Is my woodworm active or is it old damage?

Woodworm holes in an old floor set off alarm about a house that is being eaten. In most UK houses the holes are historic — the record of an infestation that ended when the timber dried out, sometimes decades ago. Treatment is often sold on the presence of holes alone, which is exactly like diagnosing rising damp from a meter reading.

Safety

What you'll need

• Torch
• Black paper or a dark cloth
• Soft brush
• Magnifier (optional)
• Pin-type moisture meter (optional)

Step by step

1. Look at the holes closely.Fresh exit holes are sharp-edged and pale inside — the colour of clean new wood. Old holes are dark, weathered at the rim, and often have dust or polish in them.
2. Look for frass.Frass is the fine powder pushed out by emerging beetles. Fresh frass is pale, gritty between the fingers, and sits in small heaps or trails below the holes.
3. Do the paper test.Clean the area thoroughly, lay black paper or a dark cloth under the suspect timber, and leave it for several weeks through late spring and summer. New frass, or dead beetles, means the infestation is live. Nothing means it is not.
4. Check the season.Most UK emergence is from around May to August. Looking for activity in January and finding none proves very little, which is why the paper test needs to span the right months.
5. Measure the timber's moisture.A pin meter in the wood is one of the few readings a meter gives honestly. Common furniture beetle, which is the great majority of UK woodworm, needs damp timber. Below about 12 per cent the timber is not a viable host.
6. Identify the timber and the hole size.Very small round holes in sapwood are typical of common furniture beetle. Larger holes, oval holes, or holes in hardwood suggest a different species — deathwatch beetle in old oak, or house longhorn in some areas — and those are assessed differently.
7. Find the moisture source if the timber is damp.This is the actual fix. A blocked airbrick, an unventilated floor void, a leak, a bridged wall. Dry the timber and the infestation ends on its own.
8. Assess whether the timber is weakened.Probe with a bradawl and look at the density of holes. A scattering of holes in a sound joist is cosmetic. Timber that crumbles or has holes over its whole section is a structural question.
9. Treat only if it is genuinely active.And treat the specific timbers, not the whole house. Where treatment is warranted, a surface application to accessible timber by a competent contractor is normal; blanket spraying of every timber in a building is rarely justified.

Tips

• Woodworm and rot travel together, because both are moisture problems. Finding one is a reason to look for the other.
• If a survey flags woodworm during a house purchase, ask specifically whether activity was demonstrated or only holes observed. It changes the negotiation and the remedy entirely.
• Furniture and floorboards can be brought into a house already infested. If new holes appear in one item and nowhere else, look at that item rather than the building.

Common mistakes

• Spraying the whole floor because there are holes — Most holes are historic. Spraying an inactive infestation puts insecticide into a home for no benefit, and it does nothing about the damp that allowed the original outbreak.
• Filling the holes to see if new ones appear — It sounds sensible but wax or filler distorts the appearance of new emergence and makes the paper test harder to read. Clean and observe instead.

Questions people ask