Woodworm survey: how surveyors identify an active infestation

Woodworm is one of the most frequently flagged timber issues in a property survey, particularly in older homes. In many cases the infestation is historic and no treatment is needed, but where it is active the situation needs proper assessment before you commit to a purchase.

This guide looks at what woodworm is, the signs surveyors look for, how they distinguish active from inactive infestations, what a survey and treatment are likely to cost and what woodworm could mean for your mortgage. For an overview of all the issues a survey can flag, see our common survey problems guide.

• Woodworm is the larval stage of several wood-boring beetle species; the common furniture beetle is responsible for the majority of UK infestations
• Surveyors look for flight holes, bore dust known as frass and signs of structural weakening in affected timber
• Many infestations found during surveys are historic and inactive, requiring no treatment
• A specialist woodworm survey typically costs £80 to £250; professional treatment ranges from £150 for a localised problem to over £3,000 for a whole-property infestation
• Active woodworm can cause a lender to pause a mortgage application until a specialist has confirmed the infestation and any necessary treatment has been completed

Despite its name, woodworm is not a worm in the biological sense. The term refers to the larval stage of several species of wood-boring beetle, which burrow through the timber in a property, feeding on the wood fibres and leaving a network of tunnels behind. A severe infestation can weaken timber to the point where it crumbles and can no longer carry the load it was designed to bear.

Several species of wood-boring beetle are found in UK properties, but a handful account for most cases:

• The common furniture beetle (Anobium punctatum) is responsible for around 75 to 80 per cent of all woodworm damage in UK homes. It is a chocolate-brown beetle, 2.5 to 5mm in length, and attacks the sapwood of both softwoods and hardwoods. It is commonly found in structural joinery and floorboards as well as older furniture.
• The house longhorn beetle is larger, up to 20 to 25mm in length, and targets the sapwood of softwood timbers such as pine and spruce. It is more prevalent in roof timbers and is notifiable to the local authority in parts of Surrey and the Home Counties.
• The deathwatch beetle targets aged hardwoods, particularly oak that has been softened by fungal decay. It is most commonly found in historic and older buildings and is identified in part by the tapping sound the adult makes against the timber surface.
• The wood-boring weevil attacks timber that is already damp and decaying, and is often found in sub-floor timbers or around persistent moisture sources. Unlike the other species, it requires no insecticide treatment; eliminating the source of moisture is sufficient.

The adult female beetle lays eggs in the cracks, crevices and old flight holes of timber. Once the eggs hatch, the larvae burrow into the wood and feed for between two and five years before emerging as adult beetles, leaving behind the characteristic holes that signal an infestation.

combination of the following:

• Flight holes: small, round holes in the surface of timber where adult beetles have emerged. The common furniture beetle leaves holes of around 1 to 2mm in diameter; deathwatch beetle holes are larger.
• Frass: the fine, powdery bore dust pushed out by emerging beetles. Fresh frass is cream-coloured and slightly gritty; older frass darkens and may be caught in cobwebs.
• Live or dead beetles: adult beetles are most likely to be found between May and September during the flight season. Their presence, or the carcasses they leave behind, can help confirm a current infestation.
• Structural weakness: in heavily infested timber, the wood may feel crumbly or friable, and visible cracking or weakening may be apparent, particularly in joists, floorboards and roof timbers.

One of the most important judgements a surveyor makes when woodworm is found is whether the infestation is active or historic. This distinction determines whether treatment is needed at all.

Signs of an active infestation include flight holes with clean, crisp edges, indicating the beetle emerged recently, fresh frass that is pale in colour and looks undisturbed, and live or dead beetles present around the affected timber, particularly in spring and summer.

Signs of an inactive infestation include flight holes with worn or darkened edges, older frass that has discoloured, dried hard or gathered in cobwebs, and no beetles visible at any time of year.

The surveyor will assess the timber carefully and use small probes to check whether the wood has retained its structural integrity. Moisture meters are also used, since damp timber is more susceptible to woodworm and other timber decay. Where access to the affected area is limited, a borescope or thermal imaging camera may be used to inspect the timber without causing disruption.

In many cases, particularly in pre-1960s properties, flight holes in softwood joists and floorboards represent a historic infestation that has since died out as the timber dried. PCA guidance is clear that treatment should only be carried out where there is evidence of active infestation. Treating old, inactive woodworm is unnecessary and a waste of money.

Yes. Both a Level 2 and Level 3 survey include a visual and physical inspection of accessible timber for signs of woodworm. A Level 2 survey will note any suspected infestation and its extent. A Level 3 survey provides a more detailed timber assessment, which is particularly worthwhile for older properties where the conditions for woodworm are more likely to have been present.

Where the surveyor suspects an active infestation, or where access is limited and the extent of the damage cannot be established, they may recommend a specialist timber report from a PCA-qualified surveyor before you exchange contracts.

Where a specialist survey confirms active woodworm, treatment options and costs depend on the extent of the infestation:

• Specialist woodworm survey: £80 to £250 depending on the size of the property and the scale of the suspected problem
• Localised treatment, such as a single room or a few floorboards: £150 to £500
• Whole-property treatment, including full fumigation where the infestation is widespread: £500 to £3,000 or more depending on property size and access

Treatment typically involves applying an insecticidal spray to the affected timber, with a deeper penetrating gel used where a more thorough treatment is needed. Structurally weakened timber may also need to be cut out and replaced, which will add to the overall cost.

DIY treatment products are available at £30 to £100, but any work carried out without a specialist will not come with a guarantee or warranty, which mortgage lenders may require.

Other cost factors include the location of the property, since south-east England tends to be more expensive than Wales or the north of England, the length and terms of any guarantee, and the cost of removing and disposing of any affected timber.

Whether woodworm affects your mortgage application depends on whether the infestation is active and whether it has caused any structural damage.

If the surveyor confirms the woodworm is inactive and there are no residual structural issues, a lender is unlikely to refuse the mortgage, though they may ask for evidence that any previous treatment was carried out by a specialist and backed by a guarantee.

If the infestation is active, the lender is more likely to pause the mortgage offer until a PCA-qualified specialist has assessed the problem and any necessary remedial work has been completed and certified. In some cases, the lender may retain part of the loan until a signed-off specialist report confirms the work is done.

A specialist report also gives you a clear basis to renegotiate the asking price with the seller, either to factor in the cost of treatment or to ask them to carry out the work before completion.

A RICS Level 2 or Level 3 survey is the right first step for any property purchase. Get a free quote or call our friendly team on .