Checking floor joists for rot doesn't always mean ripping up your floorboards. In most New Zealand homes built on piles, you can carry out a thorough inspection from the crawl space beneath the house — saving time, money, and disruption. Here's how to do it safely and what to look for.
Why Floor Joist Rot Is a Common NZ Problem
New Zealand's damp winters, high rainfall in many regions, and the prevalence of older timber-framed homes on pile foundations make subfloor rot a widespread issue. Poor subfloor ventilation, ground moisture, and leaking pipes all contribute. Left unchecked, rot can compromise the structural integrity of your floor system — and by extension, your whole home. BRANZ has extensive research on moisture-related building failures in NZ homes.
Safety First Before You Go Under
Before crawling under your house, take these precautions:
- Never enter a confined subfloor space alone. Have someone nearby who knows where you are.
- Wear a P2 respirator mask, safety glasses, gloves, and a coverall — subfloor spaces can contain dust, mould, rodent droppings, and old insulation fibres.
- Check for electrical cables or plumbing before probing or cutting anything. Electrical work is restricted — see EWRB for what homeowners can and cannot do.
- Bring a bright torch or headlamp and a charged phone.
- If the space is very confined or you're unsure, stop and call a professional.
Refer to WorkSafe NZ for guidance on working safely in confined spaces.
Tools You'll Need
How to Access the Subfloor
Most NZ homes on piles have an access hatch — often located in a hallway, laundry, or outside through a vent panel. If yours is sealed or missing, you may need to create one (check with your local council whether this requires a building consent — minor access hatches are often exempt under Schedule 1 of the Building Act, but always confirm). Find guidance at MBIE Building Performance.
What to Look For: Signs of Rot
Once you're under the floor, work systematically from one end to the other. Inspect each joist along its full length.
Visual Signs
- Discolouration — dark brown, black, or grey staining on timber
- Fungal growth — white, orange, or brown mycelium (fungal threads) on or between joists
- Cracking or cuboid checking — timber that has broken into small cube-like pieces is a classic sign of brown rot
- Soft, spongy, or stringy texture — visible even without touching
- Sagging or bowing joists that have lost their straightness
The Probe Test
Using a flat-head screwdriver or bradawl, firmly press the tip into the timber at several points along each joist, especially at the ends where they bear on piles or bearers, and anywhere near plumbing. Healthy timber will resist penetration. If the tool sinks in easily or the wood crumbles, that section is likely rotten.
The Tap Test
Tap along the joist with a small hammer. Solid timber gives a sharp, resonant knock. A dull, hollow thud can indicate internal decay — the timber may look fine on the surface but be rotted inside. This is common with white rot, which attacks the interior of the wood.
Moisture Meter Readings
A moisture meter gives you a percentage reading of moisture content in the timber. As a general guide:
| Moisture Content | Condition |
|---|---|
| Below 18% | Generally safe — low rot risk |
| 18–25% | Elevated risk — monitor closely |
| Above 25% | High risk — conditions favour rot |
| Above 30% | Active decay likely |
Note that readings can vary by timber species and meter type — use this as a guide, not a definitive diagnosis.
Above-Floor Clues to Check First
Before going under, walk your floors and note:
- Soft or springy spots underfoot
- Squeaking that has worsened over time
- Sloping or uneven floors (though this can also be pile settlement)
- Musty smell coming from floor level
These symptoms help you target your inspection once you're in the crawl space.
Improving Subfloor Ventilation
If you find damp conditions but no major rot yet, improving ventilation is your first line of defence. Ensure subfloor vents are clear of debris and not blocked by garden growth. A polythene ground vapour barrier laid over bare soil can significantly reduce moisture rising into the subfloor — this is a DIY-friendly job in most cases. Check Energywise for advice on subfloor insulation and moisture control.
When to Call a Professional
Call a Licensed Building Practitioner (LBP) if you find:
- Widespread rot affecting multiple joists or bearers
- Structural members that have lost significant section (depth or width)
- Evidence of borer alongside rot (common in older NZ homes)
- Any rot near load-bearing walls or around the perimeter
- You're unsure how serious the damage is
Replacing or sistering floor joists may be restricted building work under the Building Act if it affects the primary structure — this requires an LBP and may need a building consent. Find a qualified local builder at tradedirectory.co.nz or verify LBP credentials at LBP.
If you find leaking pipes contributing to the moisture, plumbing repairs must be carried out by a licensed plumber — see PGDB for licensing requirements.
Registered Master Builder
Licensed Building Practitioner (LBP)
