Quick answer: yes, decking in the UK can get woodworm, but it is uncommon. Properly pressure-treated, well-ventilated decking is at low risk. Most damaged decking is caused by wet rot (fungal decay), and the insects found in outdoor timber, especially wood-boring weevils, usually only attack wood that is already damp and decaying. So the fix is almost always to deal with moisture and rot first, then protect the timber.
If you’ve spotted small holes or dust on your deck, this guide from DIY Refurb explains which beetles affect outdoor timber, how to tell woodworm from rot, how to treat affected boards, and how to stop it happening again.
Quick Menu
- What is woodworm?
- Which beetles attack decking?
- Is woodworm a real risk for decking?
- Signs of woodworm in decking
- Woodworm or wet rot?
- How to treat woodworm in decking
- How to prevent woodworm in decking
- Risk comparison table
- FAQs
What Is Woodworm, Exactly?
Despite the name, woodworm isn’t a worm. It’s the larval stage of several wood-boring beetles. Adult beetles lay eggs in cracks, crevices, end grain and old exit holes. The larvae then tunnel through the wood, often for several years, before emerging as adults through small round exit holes, leaving behind gritty wood dust called frass.
Which Beetles Attack Decking?
- Common furniture beetle (Anobium punctatum): the most common woodworm in UK buildings. It attacks the sapwood of softwoods and hardwoods, and leaves neat round exit holes of about 1.5–2mm. Adults emerge in spring and summer. Larvae can feed for three years or more, so damage is usually found in older timber.
- Wood-boring weevils (Pentarthrum huttoni, Euophryum confine): the second most common in UK buildings, and very relevant outdoors. They only attack damp, at least partly decayed timber. Their exit holes are round with ragged edges, and the tunnels often run along the grain just below the surface. Where you find weevils, fungal decay is the real problem.
- Deathwatch beetle: mainly attacks old hardwoods such as oak, usually where fungal decay is present. It is rarely a concern for modern softwood decking.
- House longhorn beetle: attacks softwood sapwood but is uncommon and localised in the UK. It is mainly associated with roof timbers rather than decking.
Is Woodworm a Real Risk for Decking in the UK?
The Short Answer: Yes, But It’s Uncommon
Decking can be infested, but it’s far more likely to suffer from wet rot. Pressure treatment protects against both fungi and insects, so the risk rises mainly when that protection is missing or lost.
When Decking Becomes Vulnerable
- Poor drainage and airflow beneath the deck, keeping timber damp.
- Shaded or overgrown areas where boards never dry out.
- Untreated or under-treated softwood, or boards with sapwood exposed.
- Untreated cut ends and drilled holes, where the treated outer layer has been cut away.
- Existing rot or damage, which invites weevils.
- Neglected maintenance, where coatings wear off and water gets in.
Signs of Woodworm in Decking
- Small round exit holes: about 1.5–2mm for furniture beetle; ragged-edged holes suggest weevils.
- Frass (bore dust): fine, gritty dust around holes or beneath boards. Fresh, pale frass and sharp-edged holes suggest an active infestation. Dark, dirty holes are usually old.
- Tunnels along the grain just below or breaking the surface, typical of weevils.
- Crumbly or weak timber that gives way when probed with a screwdriver.
- Adult beetles, mostly seen in spring and summer.
Holes alone don’t prove an active problem. Check again in spring and summer for fresh holes and new dust.
Woodworm or Wet Rot?
| Sign | Woodworm (furniture beetle) | Wood-boring weevil | Wet rot |
|---|---|---|---|
| Holes | Neat, round, 1.5–2mm | Round, ragged edges | None |
| Dust | Fine, gritty frass | Coarse, gritty frass | None |
| Timber condition | Can be sound and dry around the holes | Always damp and decayed | Soft, spongy or cracked into cubes when dry; darkened |
| Main fix | Insecticidal wood treatment | Fix the damp and rot | Remove the moisture source; replace decayed timber |
On decking, weevil damage and wet rot often go together, so check for soft wood around any holes.
How to Treat Woodworm in Decking
- Inspect and assess: probe boards, joists and posts to see how far the damage goes. Pay particular attention to joists and posts, which carry the load.
- Replace weak or decayed timber: if a board or joist is soft or crumbles, replace it. Treatment won’t restore lost strength. Use exterior-rated fixings such as our green decking screws or browse the decking screws range.
- Fix the cause: improve drainage and airflow, clear debris from gaps and trim back vegetation.
- Clean and dry: brush off dirt and let the timber dry out.
- Apply an exterior-suitable preservative that also protects against insects: on clean bare wood, apply a preservative listed for exterior use against wood-boring insects, following the label. Check our wood treatments and timber treatments.
- Seal cut ends on any new timber with an end-grain preserver.
- Finish and maintain: once dry, apply your chosen decking finish, and keep up regular maintenance.
Products for Treating Decking
- EVERBUILD Lumberjack Triple Action Wood Treatment: solvent-free, for interior and exterior timber; kills and protects against wet rot, dry rot, fungal decay and wood-boring insects, can be applied to damp timber and is touch dry in about 1 hour.
- Protek Wood Preserver ++: water-based, for exterior and interior timber, protects against wood-destroying fungi and wood-boring insects; covers 6–8m² per litre and also acts as a primer for a top coat. Contains permethrin, so keep cats away during treatment and drying.
- Barrettine Wood Preserver Clear: deep-penetrating oil/spirit preservative against wet rot, wood-discolouring fungi and wood-boring insects; covers about 6m² per litre.
- Sikagard Wood Preserver Clear: solvent-free, interior and exterior, protects against wet and dry rot, decay, fungi and wood-boring insects; covers about 3–4m² per litre.
A note on woodworm killers and boron treatments: several woodworm killers and boron-based preservatives we sell, such as Lumberjack Woodworm Killer and Boracol 10, are designed for internal timbers like joists, floorboards and furniture. For decking, choose a product whose label lists exterior use.
For timing advice, read our guide to the best time of year to treat woodworm. Treat on a dry day when the timber is dry and no rain is forecast.
How to Prevent Woodworm in Decking
- Buy correctly treated timber: under the decking industry’s guidance, deck boards should be treated to Use Class 3 (exterior, above ground) and posts, joists and beams to Use Class 4 (ground contact or structural), to BS 8417. Ask for treatment details and keep them.
- Treat every cut end and drilled hole: cutting exposes untreated wood inside the board. Brush on Barrettine Timber Cut End Preserver, made for end grain on decking and pressure-treated timber, with a green tint to match tanalised wood.
- Keep it dry and ventilated: leave gaps between boards, keep the area under the deck clear, and make sure water drains away.
- Clean and recoat regularly: remove leaves and algae, and refresh protective finishes. EVERBUILD Lumberjack Wood Preserver protects against rot and fungi, and Kingfisher K-X10 Wood Stain offers up to 5 years’ protection against fungal decay, rain and UV on exterior timber. Check each product’s suitability for walked-on surfaces. See our wood stains, oils and primers.
- Use the right fixings: see does the type of wood affect which screws you should use?
For a year-round routine, follow our seasonal guide to caring for your decking.
Woodworm Risk in Decking: Comparison Table
| Condition | Risk | Why it matters |
|---|---|---|
| Pressure-treated softwood (correct Use Class) | Low | Preservative protects against fungi and wood-boring insects. |
| Untreated softwood | High | No protection against decay or insects outdoors. |
| Untreated cut ends | Medium | Exposes untreated wood inside the board. |
| Regularly cleaned and maintained deck | Low | Keeps moisture down and protection topped up. |
| Damp, shaded location | Medium to high | Slow drying encourages rot, and with it weevils. |
| Sunny, well-ventilated location | Low | Timber dries quickly after rain. |
| Decking over damp soil with no airflow | High | Persistent damp encourages both rot and insect attack. |
| Damaged or rotting boards | High | Wood-boring weevils only attack damp, decayed timber. |
Decking Woodworm FAQs
Can treated decking get woodworm?
It’s unlikely if it was treated to the right Use Class and the cut ends were sealed. Risk increases if the timber rots or the treatment is cut away and left untreated.
What do small holes in my decking mean?
Neat 1.5–2mm round holes with gritty dust usually mean furniture beetle. Ragged holes in soft, damp wood suggest weevils and rot. Holes without fresh dust may be old.
Is it woodworm or rot?
Rot leaves no holes; the wood goes soft, spongy, or cracks into cubes. Woodworm leaves exit holes and frass. On decking, both often appear together.
When is the best time to treat woodworm in decking?
Spring and summer, when beetles are active and timber is dry, but follow the product label. Our woodworm timing guide explains more.
Can I use an indoor woodworm killer on my deck?
Only if the label lists exterior use. Many woodworm killers are designed for internal timbers. Choose an exterior-rated preservative that covers wood-boring insects.
Do I need to replace boards with woodworm?
If the board or joist is soft, crumbly or weakened, yes. If damage is minor and the timber is sound, treatment and better ventilation may be enough.
Final Thoughts: Don’t Let Woodworm Wreck Your Deck
Woodworm isn’t the most common decking problem in the UK, but damp, rot and neglect make it more likely. Choose correctly treated timber, seal cut ends, keep the deck dry and ventilated, and act quickly if you see fresh holes or soft wood. Browse DIY Refurb’s wood treatments to protect your deck.
