Woodworm & Timber Treatment

Woodworm is a term used for the larval stage of several wood-boring beetles whose grubs tunnel through timber, leaving behind a network of galleries and the familiar round exit holes. In most UK properties it is a cosmetic nuisance rather than a structural emergency, but knowing how to identify the species, assess whether an infestation is active, and choose the right treatment will save you from either unnecessary expense or from ignoring a problem that could genuinely weaken timbers over time.

Which Beetles Cause Woodworm in the UK?

Four species are commonly encountered in UK homes:

The Common Furniture Beetle accounts for the overwhelming majority of woodworm encountered in residential properties. It affects both softwood and hardwood, prefers damp or slightly humid conditions, and exits the timber as an adult beetle between May and August. Its small exit holes (1–2 mm) are what most people recognise as woodworm.¹

The House Longhorn Beetle is more serious but geographically restricted in the UK: it is mainly found in north-west Surrey and neighbouring parts of Hampshire and Berkshire. It can cause significant structural damage to roof timbers and is the only UK woodworm species that Building Regulations guidance singles out. Approved Document A lists the areas, including Elmbridge, Runnymede, Woking and parts of Hart and Bracknell Forest, where new softwood roof timbers should be treated against it.²

Is the Infestation Active?

Distinguishing active from old (dead) woodworm is critical before spending money on treatment.

Signs of active woodworm:

• Fresh, clean, pale-coloured exit holes (not filled with dust or discoloured)
• Fine, creamy-yellow powdery dust (frass) in or below the holes, it looks like sawdust but finer¹
• Small oval beetles (the adult Anobium punctatum is 2.5–5 mm long, reddish-brown) appearing in the room between May and August
• The frass still feels soft or powdery, not compressed or discoloured

Signs of old, inactive infestation:

• Exit holes with dark or dirty edges (oxidised over time)
• No frass, or frass that is grey/dark and compressed
• No beetle sightings in several seasons

Old woodworm holes are extremely common in Victorian and Edwardian timber, the vast majority of Victorian floorboards have some woodworm holes from historical infestations that have been dead for decades. Treatment of inactive woodworm in non-structural timber is unnecessary.¹

When Is Treatment Necessary?

Treatment is warranted when:

• The infestation is active (fresh holes and frass present)
• The affected timber is structural, joists, rafters, purlins, ridge boards, floor bearers
• The affected area is significant (more than one or two isolated exit holes)

Treatment is not urgently necessary when:

• The woodworm appears inactive
• The timber is non-structural (furniture, skirting boards, decorative panelling)
• The affected area is a small section of isolated holes without current frass

Treatment Options

DIY Treatment

For non-structural timber and confirmed active but minor infestations, proprietary woodworm killer products are widely available. These are typically permethrin-based or boron-based liquids applied by brush or spray. Apply generously to all surfaces and into holes, allow to soak in, and repeat as the label directs: Ronseal’s Woodworm Killer, for example, asks for three coats, two hours apart.³

DIY treatment is suitable for: furniture, skirting boards, non-structural panelling, small areas of floorboarding where the boards can be lifted to treat the underside and the joist tops.

DIY treatment is not suitable for: roof timbers (access and coverage are the issues), whole-floor treatment of a suspended ground floor (impractical and inadequate coverage), or structural damage requiring repair rather than treatment.

Professional Treatment

Professional treatment uses two main approaches:

Permethrin spray treatment, the most common method, where a professional applies a water-based permethrin insecticide by spray or brush to all accessible timber surfaces. In roof spaces this typically means treating all visible softwood. Occupants and pets are normally kept out of the treated areas during application and until the surfaces are dry, as the product label directs. Firms usually give a long-term written guarantee; ask whether it is insurance-backed, so that it still holds if the firm stops trading.

Boron-based treatment, boron salts (usually disodium octaborate) are applied as a gel, paste, or liquid. Boron is less toxic to mammals than permethrin and is considered preferable in occupied homes, around food preparation areas, and where children are present. It is also effective against fungal decay. Boron treatment is slower-acting and requires the timber to have some residual moisture to carry the boron into the wood, so it is less suitable for very dry softwood.

Fumigation, treatment with a toxic gas, used only in extreme cases or for heritage buildings where spray penetration is inadequate. Requires complete evacuation and specialist contractors.

Structural Damage Assessment

Where woodworm has been active over many years, particularly Deathwatch Beetle in old oak or House Longhorn in roof timbers, structural timber may have been weakened. Before treatment, a structural engineer or chartered surveyor should assess whether any timber members need sistering (adding a new joist alongside a damaged one), reinforcing, or replacing entirely. Treatment alone will not restore the strength of heavily damaged timber.

Cost Guide

Prevention

Woodworm needs damp timber: if timber is kept at a sufficiently low moisture content, it is highly unlikely to sustain an infestation.¹ Reducing humidity in roofspaces and subfloor voids is the most effective long-term prevention. A cold roof space above an insulated ceiling should be ventilated to carry moisture away.⁴ BS 5250:2021, the code of practice on managing moisture in buildings, gives the detailed recommendations.⁵ Keep the airbricks to a suspended timber floor clear: Approved Document C asks for ventilation openings on opposite sides equal to at least 1,500mm² per metre run of external wall or 500mm² per m² of floor area, whichever is greater.⁴ Treating timbers with a preservative before installation (pressure impregnation or surface treatment) is standard on new structural softwood used externally or in contact-with-ground situations, and most modern timbers used in damp-prone areas are pretreated.¹

Any timber salvaged for reuse, particularly reclaimed floorboards or structural beams, should be inspected carefully for signs of active woodworm before installation, and treated if any doubt exists.

Choosing a Contractor

Use a contractor registered with the Property Care Association (PCA) or a surveyor who holds its CSRT (Certificated Surveyor in Remedial Treatments) qualification or the CSTDB that replaced it in 2018.⁶ Request a written survey report identifying species and extent of infestation before accepting a treatment quotation. Be wary of surveys conducted by companies offering treatment, where there is an obvious incentive to over-specify.