevention and when to act
Decades of tunnelling can genuinely weaken timber — springy floors, notched joist ends crumbling at bearings, rafters soft under probing — and a survey distinguishing cosmetic peppering from structural loss is essential before either panic or a sale. Repairs range from sistered joists to replaced sections, priced with the treatment.
Prevention is environmental: keep timber dry (ventilated voids, sound DPCs, maintained roofs), keep humidity moderate, and inspect vulnerable zones — subfloors, lofts, cellars — every few years. Buying a period property? The homebuyer's guide covers how woodworm findings should shape negotiation rather than derail purchases: active infestation is a priced, treatable condition, not a catastrophe.
Related services, guides and tools
Woodworm Treatment
The service this guide supports — diagnosis-first, with independent qualified providers.
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Use the toolReferences & further reading: Property Care Association (PCA) · BRE guidance on wood-boring insects · TRADA (Timber Research and Development Association) timber knowledge. This guide is general information for North London property owners, not professional advice for a specific building — a survey provides that.
Woodworm FAQs
Round flight holes 1–2mm across (Common Furniture Beetle) peppering timber surfaces, with active cases showing pale gritty frass spilling from holes or dusting surfaces beneath. Deathwatch makes ~3mm holes in old hardwood; House Longhorn oval holes 6–10mm in roof softwood.
Fresh frass is the key sign — pale bore dust at or below holes. Clean pale-edged new holes, beetles on sills April–September, and new holes in painted or dated surfaces confirm it. Vacuum, mark, monitor through a flight season for borderline cases, or commission a timber survey.
£400–£1,200 for typical active-infestation treatments in North London (loft or floor scope), rising with access and any structural repairs. Historic inactive woodworm costs nothing — it needs no treatment, whatever a doorstep quote says.
Usually not — most infestation is cosmetic sapwood peppering. But decades of activity can weaken joist ends, floorboards and rafters, so a survey checks bearings and sections wherever activity is confirmed. Springy floors and soft probing warrant prompt assessment.
Because it's damp: blocked airbricks raise void humidity, beetles favour moist timber, and infestations sustain where moisture content stays elevated. Restoring subfloor ventilation is part of the treatment, not an optional extra.
Surface-spraying accessible furniture, perhaps. Building timber is different: activity confirmation, species identification, structural checks and treatment of concealed faces (lifted boards, opened voids) are what make professional treatment effective — and guaranteed.
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