Rotting subfloor piles are one of the most common — and costly — structural problems in New Zealand homes, particularly in older timber houses built before the 1980s. The good news is that with a torch, a screwdriver, and a bit of confidence to get under the house, you can carry out a basic inspection yourself. Here's how to do it safely and what to look for.
Safety First Before You Go Under
Before crawling into your subfloor space, take a few precautions:
- Wear a P2 respirator mask, safety glasses, gloves, and old clothing. Subfloor spaces can contain mould, dust, rodent droppings, and fibreglass insulation.
- Take a bright torch or head lamp — you'll need both hands free.
- Check for standing water before entering. If the space is flooded or very wet, don't enter until it has dried out or been pumped clear.
- Never work alone. Have someone nearby in case you need help.
For more on working safely in confined spaces, refer to WorkSafe NZ.
What Are Subfloor Piles?
Subfloor piles (also called foundation piles or stumps) are the vertical timber or concrete posts that support the floor bearers and joists of your home. In New Zealand, older homes commonly used untreated timber piles — typically rimu, totara, or pine — which are vulnerable to rot and borer over time. Newer homes built to NZS 3604 use H5-treated timber or concrete piles, which are far more durable.
Visual Inspection: What to Look For Outside First
Start your inspection from outside the house before going under:
- Uneven or sloping floors inside the home are a classic sign of pile movement or failure.
- Sticking doors and windows can indicate the framing has shifted due to pile settlement.
- Look around the perimeter of the house for piles that appear to have sunk, tilted, or cracked at ground level.
- Check for poor subfloor ventilation — blocked or missing vents allow moisture to build up, accelerating rot. BRANZ recommends adequate cross-ventilation to keep subfloor spaces dry.
The Under-Floor Inspection
Once you're safely inside the subfloor space, work systematically from one end to the other. Inspect each pile individually:
1. Look for Discolouration and Fungal Growth
Healthy timber is generally pale to golden brown. Dark staining, black patches, or white/grey fungal growth are warning signs of moisture damage and early rot.
2. The Screwdriver Probe Test
This is the most reliable DIY test. Press the tip of a flat-head screwdriver firmly into the timber at and just above ground level — this is where rot typically starts. Healthy timber will resist the probe. If the screwdriver sinks in easily or the wood feels spongy and crumbles, the pile is likely rotting internally.
3. Check the Base of Each Pile
The ground contact zone is the highest-risk area. Look for:
- Timber that has split, cracked, or collapsed at the base
- Piles that have visibly sunk into the ground
- Moisture pooling around the base
4. Look for Borer Damage
Small round exit holes (roughly 1–2 mm diameter) in the timber surface indicate borer (wood-boring beetles), which weaken piles and make them more susceptible to rot. Borer and rot often occur together.
5. Assess the Surrounding Conditions
| Condition | Risk Level |
|---|---|
| Dry, well-ventilated subfloor | Low |
| Damp soil, no vapour barrier | Moderate |
| Standing water or pooling | High |
| Blocked vents, no airflow | High |
| Visible mould on joists/bearers | High |
What Counts as Serious?
A pile or two showing early surface discolouration may not be urgent, but the following warrant prompt professional assessment:
- Multiple piles that fail the screwdriver test
- Any pile that has visibly collapsed, cracked, or tilted
- Rot that has spread into the floor bearers or joists
- Floors that feel springy, bouncy, or noticeably uneven
When to Call a Professional
A DIY visual inspection is a useful starting point, but it has limits — rot can be hidden deep inside a pile that looks fine on the surface. If you have any concerns, it's strongly recommended to engage a licensed building inspector or LBP (Licensed Building Practitioner). They have the tools and expertise to assess structural integrity properly.
Pile replacement or significant subfloor structural work may require a building consent depending on the scope — always check with your local council before starting work. Some repairs may qualify as exempt work under Schedule 1 of the Building Act, but confirm this with your council first. You can find more guidance at MBIE Building Performance.
For finding a qualified local building inspector or tradesperson in your area, search tradedirectory.co.nz.
Preventing Future Rot
- Install or clear subfloor ventilation to maintain airflow
- Lay a polythene vapour barrier over bare soil to reduce ground moisture
- Ensure surface water drains away from the house perimeter
- Consider replacing old untreated timber piles with H5-treated timber or concrete piles
Licensed Building Practitioner (LBP)
BCITO Qualified
