Exterior cladding
Borer in weatherboards
Weatherboards on older Auckland villas and bungalows can date from before boron treatment of building timber began in 1952, and many have been painted a dozen times since. Borer in them tends to turn up when a repaint starts and scraping exposes the holes. A borer inspection looks at the cladding from outside and, where it can, at the framing behind it.
Borer in weatherboards is usually the common house borer, and it is often worse than the painted face suggests. Consumer NZ notes that borer tend to attack weatherboards from the inside face, so the holes you can see outside may be the last part of the damage to show. Lower boards, sills and areas where paint has failed and the timber stays damp are the usual places to find it.
Paint seals the surface but does not kill larvae already inside a board. Badly affected boards are replaced and the framing behind treated; sound boards where borer is still active can be treated bare and then repainted.
Where does borer show up in weatherboards?
Borer in cladding concentrates where timber stays damp longest: the bottom two or three boards near the ground, boards under windows and sills, around downpipes and overflowing gutters, and on shaded south walls. It also shows where paint has cracked and peeled, because bare weathered timber takes up water and stays damp. The common house borer develops best in humid air, above roughly 70% relative humidity according to NZFFA.
The framing behind the boards may be untreated too. Houses framed before 1952 usually are. Later, untreated kiln-dried framing was allowed from September 1995 and cited in the Building Code's Acceptable Solution from February 1998, until treated framing was required for consents issued from 1 April 2004. Borer in the cladding is a reason to check whatever framing can be seen from the roof space and subfloor.
Is borer in weatherboards worse than it looks?
Often, yes. Larvae can feed through the back of a board for years while the painted face stays intact, and holes only appear where adults chew their way out. Consumer NZ reports that weatherboard borer usually attacks from the inside face, and that the long-term fix is often replacing the boards and treating the framing behind them. A few holes outside can sit over a board hollowed from behind.
Pressing a board with a thumb or a probe is the quick test. Sound timber resists; damaged boards give, crack or crumble. When boards come off during a repair, the back face shows the full extent, and it is worth looking at the framing while it is exposed.
Does paint stop borer in weatherboards?
No. Paint seals the face of a board, but it does not reach larvae already inside, and adults still chew out through it. Painting over live borer hides the holes for a season or two while feeding continues behind the new coat. Paint also blocks residual borer products, which only work on bare timber, so any treatment belongs at the preparation stage, before the repaint rather than after it.
If you are repainting an older house, the scraped-back stage is a good time for a check: holes are easy to see and bare timber can take a surface treatment. Filling and painting without a check leaves the next repaint, or the next owner, to find the same borer with more damage behind it.
Should weatherboards be replaced, treated or repainted?
That depends on two things: whether the boards are still strong and whether the borer is still active. Firm boards with old, inactive holes can be filled and repainted. Firm boards where borer is still active can be treated while bare and then painted. Boards that are soft, crumbling or split along lines of holes need replacing, and the framing behind them should be checked and treated where it is exposed.
| What the boards show | Usual outcome |
|---|---|
| Old holes, no fresh dust, boards firm | Fill and repaint; look again at the next repaint |
| Fresh holes or dust, boards firm | Treat the bare timber before painting; check after the next flight season |
| Soft, crumbling or hollow boards | Replace like for like; check and treat the framing behind |
| Borer damage plus leaks or rot | Builder first: the moisture source, framing and any consent question |
Products and timing are set out under treating live borer.
Does replacing borer-damaged weatherboards need a building consent?
Usually not, when it is a repair. Replacing some damaged weatherboards with comparable boards in the same position is generally exempt under Schedule 1 of the Building Act 2004. The exemption does not cover repairing or replacing cladding that has failed the Building Code's durability requirement, which for cladding is 15 years. Exempt work still has to comply with the Building Code.
Whether the durability limit applies depends on the cladding's age and history, so a builder confirms whether a particular job needs a consent before work starts, and Auckland Council can be asked when it is unclear. Where more than a few boards are involved, borer damage repair explains how the builder and consent side fits with treatment.
How can you tell if borer in weatherboards is still active?
Fresh activity shows as pale, sharp-edged holes in otherwise weathered paint, fine dust on the sill or the board below, and new holes through a recent coat of paint. Old holes are dark, softened with grime or filled by earlier coats. The common house borer emerges mostly from November to January, so new holes and dust are most likely to show over summer.
A simple test: photograph a patch of holes in winter, with something for scale, and look again in late summer. New holes through the paint mean the infestation is still going.
Questions people ask
Is borer damage in weatherboards bad?
It can be, because the damage is often behind the face of the board. A few old holes in firm boards are a cosmetic issue. Live borer in lower boards, or boards that give under a thumb, mean feeding is continuing out of sight and the boards may need replacing, with the framing behind checked. The amount of damage is judged board by board, not from the number of holes.
Can you treat borer in weatherboards yourself?
Retail borer products need no licence, and treating bare, scraped boards before a repaint is possible for an owner who follows the label. What a product applied outside cannot reach is the inside face of the board and the framing behind, where weatherboard borer usually starts. The borer bombs and sprays guide sets out what each product type reaches.
Are cedar weatherboards affected by borer?
Less often. The NZ Farm Forestry Association notes that western red cedar is rarely damaged by the common house borer, so cedar cladding is low risk for that beetle. Older native boards such as rimu and kahikatea, and untreated radiata pine, are its usual hosts. If you do find holes in cedar boards, have the beetle identified before anything is treated.
Will borer in weatherboards spread into the framing?
It can where the framing behind is untreated sapwood, because adults emerging from the boards can reach it. That is the reason Consumer NZ's long-term fix includes treating the framing behind replaced boards. Houses framed before 1952, or with untreated kiln-dried framing from the late 1990s and early 2000s, are the ones where this matters most.
Does borer in the weatherboards affect selling or buying a house?
A pre-purchase building report usually notes visible borer in cladding, and buyers then ask whether it is live and how far it goes. Old, inactive holes in firm boards rarely change much. Live borer, soft lower boards or damage near leaks raise questions about cost. Whether it should change an offer is covered in is borer a deal breaker.
Outside the house
- Treating live borerWhat treatment involves after a live borer finding: bare and finished timber, timing, moisture and follow-up
- Borer damage repairWhen borer damage needs a builder, which repairs are exempt from consent, re-piling and the order of work
- House borer by build yearWhich New Zealand houses carry house borer, era by era, and why the framing timber decides it
- Is borer a deal breaker?When borer should change an offer on a house, and when it should not