Borer is one of the most common timber pests in New Zealand homes, particularly in older properties built before the 1950s. Checking your subfloor regularly can catch an infestation early — before it causes serious structural damage. Here's how to do a thorough inspection yourself, and when to bring in a professional.
What Is Borer and Why Does It Matter?
Borer is the common name for the larvae of several wood-boring beetles. In New Zealand, the most widespread species is the common house borer (Anobium punctatum). The larvae tunnel through timber for years before emerging as adult beetles, leaving behind a network of galleries that can seriously weaken structural timbers. Subfloor framing — bearers, joists, and piles — is particularly vulnerable because it is often damp, poorly ventilated, and made from untreated native or early plantation timber.
BRANZ research highlights that borer damage is especially prevalent in homes with poor subfloor ventilation and high moisture levels, both common in New Zealand's varied climate.
Preparing for Your Subfloor Inspection
Before you head under the house, gather the right gear:
- A bright torch or headlamp (the brighter the better)
- Knee pads and old clothing you don't mind getting dirty
- A dust mask (P2 rated) and safety glasses
- A sharp knife or bradawl for probing timber
- A camera or smartphone to photograph any findings
Check WorkSafe NZ guidance before entering a confined subfloor space — make sure there is adequate ventilation and that you have a way to communicate with someone above ground.
How to Access Your Subfloor
Most NZ homes have a subfloor access hatch, usually located in a hallway, laundry, or outside the house perimeter. If yours is sealed or missing, you may need to create one — check with your local council whether this requires a building consent (most minor access hatches fall under Schedule 1 exemptions, but always confirm with your council first). You can find guidance on consenting requirements at MBIE Building Performance.
Key Signs of Borer Activity to Look For
Once you're under the house, work systematically from one end to the other. Look for the following:
1. Exit Holes
Small, round holes (typically 1–2 mm in diameter for common house borer) in the surface of timber are the most obvious sign. These are where adult beetles have emerged. Fresh holes will have clean, sharp edges; older holes may be dusty or discoloured.
2. Frass (Bore Dust)
Frass is the fine, powdery sawdust-like material that borer larvae push out of their tunnels. Look for small piles of cream or pale yellow gritty powder beneath or around timber members. Fresh frass is a strong indicator of active infestation.
3. Soft or Crumbling Timber
Use your bradawl or knife to gently probe suspect timbers. Healthy timber is firm and resists penetration. Borer-damaged timber will feel soft, spongy, or crumbly — in severe cases it may collapse under light pressure. Pay close attention to bearers, joists, and the base of piles.
4. Tunnelling Beneath the Surface
Slice a small section of suspect timber with your knife. Active borer damage will reveal a honeycomb of tunnels just beneath the surface, often packed with frass.
5. Adult Beetles
Between October and February (the NZ flight season), you may spot live adult beetles — small, brown, and roughly 3–5 mm long — on or near timber surfaces.
| Sign | What It Means |
|---|---|
| Round exit holes (1–2 mm) | Past or current borer activity |
| Fresh frass beneath timber | Likely active infestation |
| Soft, crumbling timber | Structural damage present |
| Honeycomb tunnels inside timber | Significant internal damage |
| Live beetles (Oct–Feb) | Active adult emergence |
Checking Moisture Levels
Borer thrives in damp conditions. While you're in the subfloor, look for signs of excess moisture: condensation on the underside of the floor, mould or mildew on timber, or standing water. Poor subfloor ventilation is a key contributing factor. The NZ Building Code clause E3 covers internal moisture, and improving subfloor ventilation can help deter future infestations. Energywise has useful information on managing moisture in NZ homes.
What to Do If You Find Borer
If you spot signs of borer activity:
- Photograph everything — document hole locations, frass, and any soft timber.
- Note the extent — is it isolated to one area, or widespread across multiple members?
- Do not ignore it — borer damage is progressive and can compromise structural integrity over time.
- For minor, localised surface activity in non-structural timber, DIY borer treatment products (available at Bunnings or Mitre 10) can be effective. Always follow the manufacturer's instructions carefully.
- For widespread or structural damage, seek professional assessment immediately.
When to Call a Professional
You should engage a licensed building inspector or registered pest management professional if:
- Borer damage appears widespread or affects structural timbers (bearers, joists, piles)
- You are unsure how to assess the severity of the damage
- You are buying or selling a property and need a formal report
- DIY treatment has not resolved the problem
- You notice significant moisture issues alongside borer activity
A qualified building inspector can provide a written report and recommend appropriate remediation. You can find experienced local building inspectors and pest specialists on tradedirectory.co.nz.
Note: Treatment of structural timber and any repairs to subfloor framing may require a licensed building practitioner (LBP). Check the requirements at Licensed Building Practitioners.
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