Borer in exterior timber: weatherboards, joinery and verandahs

Why is weatherboard borer worse than it looks?

Weatherboard borer is worse than it looks because the larvae feed from the back of the board. Paint protects the outside face, so eggs are laid on the bare inside face, behind the cladding, and the larvae eat outwards. By the time holes show through the paint, the board may be a thin shell. Consumer NZ notes that the long-term fix is often replacing affected boards and treating the framing behind.

A simple test: press a blunt tool or thumbnail into the board near the holes. Sound timber resists. Soft, crumbly or hollow-sounding timber usually means the board has gone too far to save by treatment alone.

How is exterior timber treated?

Exterior timber is treated in three parts: inject the flight holes in painted boards and joinery, coat every bare face you can get at (backs of boards during repairs, barge board backs, framing exposed in the roof or subfloor) with a residual product, and replace boards that are no longer sound. Then the paint system goes back on. Water-based permethrin products are labelled for unsealed timber, so painted faces rely on injection.

When do boards need replacing instead?

Boards need replacing when the timber has lost strength: it crumbles, flexes, sounds hollow or lets a screwdriver sink in easily. Replacement also makes sense when the holes are so dense that filling them leaves more filler than wood. Treating a ruined board kills the borer but doesn't make the board sound. Replacement boards should be treated timber, primed on all faces before fixing.

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

Replacing some boards like for like is generally building work that doesn't need consent under Schedule 1 of the Building Act 2004, as long as it's not a complete or substantial replacement. Re-cladding a whole wall or house is different, and any substantial structural replacement needs a consent and a Licensed Building Practitioner. A borer treatment visit doesn't include building repairs; they're priced separately by a builder.

Can you treat exterior borer yourself?

You can treat a few holes in sound weatherboards or joinery yourself with an injector, a filler and paint, working from the ground or a short ladder. Higher work, barge boards and gables need safe access, and anything soft or widespread needs a closer look before you spend money on paint. If holes show on the outside, also check the subfloor and roof space, where the same borer is often more active.

Our page on flight-hole injection explains the injecting, and filling borer holes covers making good before you repaint.

Does paint stop borer getting in?

Paint stops borer laying eggs on the painted face, because the beetle needs bare timber, but it doesn't stop larvae already inside, and they chew straight out through it. It also doesn't protect the unpainted back of a weatherboard. Keeping the paint system sound helps, and priming all faces of new boards before they go up helps more. See does painting stop borer.

Do heritage rules affect exterior repairs?

In some Auckland suburbs, heritage and special character rules in the Auckland Unitary Plan can affect changes to a house's exterior. Treating borer and replacing a few boards like for like rarely raises an issue, but changing cladding, joinery or verandah details might. If you're in a villa suburb such as Grey Lynn and Ponsonby or Devonport, check with Auckland Council before larger repairs.