Timber rot is one of the most common — and most damaging — problems found in New Zealand homes, particularly in older weatherboard houses and buildings in high-rainfall or coastal regions. The good news is that if you catch it early, repair costs are far more manageable. Here's how to spot the warning signs before rot takes hold.
What Causes Timber Rot?
Timber rot is caused by fungi that break down wood fibres when moisture levels in the timber remain consistently high — generally above about 20% moisture content. In New Zealand's often damp climate, this can happen quickly if:
- Weathertightness is compromised (leaky roofs, failed cladding joints, or poor flashing)
- Subfloor ventilation is inadequate, trapping moisture under the house
- Timber is in direct ground contact without proper treatment or separation
- Gutters and downpipes are blocked, causing water to overflow onto cladding or fascia boards
- Paint or sealant has failed, exposing bare timber to rain
New Zealand's BRANZ research consistently identifies moisture ingress as the leading cause of building defects in residential construction.
Where to Look First
Focus your inspection on areas where water is most likely to collect or penetrate:
Exterior Cladding and Joinery
- Run your hand along weatherboard edges, particularly at the bottom courses closest to ground level
- Check window and door frames — press firmly with your thumb; soft or spongy timber is a red flag
- Inspect sills, architraves, and reveals for paint bubbling, cracking, or dark staining
- Look at corner junctions and anywhere two cladding materials meet
Roof and Fascia Areas
- Examine fascia boards and barge boards for soft spots, especially at the ends
- Check roof penetrations (chimneys, skylights, vents) for failed flashings
- Look inside the roof space for dark staining or soft timber in rafters and purlins
Subfloor and Foundations
- If you have a suspended timber floor, get under the house with a torch and a sharp tool (like a screwdriver or bradawl)
- Probe the bearer and joist ends near the foundation walls — these are prime rot locations
- Look for white or grey fungal growth, a musty smell, or timber that crumbles when probed
- Check that subfloor vents are open and unobstructed; poor ventilation is a major contributor to rot in NZ homes
Decks and Exterior Timber
- Deck boards and posts are highly vulnerable, especially where they meet concrete or soil
- Check post bases — if the post sits directly on concrete without a galvanised post shoe, rot is common
- Look for dark discolouration, surface checking, or a soft texture when pressed
Key Visual and Physical Signs of Rot
| Sign | What It Means |
|---|---|
| Soft, spongy timber when pressed | Active wet rot or dry rot present |
| Dark brown or black staining | Moisture damage, possible fungal activity |
| Paint bubbling or peeling | Moisture pushing through from behind |
| Crumbling or cuboid cracking | Classic dry rot (Serpula lacrymans) |
| White or grey fungal threads | Active fungal growth — act quickly |
| Musty or earthy smell | Elevated moisture and likely fungal activity |
| Visible gaps or shrinkage in timber | Timber has dried after being wet — check for rot |
Two Main Types of Rot to Know
- Wet rot is the most common in NZ. It stays localised to the damp area and causes timber to feel soft and look darker than surrounding wood. Remove the moisture source and the rot stops spreading.
- Dry rot (caused by Serpula lacrymans) is more serious. Despite its name, it starts with moisture but can spread through masonry and dry timber. It produces distinctive brick-like cracking and white mycelium strands. If you suspect dry rot, get a professional assessment immediately.
DIY Inspection Tips
- Use a sharp screwdriver or bradawl to probe suspect timber — healthy wood resists penetration; rotten wood gives way easily
- A moisture meter (available from Bunnings or Mitre 10) can confirm elevated moisture levels in timber without causing damage
- Inspect after heavy rain when leaks are most visible
- Take photos of anything suspicious to share with a professional
- Check your building consent records — homes built before the 1990s may use untreated timber in areas now required to use treated timber under the NZ Building Code
When to Call a Professional
If you find soft timber, visible fungal growth, or widespread staining, it's time to bring in a qualified expert. A Licensed Building Practitioner (LBP) or registered building inspector can carry out a thorough assessment, identify the moisture source, and advise on the scope of repairs. You can find qualified local inspectors and builders on tradedirectory.co.nz.
Keep in mind:
- Structural repairs to framing, bearers, and joists may require a building consent — always check with your local council before starting work
- Some repairs (such as replacing wall framing) fall under restricted building work and must be carried out or supervised by an LBP — see Licensed Building Practitioners for details
- If rot is found near electrical wiring or plumbing, a licensed electrician (EWRB) or plumber (PGDB) must assess and repair those components
Safety note: Working in subfloor spaces can involve confined spaces, asbestos (in homes built before the 1990s), and unstable ground. If in doubt, have a professional carry out the inspection.
Licensed Building Practitioner (LBP)
BCITO Qualified
