Woodworm is one of those property problems that sounds alarming but is frequently misunderstood. Most woodworm found in UK homes is either inactive, confined to furniture, or limited to non-structural timber — and in many cases requires no treatment at all. The minority of cases that involve structural timber or the more serious species do need professional attention. This guide explains how to tell the difference.
What Woodworm Actually Is
"Woodworm" is not one species but several. The damage is caused not by worms but by the larvae of wood-boring beetles, which tunnel through timber feeding on the cellulose and starch content. The characteristic round exit holes appear when the adult beetle emerges to mate, typically during the summer flight season.
The three species that matter in UK homes are:
Common furniture beetle (Anobium punctatum) — responsible for the vast majority of woodworm in UK housing. Prefers sapwood of softwood and European hardwoods. Exit holes are 1–2mm in diameter. Risk level: moderate. Rarely causes structural failure in well-seasoned, well-ventilated timber.
Deathwatch beetle (Xestobium rufovillosum) — found almost exclusively in hardwood, particularly old oak in historic or listed buildings. Exit holes are 3mm in diameter. The characteristic tapping sound is the adult beetle knocking its head against timber walls. Risk level: high. Can cause significant structural deterioration over time if untreated.
House longhorn beetle (Hylotrobus bajulus) — the most destructive species. Restricted primarily to parts of Surrey and the surrounding counties in the UK, and notifiable under the Forestry Commission's rules. Attacks softwood roof timbers, potentially causing catastrophic structural damage. Exit holes are 6–10mm oval-shaped. Risk level: very high.
How to Identify Active vs Inactive Infestation
This distinction is critical — treating a long-dead infestation is a waste of money, while leaving an active one risks cumulative timber damage.
Active infestation:
- Fresh exit holes with fine, cream-coloured bore dust (frass) visible around or beneath the holes — like fine sawdust
- Frass feels gritty and powdery between your fingers (old frass becomes hard and compacted)
- New holes have clean, sharp edges with a pale interior
- Active flight season is May to September — inspect timber closely during and after this period
Inactive infestation:
- Exit holes are present but no frass visible (or only hard, compacted old frass)
- Holes have a darkened interior from years of air exposure
- No new holes since last inspection
When in doubt, photograph the affected area in May and again in September. New holes appearing over the summer confirm activity.
Which Timber Is Most at Risk?
The common furniture beetle strongly prefers sapwood — the outer, lighter-coloured wood of a log — over denser heartwood. Poorly ventilated, slightly damp timber accelerates activity.
Most at-risk timbers:
- Subfloor joists — particularly in older properties with inadequate subfloor ventilation
- Floorboards — especially in the sapwood edges
- Roof timbers — purlins, rafters, and sarking in Victorian and Edwardian roofs
- Old furniture and joinery — particularly items brought from storage or purchased second-hand
- Staircase timbers — spindles, handrails, and older softwood strings
Modern kiln-dried structural timber treated with preservative is significantly less vulnerable than Victorian-era softwood.
DIY Treatment: Products, Process, and Safety
DIY treatment is appropriate for non-structural timbers with an active common furniture beetle infestation. It is not appropriate for structural timbers, deathwatch or longhorn beetle, or extensive roof infestations.
Products: Permethrin-based woodworm treatment fluid is the standard DIY option. Widely available brands include Cuprinol, Rentokil, and Barrettine. These are oil-borne solvent products that penetrate the timber and kill larvae on contact.
Application process:
- Remove furniture and cover flooring — the product stains and is difficult to remove
- Apply by brush, roller, or low-pressure garden sprayer to all exposed timber surfaces, paying particular attention to flight holes, joints, and end grain
- Apply two coats, allowing the first to dry according to the manufacturer's specification before applying the second
- Ensure larvae at depth are reached — end grain is the primary entry point
Safety (HSE COSHH considerations): Permethrin-based fluids are classified as hazardous substances. Comply with the product's Control of Substances Hazardous to Health (COSHH) data sheet:
- Wear nitrile or PVC gloves, safety goggles, and a P2 respirator (dust/mist mask is not adequate)
- Ensure thorough cross-ventilation — open all windows and doors; do not remain in the room while the product is wet
- Keep children and pets out of the area until fully dry (typically 24–48 hours)
- Dispose of excess product in accordance with local authority hazardous waste guidance — do not pour down the drain
Coverage is approximately 4–6m² per litre per coat on bare timber; less on absorbent or rough-sawn surfaces.
When to Call a Professional
Call a PCA-accredited woodworm treatment specialist if:
- Structural timbers are affected — joists, rafters, bressumers, or any load-bearing element
- Deathwatch or longhorn beetle is present or suspected — the timbers involved and the treatment approach differ significantly from common furniture beetle
- An active infestation is found in roof timbers — safe access and heat treatment or professional insecticide may be needed
- The infestation is extensive — whole-house treatment requires consistent, professional application
- You need a guarantee for mortgage or sale purposes — a surveyor or lender will require a guarantee from a PCA-accredited contractor, not DIY evidence
What Does Woodworm Treatment Cost?
| Work | Cost |
|---|---|
| DIY treatment products (per litre) | £15–£50 |
| DIY product coverage | ~4–6m² per litre per coat |
| Professional treatment, single room | £150–£300 |
| Professional treatment, whole house | £500–£1,500 |
| Roof void treatment (professional) | £300–£800 |
| Structural timber replacement (if required) | £1,000–£10,000+ |
Professional treatment typically includes a 20-year guarantee from a PCA-accredited contractor, backed by an insured warranty scheme. This is the standard required by mortgage lenders.
Surveyor and Mortgage Lender Perspective
A RICS Level 3 Building Survey (formerly Full Structural Survey) will flag active woodworm in any accessible timber, and surveyors are required to note it in their report. Common furniture beetle in non-structural timber will typically be flagged as a defect but not a major structural concern. Deathwatch or longhorn beetle, or extensive structural involvement, will be flagged as a high priority or urgent defect.
Mortgage lenders will often require evidence that active woodworm in structural timbers has been treated by a PCA-accredited contractor with a minimum 20-year guarantee before releasing funds.
If you are selling, commissioning pre-emptive treatment before marketing will avoid renegotiation later.
Prevention: Keeping Woodworm Out
The common furniture beetle thrives in damp, poorly ventilated timber. Addressing the conditions that favour it is the most effective long-term prevention:
- Ventilate the subfloor — ensure airbricks are clear and unobstructed; a subfloor relative humidity above 60% creates ideal conditions for activity
- Control indoor humidity — aim for 40–60% relative humidity throughout the property; use a dehumidifier if necessary
- Inspect second-hand furniture before bringing indoors — a classic route of introduction; check thoroughly for exit holes and treat any active infestation before it can spread to structural timber
- Ventilate roof spaces — inadequate eaves ventilation creates warm, damp conditions that attract egg-laying beetles
- Treat exposed timbers in outbuildings and garages — a preventive coat of permethrin-based fluid on any untreated external softwood is low cost insurance
Managing the Process with Renovate Me
Woodworm treatment is rarely a standalone job — it often sits alongside damp proofing, subfloor ventilation works, or roof repairs that need sequencing correctly. Renovate Me maps the full renovation process, helping you understand what needs to happen and in what order, so nothing gets missed and no contractor gets called before the work is ready for them.