Checking your subfloor timber for borer damage is something most NZ homeowners can do themselves with a torch, a sharp tool, and a bit of know-how. Catching borer early can save you thousands in structural repairs, so a regular inspection — ideally every year or two — is well worth the effort.
Understanding Borer in NZ Homes
The most common culprit in New Zealand homes is the common house borer (Anobium punctatum), a small beetle whose larvae tunnel through untreated timber for several years before emerging as adults. Older homes built before the widespread use of treated timber are particularly vulnerable. Borer tends to be more active in damp, poorly ventilated subfloor spaces — a common issue in many parts of NZ, especially in cooler, wetter regions like the West Coast, Otago, and parts of the Waikato.
BRANZ research highlights that subfloor moisture and inadequate ventilation are key drivers of timber decay and pest damage in New Zealand housing stock.
Safety First Before You Go Under
Before crawling under your house, take these precautions:
- Wear a dust mask or P2 respirator — subfloor spaces can contain dust, mould spores, and old insulation fibres.
- Wear old clothes, knee pads, and gloves.
- Use a bright LED torch — you need good visibility in tight spaces.
- Never work alone; let someone know you're going under.
- Check for hazards like old asbestos insulation, rodent droppings, or unstable ground.
For health and safety guidance, refer to WorkSafe NZ.
Tools You'll Need
- Bright LED torch or head torch
- Flat-blade screwdriver or bradawl (sharp probe)
- Pocket knife
- Notepad or phone to photograph findings
- Dust mask (P2 rated)
- Knee pads and gloves
What to Look For: Key Signs of Borer Damage
1. Exit Holes
Small, round holes (roughly 1–2 mm in diameter) on the surface of timber are the most obvious sign. These are where adult beetles have emerged. Fresh holes look clean and pale inside; older holes may be darker and dusty.
2. Fine Powdery Dust (Frass)
Look for small piles of fine, cream-coloured powder (called frass or bore dust) beneath or around timber. Fresh frass is a strong indicator of active infestation. Run your finger along the underside of joists and bearers — if powder falls, take note.
3. The Probe Test
This is the most reliable DIY method. Press the tip of a screwdriver or bradawl firmly into the timber at various points — especially along the grain and near any holes. Healthy timber will resist the probe. If the tool sinks in easily, or the surface crumbles or feels spongy, the timber has been significantly weakened by borer tunnelling.
4. Tunnels and Galleries
If you can see a cross-section of damaged timber (e.g., a split or broken piece), look for a network of small tunnels running through the wood. Borer larvae create these galleries as they feed.
5. Structural Members to Prioritise
| Timber Member | What to Check |
|---|---|
| Bearers | Main horizontal supports — probe every 300–500 mm along the length |
| Floor joists | Check top and bottom edges, especially near ends |
| Piles/posts | Inspect at ground level and where they meet bearers |
| Blocking and noggins | Often overlooked but can be heavily affected |
Mapping Your Findings
As you inspect, sketch a rough plan of the subfloor and mark where you find holes, frass, or soft spots. Note whether damage appears isolated (a few joists) or widespread (multiple bearers and joists). This information will be invaluable if you need to bring in a builder or pest specialist.
Active vs. Old Damage
Not all borer damage is current. To help determine if an infestation is active:
- Place a sheet of dark paper under suspect areas and check after a few weeks — fresh frass falling onto it suggests live larvae.
- Fresh exit holes (appearing in spring/summer, when adults emerge) will look bright and clean inside.
- Old damage may have holes filled with grime and no fresh frass.
Even old, inactive damage can still compromise structural integrity, so don't ignore it.
Ventilation and Moisture — The Root Cause
Borer thrives in damp timber. While you're under the house, check that subfloor ventilation is adequate — vents should be unblocked and evenly spaced around the perimeter. Ground moisture barriers (polythene sheeting) help reduce humidity. The MBIE Building Performance guidance on NZ Building Code clause E3 covers subfloor moisture management.
If you're a landlord, note that the Healthy Homes Standards also require adequate subfloor moisture control in rental properties.
When to Call a Professional
DIY inspection is a great first step, but you should engage a Licensed Building Practitioner (LBP) or qualified builder if:
- The probe test reveals soft, crumbling timber in bearers or multiple joists — this is a structural concern.
- You find widespread damage across a large area of the subfloor.
- You're unsure whether the damage is structural or cosmetic.
- You want a formal report for insurance, sale, or renovation purposes.
A builder can assess whether timber needs sistering (adding new timber alongside damaged members), partial replacement, or full replacement — and will know what requires a building consent under the NZ Building Code. Always confirm consent requirements with your local council, as Schedule 1 exemptions may apply to minor repairs but not to significant structural work.
For borer treatment (chemical application), a licensed pest control operator is recommended. Builders and pest specialists can be found through tradedirectory.co.nz.