Borer library

House borer in New Zealand: which houses are at risk

The common house borer does not pick houses at random. Whether yours carries it depends mostly on what the framing was cut from and whether it was treated, which comes down to when the house was built and what has been added since. A borer inspection tells you what is in your own timber; this guide shows where to expect it.

Updated 6 October 2026

In New Zealand, house borer means Anobium punctatum, a beetle a few millimetres long whose grubs bore into the sapwood of untreated timber. Risk follows the build year: houses framed in untreated timber before 1952 carry the most, boron-treated houses the least, and homes framed between about 1995/1998 and 2004/2005 sit in a window when untreated kiln-dried framing was allowed.

Renovated houses mix eras, so one house can hold old rimu floors beside new treated framing. The build year narrows the question. It does not answer it for any single house.

What is house borer in New Zealand?

House borer is the everyday New Zealand name for Anobium punctatum, known internationally as the common furniture beetle. Adults are brown and only 2.5–5 mm long, and they leave round exit holes about 1–2 mm across. The damage is done by the larvae, which spend about three years in kahikatea sapwood and about four in rimu or radiata pine before they emerge as adults.

Years inside the timber, weeks outside it. Adults of the common house borer mostly emerge from November to January.

That imbalance is why a house can carry borer for a long time before anyone notices. Adults mostly appear between November and January, sometimes from October and into March, and survive for just a few weeks. The holes you see are the doors they left by. The larvae that matter are still inside, which is why holes are always read together with dust and timber type. Month-by-month timing for all four beetles is in the borer flight season calendar.

Which houses are most at risk of house borer, by build year?

Risk follows the framing timber, and the framing timber follows the rules in force when the house was consented. Boron treatment of building timber began in 1952. Untreated kiln-dried framing was allowed from September 1995 and stopped being acceptable for new consents from 1 April 2004. Houses built outside the treated periods, and any house with older timber added later, carry more risk.

House borer risk by build era (framing only: floors, cladding and furniture vary)
Build era	Typical framing	House borer risk
Before 1952	Untreated native timber such as rimu and kahikatea, and some early untreated radiata pine	Highest. Sapwood in framing, floors and weatherboards is susceptible
1952 to mid-1990s	Boron-treated framing, standard practice through this period	Low in the framing. Untreated floors, joinery or later additions can still carry it
About 1995/1998 to 2004/2005	Untreated kiln-dried framing permitted (NZS 3602:1995 from September 1995; cited in B2/AS1 from February 1998)	Raised where untreated framing was used, more so if it has stayed damp
Consents from 1 April 2004	Treated framing required again; boron-only H1.2 became the standard framing treatment in 2011	Low in the framing. Recycled timber and old furniture are the usual way in
Renovated houses	Original timber beside newer treated framing	Judged room by room, not by the date on the title

The middle window needs care with dates. NZS 3602:1995 allowed untreated framing from 11 September 1995 if it was kiln-dried, and the Building Code's Acceptable Solution B2/AS1 cited that change from 28 February 1998. A directive then required treated timber for building consents issued from 1 April 2004, and homes already consented with untreated framing could be finished up to 1 April 2005. The exposed group is roughly 1995/1998 to 2004/2005, not an exact span.

Treat 1952 as the year treatment began, not as a line after which every house is safe. If you are buying in one of these windows, the pre-purchase borer inspection page covers how a check fits inside your conditions.

Why does house borer attack sapwood and not heartwood?

The common house borer develops in sapwood, the lighter outer layer of the trunk, and normally leaves heartwood alone. In rimu, kahikatea and radiata pine the sapwood is the susceptible part, while Douglas fir and western red cedar are rarely damaged at all. A joist with a sapwood edge can be riddled along one face and still be sound through its core.

This is the main reason borer in a house often looks worse than it is. Holes clustered near the edge of a rimu board may sit entirely in sapwood, while the heartwood carrying the load is untouched. It is also why the native longhorn is read differently: it tunnels through heartwood too, so the same number of holes can hide much more damage. The guide to the two-toothed longhorn explains how to tell the two apart.

Moisture matters as well. The NZ Farm Forestry Association notes that the beetle develops fastest where relative humidity stays above about 70%, so damp subfloors and poorly ventilated roof spaces tend to hold the worst of it.

Where does house borer show up in an older house?

The first places to look are the timbers that are untreated and rarely disturbed: subfloor joists and bearers, the underside of floorboards, roof framing and the lower weatherboards. Paint and polish hide holes on the surfaces you live with, so the bare faces in the subfloor and roof space usually give the clearest picture of what the rest of the house is carrying.

Floors. Rimu and pine boards often carry holes near their edges. Borer in floorboards covers finished boards and when they need replacing.
Subfloor. Joists, bearers and timber piles in a damp subfloor. See borer in piles, bearers and joists.
Cladding. Lower weatherboards and sills where paint has failed. See borer in weatherboards.
Furniture. Second-hand and antique pieces can bring borer into a house of any age. See borer in furniture.

Does treated timber stop house borer?

Boron-treated framing is treated to resist borer, and BRANZ describes boric treatment as having a long record of protecting timber against insects and decay. It is not a promise that nothing can happen. Boron dissolves in water and can leach out of timber left rain-wet for long periods, and the Ministry for Primary Industries warns that exotic termites can damage treated timber.

In practice, treated framing in a dry house is the least likely place to find live borer. When it does turn up in a newer house, the source is more often untreated timber added later, recycled boards, or furniture.

How do you know whether house borer is still active?

Look for fresh signs rather than old holes. Pale, clean-edged holes, fine dust that returns after you vacuum, and small brown beetles collecting on windowsills in early summer all point to activity. Dark, weathered holes with no new dust usually mean the adults left years ago. When the evidence is unclear, an area can be marked and checked again after the next flight season.

Holes alone cannot date an infestation. What matters to an owner or a buyer is which timbers are live, which are historic and which are weakened, and that takes someone looking at the timber. Typical prices for inspection and treatment are in the borer cost guide.

Questions people ask

Is house borer the same as woodworm?

Yes, in practice. Woodworm is the British word for the grubs of the same insect, Anobium punctatum; in this country it is the common house borer, and internationally it is listed as the common furniture beetle. The insect, the round 1–2 mm holes and the gritty dust are all the same. Only the name changes with the country.

How bad is borer in a house?

It ranges from cosmetic to structural. Scattered holes in sapwood edges with no fresh dust are usually old and need no work. Live borer in subfloor framing needs treatment and a moisture fix. Timber that crumbles or dents under a probe, or longhorn damage in bearers, is a builder's matter. The finding decides which, not the number of holes.

Are villas and bungalows more likely to have borer?

Often, because a villa or California bungalow framed before 1952 was built from untreated timber, much of it rimu and kahikatea. That does not mean every one is badly affected: a dry, well-ventilated house may carry only historic holes. It does mean the subfloor, roof space and lower weatherboards deserve a close look before you buy or renovate.

Can a new house get house borer?

It is uncommon in treated framing. In modern houses borer usually arrives with untreated additions: recycled timber, old boards relaid as a feature, or antique furniture. Houses consented in the untreated kiln-dried window, roughly 1995/1998 to 2004/2005, are the main exception, particularly where that framing has stayed damp.

Does fixing damp help with house borer?

Yes, as part of the answer. Humid timber speeds the beetle up, so clearing blocked subfloor vents, fixing leaks and keeping ground moisture down make the timber less favourable. Drying out is not a treatment on its own, so live borer found in framing still needs one, but moisture work helps stop a treated house slipping back.