Remedies: Borer, rot and mould

Borer damage is frequently found in old villas, especially where sapwood rimu and untreated air dried pine have become damp. Rot and mould are also common when moisture is present.


Types of borer

There are two types of borer – the common house borer and the much rarer two-tooth long-horn borer. The easiest way to identify which one is affecting timber is to look at the holes they leave: the common house borer leaves a small round hole, the two-tooth long-horn borer an oval hole up to 5 mm long.

Both types of borer lay their eggs on a rough timber surface or in cracks or holes. The larvae bore into the timber, sometimes for up to three years. As adults, they bore their way back to the surface. 
Fresh holes and dust that appear between November and March indicate that the infestation is still active.

Infestation by the common house borer generally tends to cease before timber becomes structurally unsound because these borer attack only the sapwood in the timber – most framing timber has sufficient heartwood to avoid serious weakening.

Infestation by two-tooth borer is far more serious as they can attack heartwood as well as sapwood. Structural failure sometimes occurs before their presence becomes evident.

Borer in structural timber

For structural timber such as rafters and floor joists, the difficult decision is determining the extent of the damage and its effect on structural performance. If two-tooth borer is present, the timber should be replaced. If the common house borer is present, the timber can often be left, or it may be able to be strengthened. If in doubt, strengthen or replace.

Borer in finishing timber

For internal finishing timbers, the decision to replace infested timber is usually based on aesthetic reasons –  whether the appearance of borer holes is acceptable or not. As the borer holes in finishing timbers are generally caused by the common house borer, major damage is unlikely to be a problem. 

Prevention and treatment

Precautionary measures against both types of borer are similar. Treat infested timber with a residual insecticide by brushing on or spraying surfaces, ensuring that flight holes are drenched or preferably injected. Using a small spray nozzle to inject the liquid into the holes is effective, but wear eye protection as the spray may shoot out another hole towards you.

If the infestation is extensive or fumigation is being considered, consult a specialist pest destruction firm that is a member of the Pest Management Association of New Zealand.

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Almost all timber will deteriorate when exposed to moisture for long periods of time. Fungal decay or rot is the result.

Types of rot

There are three broad categories of rot.

Brown rot

Brown rot, which can be dry or wet rot, tends to make the timber look darker. It is more common on soft woods and rare in hardwoods. 

Timber affected by brown rot can easily be penetrated with a knife, and when the timber is dry, the affected area appears dark and has cross-grain cracks.

Once started, wet rot can continue to grow at lower moisture levels than other rots and will decay timber rapidly. Dry rot can transport the moisture it needs, allowing it to attack even dry timber. It is extremely destructive, so all timber affected by dry rot must be removed completely.

White rot

White rot, which is a wet rot only, gives timber a yellowish-white fibrous appearance. It prefers hardwoods and requires moderate to higher moisture levels to grow.

Soft rot

Soft rot may cause timber to darken or appear greyish but cannot always be seen from the outward appearance of the timber. In advanced stages of decay, the timber can easily be penetrated with a sharp knife. Soft rot requires high moisture levels to grow and is more commonly found on timber in contact with the ground.

Location of rot

Rot in timber is most likely to be found:

  • around brick chimneys
  • around windows
  • on weatherboards on the side of the house most exposed to the weather
  • in framing or weatherboards close to the ground
  • under internal gutters, and
  • in bathrooms.

Dealing with rotted timber

When dealing with rotted timber:

  • check that the source of the moisture has been identified and remedied. This is crucial: if it isn’t done, the problem could return
  • remove all visible rot
  • remove at least one metre of timber past the last visible sign of rot damage as the root system of the rot may be present in apparently sound timber – in some cases, it may prove easier to replace the entire piece of affected timber rather than trying to replace and strengthen a portion 
  • treat cut timber with a proprietary paint-on preservative.

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Where moisture levels have been high or there has been a leak, mould may initially be seen on painted and papered walls and ceilings, and on fabrics. Mould may also be found when structural elements are exposed during demolition and renovation work, inside walls, under the floor or behind linings. 

Moulds are fungi and require moisture and a food source to grow. They reproduce by releasing vast numbers of tiny spores. There is no effective way of eliminating mould, but it can be controlled by controlling indoor moisture levels.

If inhaled in large quantities, some mould spores may cause health problems such as allergic reactions, breathing difficulties, eye irritation, skin rashes and, occasionally, more serious symptoms. Appropriate precautions must be taken to ensure that building occupants and anyone working on the building are not exposed to health hazards from mould during renovation or repair work.

Removal and clean-up for non-toxic moulds

The moulds most commonly seen on surfaces around the house are generally not toxic. To remove them, wash the surface with warm water and household detergent, using a cloth or scrubbing brush depending on the surface. Rinse with clean water and allow the surface to dry thoroughly.

If you wish you can then disinfect or sanitise the surface by repeated treatments with methylated spirits, but ensure the area is well ventilated.

Mould may be removed from fabrics by washing. 

Stachybotrys chartarum

Some types of moulds produce toxic compounds. Stachybotrys chartarum is a toxic mould that is associated with the leaking building problems that New Zealand has experienced in recent years. Leaks originating from outside the building and from wet areas in the building provide the environment suitable for Stachybotrys to grow.

Stachybotrys is a greenish-black mould that grows on materials containing cellulose such as wood fibreboard, fibre-cement, the lining paper of gypsum board, kraft paper wall and roof underlays, wallpaper and timber when it is subject to repeated wetting. It is almost always within the wall cavity, not within the rooms.

Finding Stachybotrys in a building does not immediately mean that the building occupants have been exposed to allergens or toxins. While it is growing, a wet slime covers the Stachybotrys spores, preventing them from becoming airborne. Exposure only occurs when the mould has died and dried up.

Testing for Stachybotrys 

If Stachybotrys is suspected, investigate from outside if possible, by carefully removing a small portion of cladding (or lining, if access is easier from the inside) so a sample of the mould can be taken for testing. Wear a mask or breathing filter and disposable gloves and ensure that no skin is exposed.

Follow the procedure described below to take a sample:

  1. Take a strip of clear adhesive tape about 100 mm long, place it over the mould and press firmly. 
  2. Remove the tape and place onto non-stick baking paper. Fold the paper around the tape and place in a plastic bag.
  3. Securely seal the bag. 
  4. Send the sample to a testing laboratory such as Biodet Services Ltd ( Ltd ( or Plant Diagnostics (

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Removing and cleaning up procedures for toxic moulds

If toxic mould is found in a building, employing a specialist contractor to carry out the removal is recommended.