Damp & timber guides
How to tell if woodworm is active
Written by Damptech · Updated August 2026
Key points
- Active woodworm: clean, pale, sharp-edged exit holes and fresh gritty bore dust (frass).
- Inactive: dark, weathered holes, no dust, old paint/polish inside them.
- Live or freshly-dead beetles indoors May to August is a strong active sign.
- Damp timber attracts it: an infestation is often a damp problem too.
Not every woodworm hole means you have a live problem. Plenty of houses have old, long-dead infestations that need no treatment at all. The question that matters is whether beetles are currently tunnelling. Here’s how to judge it.
Signs the infestation is active
- Fresh exit holes. Clean, round, roughly 1 to 2mm across, with light-coloured, sharp edges the same pale colour as freshly broken timber.
- Bore dust (frass). Fine, gritty, biscuit-coloured powder on or below the timber, on skirtings, or on surfaces under joists and rafters. Fresh frass is the single best indicator of activity.
- Live or freshly dead beetles. Small reddish-brown beetles (2 to 5mm) on windowsills, in cobwebs or on the floor near timber, in late spring and summer.
- Tunnels in broken timber that look clean and pale rather than dirty and oxidised.
Signs it’s old and inactive
- Holes with dark, weathered, rounded edges
- Old paint, polish, limewash or stain visible inside the holes (they pre-date the finish)
- No frass at all, even after you’ve cleaned nearby surfaces and re-checked weeks later
- Cobwebs across the holes; dust in them that is grey household dust, not pale frass
A useful test: vacuum and wipe the area, lay down white paper or masking tape below suspect timber, and check after a few weeks in spring/summer. New frass on the paper means activity.
Which beetle?
Most UK woodworm is Common Furniture Beetle. It’s worth identifying others because they’re handled differently:
- Deathwatch Beetle: larger holes (~3mm), typically in old oak and hardwood in historic buildings, often with fungal decay.
- House Longhorn Beetle: large oval holes, serious structural damage to softwood roof timbers; restricted to certain areas.
- Wood-boring weevil: associated with already-decayed, damp timber; usually resolves once the timber dries.
Why damp matters here
Furniture beetle larvae do far better in timber with a raised moisture content. A genuinely active infestation is often a sign that timber is getting damp: a slipped tile over the loft, blocked sub-floor airbricks, or penetrating damp at a gable. Treating the beetles without drying the timber is only half a solution.
If it’s active
Active infestations are treated by applying a Biokil Crown approved insecticide to all accessible timber, after clearing lofts and lifting insulation, and with any moisture source addressed. Where beetle attack has combined with decay to weaken joists or rafters, structural repair may be needed too. A survey, often requested by a mortgage lender on a purchase, will confirm whether treatment is actually required.
This guide is general information, not a diagnosis. If you’re dealing with damp or timber problems in North Somerset, book a survey and we’ll tell you what’s actually going on.
Common questions
I found old holes but no dust. Do I still need treatment?
Possibly not. Holes with no fresh frass, weathered grey edges and old paint or polish inside them often indicate a historic, inactive infestation. A surveyor may recommend monitoring rather than treatment.
When do woodworm beetles emerge?
For Common Furniture Beetle, mainly May to August. Finding live or freshly dead beetles indoors in that window, near affected timber, is a strong sign of an active infestation.
Does woodworm mean I have a damp problem too?
Often. Furniture beetle strongly prefers timber with a raised moisture content, so an active infestation frequently points to a damp issue (a roof leak, poor sub-floor ventilation or penetrating damp) that needs dealing with too.
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