Sanding a floor that has visible mould or mildew on the boards is not recommended without treating the mould first. Running a drum or orbital sander over affected timber will spread mould spores through the air and across your home, creating a serious health hazard — and it won't fix the underlying problem. The good news is that with the right preparation, many mouldy timber floors can be successfully remediated and sanded to a beautiful finish.
Why Mould on Timber Floors Is a Serious Issue
Mould and mildew thrive where there is moisture, poor ventilation, or both. In New Zealand homes — particularly older villas, bungalows, and houses with subfloor spaces — damp conditions are common, especially in cooler, wetter regions. Mould on floorboards is often a symptom of a deeper moisture problem rather than just a surface issue.
From a health perspective, airborne mould spores can trigger respiratory problems, allergic reactions, and other health issues. WorkSafe NZ recognises mould as a workplace hazard, and the same risks apply in your home. Sanding without containment and prior treatment will aerosolise spores at high concentration — far worse than simply leaving the mould alone.
Step 1 — Identify and Fix the Moisture Source
Before you do anything to the floor surface, find and fix the cause of the moisture. Common sources include:
- Subfloor condensation or rising damp (very common in NZ homes without a vapour barrier)
- Plumbing leaks from pipes or wet areas
- Poor subfloor ventilation
- Roof or wall leaks that have tracked down
If you suspect a plumbing leak, you'll need a licensed plumber — plumbing work is restricted work under the Plumbers, Gasfitters and Drainlayers Act. Check the Plumbers Gasfitters & Drainlayers Board to find a registered tradesperson. Improving subfloor ventilation may also require a builder or specialist.
The BRANZ website has excellent guidance on moisture management in NZ homes, and MBIE Building Performance outlines requirements under the NZ Building Code (particularly Clause E3 – Internal Moisture).
Step 2 — Assess the Extent of the Mould
Not all mould situations are equal. Use this as a rough guide:
| Situation | Likely Action |
|---|---|
| Light surface mildew, boards structurally sound | Clean, treat, then sand |
| Moderate mould, boards discoloured but firm | Professional remediation recommended before sanding |
| Deep mould, boards soft, spongy or rotting | Board replacement required — consult a builder |
| Mould covering large area (>1 m²) | Professional mould remediation strongly advised |
If boards are structurally compromised — soft, crumbling, or with visible rot — sanding won't save them. They'll need to be replaced before any finishing work begins.
Step 3 — Treat the Mould Before Sanding
For light to moderate surface mould on structurally sound boards, you can treat the area yourself before sanding:
- Ventilate the space — open windows and use fans to exhaust air outside.
- Wear PPE — at minimum, an N95 respirator, safety glasses, and gloves. For larger areas, consider a full-face respirator.
- Apply a mould treatment — products such as those in the Resene mould treatment range, or similar products available at Bunnings or Mitre 10, can kill surface mould. Always follow the manufacturer's instructions and allow full drying time.
- Allow the timber to dry completely — this is critical. Sanding damp timber clogs sandpaper and can drive moisture deeper into the wood. Depending on conditions, this may take several days.
- Contain the work area — seal doorways with plastic sheeting before sanding to prevent spore spread.
Step 4 — Sand and Finish
Once the mould is treated, the timber is dry, and the moisture source is fixed, sanding can proceed. A professional floor sander will use a drum sander on the main field and an edge sander around the perimeter, working through progressively finer grits. After sanding, a suitable timber finish — oil, polyurethane, or hardwax oil — will seal the surface and help resist future moisture ingress.
Note that some staining may remain in the timber grain even after sanding, particularly with black mould. A professional can advise whether bleaching or a pigmented finish is appropriate.
Healthy Homes and Rental Properties
If this is a rental property, mould and dampness may also be relevant to your obligations under the Healthy Homes Standards. Tenancy Services has detailed information on what landlords are required to provide in terms of ventilation, drainage, and moisture control.
When to Call a Professional
Call a qualified professional if:
- The mould covers a large area or has penetrated deeply into the timber
- You or anyone in the household has respiratory conditions or allergies
- The boards are soft, rotting, or structurally unsound
- You're unsure of the moisture source
- The subfloor or wall cavities appear to be affected
A professional floor sander will have the right containment equipment, industrial dust extraction, and experience to assess whether the floor is ready to sand. You can find qualified local floor sanders and related tradespeople on tradedirectory.co.nz.
