Subfloor moisture is one of the most common — and most damaging — problems in New Zealand homes, particularly in older timber-framed houses. The good news is that you can carry out a basic inspection yourself to spot early warning signs, even before calling in a building inspector. Here's how to do it safely and what to look for.
Why Subfloor Moisture Matters
New Zealand's climate — especially in wetter regions like the West Coast, Northland, and parts of the South Island — means many homes are at risk of moisture accumulating under the floor. Left unchecked, excess moisture can cause timber rot, mould growth, structural damage to floor joists and bearers, and poor indoor air quality. The NZ Building Code sets requirements (clause E3) to manage subfloor moisture, but many older homes predate these standards.
Step 1: Look for Warning Signs Inside the House
Before you even go under the house, walk through your rooms and look for:
- Soft, springy, or bouncy floorboards — a classic sign of timber decay
- Lifting, bubbling, or warping floor coverings (vinyl, carpet, or timber)
- Musty or earthy smells coming from floor level
- Visible mould on skirting boards or at floor edges
- Condensation on windows or walls at floor level
- Staining or discolouration on timber floors
These symptoms don't always mean a serious problem, but they're worth investigating further.
Step 2: Access the Subfloor
Most NZ homes built on piles have a subfloor access hatch — often found outside under a deck, in a cupboard, or along the exterior cladding line. Before entering:
- Wear a P2 respirator mask, safety glasses, and old clothes — subfloor spaces can contain mould spores, dust, and in older homes, possibly asbestos insulation or pipe lagging. If you suspect asbestos, do not disturb it; contact a licensed asbestos assessor instead. Check WorkSafe NZ for guidance.
- Bring a bright torch or head lamp
- Take your phone for photos
- Never enter a subfloor space that is flooded or has standing water
Step 3: Inspect the Subfloor Space
Once inside (or peering in if the space is too tight), check the following:
Ground Condition
- Is the ground wet, damp, or muddy? You should ideally see dry, firm soil or a polythene vapour barrier.
- Is there standing water or evidence of recent pooling (tide marks on piles)?
- Is there visible mould or fungal growth on the soil surface?
Timber Condition
- Press a screwdriver or sharp object gently into joists, bearers, and the underside of flooring. Sound timber is firm; rotten timber will feel soft or crumble.
- Look for dark staining, white fungal threads (mycelium), or bracket fungi — all signs of decay.
- Check for sagging joists or bearers that look bowed or twisted.
Ventilation
- Are the subfloor vents clear and unobstructed? Blocked vents are a leading cause of moisture build-up. NZS 3604 specifies minimum ventilation requirements for timber-framed buildings.
- Is there adequate cross-ventilation — vents on opposite sides of the house?
Vapour Barrier
- If a polythene ground sheet is present, check it's intact, overlapping at joins, and weighted down. Gaps or tears allow moisture to evaporate directly into the subfloor space.
Step 4: Use a Moisture Meter
A pin-type moisture meter (available from Bunnings or Mitre 10 for a broad guide price of roughly $30–$150) lets you measure moisture content in timber. As a general guide:
| Moisture Content | Condition |
|---|---|
| Below 18% | Acceptable for structural timber |
| 18–25% | Elevated — monitor closely |
| Above 25% | High risk of decay — action needed |
| Above 28% | Active decay likely |
Take readings from several joists and bearers, especially near exterior walls and in corners where moisture tends to concentrate. Note that consumer-grade meters give a useful indication but are not as accurate as professional instruments.
Step 5: Check Drainage Around the House
Step back outside and look at:
- Ground slope — does the ground fall away from the house, or does water pool against the foundation?
- Downpipes and spouting — are they discharging water away from the subfloor area?
- Garden beds or paving that may be directing water toward the house
Poor site drainage is one of the most common causes of subfloor moisture in NZ homes.
Simple Fixes You Can Do Yourself
- Clear blocked subfloor vents of debris, soil, or vegetation
- Lay or repair a polythene vapour barrier (minimum 0.25 mm thick) over bare soil
- Redirect downpipes away from the foundation
- Improve ground drainage by regrading soil away from the house (check with your council if earthworks require consent)
When to Call a Professional
If you find soft or rotten timber, significant mould, standing water, or moisture readings consistently above 20%, it's time to bring in a qualified professional. A licensed building inspector can carry out a thorough assessment with calibrated equipment and provide a written report. Structural repairs to joists, bearers, or piles may require a Licensed Building Practitioner (LBP) — check LBP.govt.nz — and depending on the scope, a building consent from your local council.
If you discover plumbing leaks contributing to the moisture, that repair work must be carried out by a licensed plumber registered with the Plumbers, Gasfitters and Drainlayers Board.
You can find qualified local building inspectors and tradespeople on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
BCITO Qualified
