Subfloor borer treatment: piles, bearers and joists

Why does borer concentrate under the floor?

Borer concentrates under the floor because the timber there is exactly what the common house borer wants: unpainted sapwood, humid air and no disturbance. The larvae grow fastest in timber kept above about 70% relative humidity, and an enclosed subfloor over damp ground often sits there for months. In a house built before treated framing became standard, the joists and bearers are usually untreated native timber or early pine.

The result is that a few holes in a hallway floor often mean a lot more activity on the hidden underside of the same boards. A subfloor treatment works on that hidden face, which is where most eggs are laid.

What does a technician actually do in the subfloor?

A technician checks how far the borer goes, then sprays or brushes a residual borer treatment onto every reachable bare surface: pile tops, bearers, joists, blocking and the undersides of the boards. Most labels ask for a liberal first coat and a second once the first has soaked in. Flight holes in finished timber upstairs are injected separately if they show activity.

A typical visit runs in this order:

1. Look first. Fresh pale holes, frass on the ground sheet or piles, soft timber, damp patches and old repairs are noted. You get a written quote before anything is treated.
2. Clear the way. Stored items, loose timber and rubbish near the access are moved, because they block spray and often carry borer themselves.
3. Treat the timber. Coarse low-pressure spray reaches the framing; brush work covers places a spray can't.
4. Note what isn't treatment. Rotten or badly weakened members are pointed out as building work, priced separately by a builder.

What makes a subfloor hard to treat?

Clearance is the biggest factor. A subfloor you can crouch in is quick; one with 300 mm between ground and joists means crawling on your back with a sprayer. Underfloor insulation, ducting, plumbing, wiring and a ground moisture barrier all have to be worked around without damage. Damp ground, sharp rock and old debris slow the work too.

Can you treat your own subfloor?

You can treat a roomy, dry subfloor yourself with a retail borer product, a pump sprayer and proper protective gear. It gets much harder in a low or tight subfloor: two coats over every surface use more product than expected, the mist settles on you, and lying in a confined space with an insecticide is genuinely unpleasant. Many people treat the easy end and get the rest done.

Borer under the floor or in the roof space? Get a written quote

If you go DIY, read the label before you buy, wear a respirator suited to the product plus eye protection and coveralls, and keep children and pets well clear. Our DIY cost guide works out how much product a subfloor takes, and the product safety notice covers the precautions.

What does subfloor borer treatment cost?

Published Auckland price lists put a subfloor borer treatment at roughly $450 to $750, more for large, low or awkward subfloors. Doing the subfloor and roof space together usually lands around $800 to $1,350. Some businesses advertise "from" prices around $210 to $375 plus GST for both areas on small houses. Your price is set in writing after the subfloor has been seen.

The main things that move the price are floor area, clearance, insulation, how many places show live borer and whether finished timber upstairs needs injecting too. The borer treatment cost page puts these figures alongside DIY shelf prices.

Does fixing damp matter as much as the spray?

Fixing damp matters about as much as the spray, because damp timber is what lets borer thrive and what draws in huhu and rot. A residual treatment protects the surface; a dry subfloor slows any survivors down. A polythene ground cover, clear vents, sound downpipes and no garden beds banked against the base boards all help keep the timber drier.

For rentals, the Healthy Homes standards require a ground moisture barrier under suspended floors with an enclosed subfloor, where it's reasonably practicable to install one. Our landlord guide covers that, and the prevention guide covers ventilation and drainage for any house.

Will a fogger under the house do the same job?

A fogger under the house does not do the same job. A fogger's mist only kills adult beetles flying at that moment, during the flight season from about October to March. It leaves nothing on the timber that stops larvae, which live inside the wood for three to four years. A residual surface treatment is what lasts. Our answer on whether borer foggers work explains where a fogger still has a use.