Borer in floors

Borer in floorboards and timber floors

Floorboards are where most owners first notice borer, usually when a rug comes up or a sander goes over the boards. The holes on the top face tell only half the story. The same boards are bare underneath, in the subfloor, and that is where a borer inspection gets the rest of it.

Borer in floorboards is usually the common house borer, Anobium punctatum, feeding in the sapwood of rimu, kahikatea or pine boards, or the powderpost beetle in hardwood floors such as tawa and oak. Round holes of 1–2 mm point to the first, smaller pinholes with talc-fine powder to the second. Whether the floor needs treatment, a builder or nothing depends on activity and on how firm the boards still are.

Filling the holes improves the look of a floor but leaves any larvae in the board untouched, and it hides the evidence that shows whether the borer is still working.

Which borer is in my floorboards?

Two beetles cause most floorboard damage in Auckland houses. The common house borer feeds in the sapwood of softwood boards such as rimu, kahikatea and radiata pine and leaves round holes of 1–2 mm. The powderpost beetle, Lyctus brunneus, is confined to the sapwood of hardwoods like tawa, rewarewa and oak, where it leaves 0.8–1.6 mm holes and a powder close to flour.

Floorboard borer at a glance
Beetle	Floor timbers	Exit hole	Larval life
Common house borer	Sapwood of rimu, kahikatea, radiata pine	Round, 1–2 mm	About 3 years in kahikatea, 4 in rimu and pine
Powderpost beetle	Sapwood of tawa, rewarewa, oak, ash	Round, 0.8–1.6 mm	Usually 1 year
Two-toothed longhorn	Rimu, kahikatea, macrocarpa, heartwood included	Oval, 3–7 mm	2 to 11 years

The third row is the one to take seriously. Kiwicare, which makes borer products, describes the native two-toothed longhorn as a major pest of building timber and singles out rimu floors. Its larvae tunnel through heartwood as well as sapwood, so a handful of oval holes in a rimu floor can mean more damage than a scatter of round ones. Hardwood floors with pinholes are covered in the guide to the powderpost beetle.

Kauri floors are common in older Auckland houses as well. The published host lists for the common house borer name rimu, kahikatea and radiata pine rather than kauri, so treat holes in a kauri board as something to identify before assuming which beetle made them.

What are the signs of borer in floorboards?

Look along board edges and in the gaps between boards, because sapwood can sit at the edge of a board and dust settles in the joints. The signs are small holes on the top face, powder in the gaps or under furniture that has not moved in a while, and boards that feel soft or dent under a heel near the holes. Then look at the same boards from below, in the subfloor.

Powder that comes back. Vacuum the gaps and look again a few weeks later. Fresh powder is the clearest sign larvae are still feeding.
Clean holes. Pale, sharp-edged holes are recent. Holes dark with old polish or grime are usually historic.
Soft spots. A board that flexes more than its neighbours, or crumbles at the edge, has lost timber inside.
Beetles in season. Adult common house borer are 2.5–5 mm long and emerge mostly between November and January.

Emergence can run from October into March, according to Consumer NZ and Kiwicare, so small beetles on window sills across that period are worth noting. Overseas woodworm advice gives Northern Hemisphere months, which are wrong for New Zealand. The borer flight season guide sets out the months for each beetle.

Does a polished or varnished floor change the treatment?

Yes. Polyurethane, varnish, oil and wax seal the top face of the boards, and residual borer products only work on bare timber that can absorb them, so spraying a finished floor reaches very little. On finished boards the usual route is to inject a borer product into the exit holes, while the bare underside of the same boards, and the joists below, are treated from the subfloor.

That is why a floor is assessed from both sides. The top face shows where beetles have come out; the subfloor shows how far the infestation runs through the joists and bearers that hold the floor up. What each step involves is set out under treating live borer.

What does sanding reveal in a borer-affected floor?

Sanding takes off the top skin of the boards along with decades of polish, so a freshly sanded floor often shows more borer than anyone expected. The sander does not cause the damage. It opens up tunnels the larvae made just under the surface and clears filler from old holes, which makes a bare floor one of the easiest places to judge how widespread the borer is.

If you are about to refinish, the bare stage is the moment to have the boards looked at. Bare timber can take a surface treatment that a sealed floor cannot, so any treatment the floor needs is simpler before the new coat goes on than after it.

Should you fill borer holes in floorboards?

Filling borer holes is cosmetic only. Wood filler or coloured wax makes a floor look sound, but it does nothing to larvae still feeding inside, and it removes the evidence anyone would use to judge activity, including a building inspector working for a future buyer. Fill holes after the floor has been checked and any live borer treated, and keep a written note of what was found.

A new hole that appears through filler in a later summer is a useful signal: something has emerged since the work was done, and it is worth reporting back. How borer is described in a pre-purchase report is covered under borer in a building report.

When do floorboards need replacing rather than treating?

Boards need replacing when they have lost strength: soft or crumbling under a probe, cracked along a line of holes, or flexing underfoot. Treatment stops further feeding but cannot restore timber the larvae have eaten. Replacing some damaged boards with comparable boards in the same position is generally building work that does not need a consent, but a springy floor means checking the joists and bearers first.

A bouncy floor is often a subfloor problem rather than a board problem. Where joists or bearers are weakened, the job moves to a builder and, if the replacement is substantial, to a building consent. Borer damage repair explains where those lines fall, and borer in piles, bearers and joists covers what the subfloor check looks for.

Questions people ask

How bad is borer in floorboards?

It depends on activity and depth. Old holes in a sanded rimu floor are often historic and need nothing more than a check. Live common house borer in the sapwood edges of boards damages them slowly, over years. Oval longhorn holes, or boards that are soft and crumbling, are more serious and usually mean some timber has to be replaced. Which case you have can only be judged in your own floor.

What does NZ borer look like?

The adult common house borer is a small beetle, 2.5 to 5 mm long, seen mostly between November and January. Most people never see one. What they see is the round exit hole of about 1–2 mm, the size of a pencil lead, and the powder that falls from it. The native two-toothed longhorn leaves larger oval holes, and the powderpost beetle leaves pinholes in hardwood.

Can you treat borer in floorboards yourself?

Retail borer products need no licence, and injecting holes or brushing a product onto bare timber is within reach of a careful owner who follows the label. The limits are reach and judgement: the joists and the underside of the boards matter more than the top face, and no product tells you whether a board is still strong. The borer bombs and sprays guide compares what each product type reaches.

Is woodworm the same thing as borer?

In practice, yes. British usage calls the grubs woodworm; here the same insect goes by common house borer, Anobium punctatum, known overseas as the common furniture beetle. Advice about woodworm in floorboards describes the same insect, but overseas emergence months are Northern Hemisphere ones and need shifting by about half a year for New Zealand.