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Subfloor

Borer in piles, bearers and joists

The subfloor is the part of an older Auckland house most owners never crawl into, and the part an inspector usually spends longest in. It holds the untreated timber that carries the floor, and it is often damper than anywhere else in the house, which suits several beetles. A borer inspection treats it as the first place to look.

Borer in piles, bearers and joists matters more than borer anywhere else in a house, because these members carry the floor. Common house borer in sapwood edges is usually slow damage that treatment and better ventilation can stop. Oval holes from the two-toothed longhorn, which tunnels through heartwood, and timber that is soft under a probe are more serious and can mean replacement.

A subfloor too low to move through safely is recorded as not inspected, so a report with that line has not ruled anything out.

What does a subfloor borer check look at?

It covers every timber member under the floor that can be reached: the piles, the bearers that sit on them, the joists that span between bearers, and the underside of the floorboards. Each is checked for exit holes and dust, probed where holes cluster, and read together with the ground beneath, the vents in the base of the walls and any sign of leaks from plumbing above.

  • Piles

    Timber piles at ground level, where they stay wettest, and at the top, where the bearer sits.

  • Bearers

    The heavy members along the pile lines. Longhorn tunnels here are the finding most likely to need a builder.

  • Joists

    Sapwood edges are where the common house borer usually feeds. Ends and notches are probed.

  • Ground and air

    Bare or wet soil, blocked vents, leaking pipes and the clearance under the floor.

Where the gap under the floor is too tight, or parts are boxed in, those areas are listed as not inspected. That is normal practice rather than a shortcut, but worth knowing before you rely on a report, particularly when you are buying a house with an older subfloor.

Why does a damp subfloor make borer worse?

Moisture favours every beetle found under Auckland houses. The common house borer develops fastest when relative humidity is above about 70%, the two-toothed longhorn's larval life varies with moisture and temperature, and huhu larvae keep feeding only while timber stays at about 25% moisture content or more. A damp, still subfloor with blocked vents gives all three what they need.

That is why ventilation and ground moisture belong in the plan, not as an extra. Clearing vents, fixing leaking pipes and dealing with water pooling under the house are typical recommendations. Huhu in floor framing is a sign of wet timber first and an insect problem second; see huhu in the house.

Is it borer or rot in the piles?

Not every soft pile has borer. Rot is decay caused by fungi in timber that stays wet, and it tends to show where a timber pile meets damp ground. Borer shows as exit holes and dust, with damage that follows the larval tunnels. A pile can have rot, borer or both, and the findings record which, because a borer treatment does nothing for timber that has decayed.

Searches for rotten piles usually end with a builder rather than a pest treatment. Where both are present, the moisture that caused the rot is often what let the borer thrive, so fixing the water source protects the new piles as well as the old bearers.

What does longhorn damage in bearers and joists mean?

The two-toothed longhorn, Ambeodontus tristis, is the native beetle that matters most under a house. Its larvae live for 2 to 11 years in the timber and tunnel through heartwood as well as sapwood, so a bearer that looks sound can be eaten out until only a skin of timber remains. Its oval exit holes, about 3–7 mm long, appear in autumn or early winter, and only probing shows what is inside.

Consumer NZ describes longhorn damage as usually severe, with the timber often needing replacement, while noting the beetle is found in buildings less often than the common house borer. Rimu, kahikatea and macrocarpa are its most frequent hosts. More on the beetle is in the guide to the two-toothed longhorn.

When do piles and bearers need replacing?

Replace a member when it has lost strength: it crumbles or a probe sinks in, longhorn tunnels run deep, or a pile no longer supports its bearer. Replacing a few damaged piles or a length of bearer with comparable timber in the same position is generally exempt from building consent. Complete or substantial replacement of subfloor structure is not exempt, and a full re-pile needs a consent.

The Building Act gives no fixed number of piles. A Marlborough District Council newsletter, relaying a BRANZ summary of MBIE determinations, suggests up to about 20% of piles as a rough guide, with ground conditions and each pile's structural role also weighed, and says staging the work does not get around the rule. Treat that as guidance only and check with Auckland Council. Borer damage repair and re-piling covers the consent and builder side.

Can subfloor borer be treated without replacing timber?

Yes, wherever the timber is still strong. Subfloor framing is usually bare, so a residual surface product or a borate preparation can go onto piles, bearers, joists and the underside of the floorboards. The work is timed ahead of emergence: before early summer for the common house borer, before autumn for the longhorn. Moisture fixes go in at the same time, so the treated timber dries out.

Published Auckland prices for subfloor treatment run about $450–$750 + GST, a guide only, and depend on access and the size of the house. The borer cost page breaks this down, and treating live borer explains each method.

Questions people ask

How do you know if you have borer under the house?

Look at the subfloor timber with a good torch. Small round holes in joists and bearers, fine dust on top of bearers or on the ground below, and pale, clean-edged holes that look recent are the signs. Oval holes in heavy timbers suggest the two-toothed longhorn. If you cannot get under the floor, or cannot tell old holes from new, an inspection is the practical answer.

How serious is borer in piles?

It ranges from minor to structural. A few round holes in the sapwood edge of a pile usually leave the core carrying the load, and treatment with better ventilation can be enough. A pile that is soft right through, or a bearer with longhorn tunnels, has lost strength and needs a builder. The only way to tell which applies to a particular house is to probe the timber.

Can borer get into concrete piles?

No. Borer feeds on wood, so concrete piles cannot be attacked. The timber resting on them still can: bearers, joists and floorboards. Houses re-piled in concrete years ago may still have their original untreated native bearers and joists, so a subfloor on concrete piles still needs its timber checked.

Does a building report check the subfloor for borer?

A standard pre-purchase building report follows NZS 4306 and is a visual inspection, so it usually notes signs of borer the inspector can see, including in an accessible subfloor. It is not a specialist timber-pest assessment and may not say whether the borer is active. Borer in a building report explains what the usual wording can and cannot tell you.

Under the floor

Sources

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