Yes, you can use a paint-on (liquid-applied) membrane to waterproof a laundry floor — and in many NZ homes it's a practical, cost-effective solution. However, the job needs to be done correctly to comply with the NZ Building Code and to give you a durable, leak-free result. Here's what you need to know before you crack open a tin.
Why Waterproofing a Laundry Floor Matters
Laundries are classified as wet areas under the NZ Building Code, specifically under Clause E3 (Internal Moisture). This means the floor and wall junctions must be waterproofed to prevent moisture damage to the building structure. A washing machine leak, overflowing tub, or splashing sink can cause serious damage to subfloors, framing, and even neighbouring rooms if there's no proper membrane in place.
For guidance on what the Building Code requires, MBIE Building Performance is the authoritative source.
What Is a Paint-On Membrane?
A liquid-applied or paint-on membrane is a waterproofing compound brushed or rolled onto a prepared surface, where it cures to form a seamless, flexible waterproof layer. Common types used in NZ include:
- Acrylic-based membranes – water-based, easy to apply, good for DIY
- Polyurethane membranes – more flexible and durable, often used in commercial settings
- Cementitious membranes – cement-based, rigid, good for tiling over
Well-known products available in NZ include options from Ardex, Mapei, and Sika. Always check the manufacturer's datasheet to confirm the product is suitable for wet area floors and compatible with your intended floor finish (tiles, vinyl, etc.).
Does It Need a Building Consent?
In most cases, waterproofing a laundry floor as part of a like-for-like repair or renovation falls under Schedule 1 exemptions of the Building Act — meaning no consent is required. However, if you're doing structural work, changing the layout, or the laundry is part of a larger project, consent may be needed. Always check with your local council before starting work to confirm whether consent applies to your specific situation.
Step-by-Step: How to Apply a Paint-On Membrane
1. Prepare the Surface
The surface must be clean, dry, and structurally sound. Remove any old adhesive, grease, or loose material. Fill cracks and holes with a suitable filler and allow to cure fully. For timber subfloors, check for flex — excessive movement can crack a rigid membrane.
2. Prime if Required
Many membrane systems require a primer coat first. Check the product instructions — skipping this step can cause adhesion failure.
3. Reinforce Junctions and Corners
This is a critical step that DIYers often miss. Use reinforcing tape or fabric (supplied or recommended by the membrane manufacturer) at all floor-to-wall junctions, internal corners, and around drain flanges. These are the most common failure points.
4. Apply the Membrane
Apply the membrane by brush or roller in even coats, following the manufacturer's recommended coverage rate and number of coats (typically two or more). Allow each coat to dry fully before applying the next.
5. Check the Dry Film Thickness
The finished dry film thickness (DFT) must meet the manufacturer's specification — commonly 1–1.5 mm for wet areas. Applying too thinly is one of the most common DIY mistakes.
6. Allow Full Cure Before Tiling or Finishing
Don't rush this step. Applying tiles or vinyl before the membrane has fully cured can trap moisture and cause failure.
Key Tips for a Long-Lasting Result
| Step | Common Mistake | Best Practice |
|---|---|---|
| Surface prep | Skipping crack repairs | Fill and prime properly |
| Corner reinforcement | Leaving junctions bare | Always use reinforcing tape |
| Coat thickness | Applying too thinly | Check DFT with a wet film gauge |
| Curing time | Tiling too soon | Follow manufacturer's cure times |
| Drain detail | Forgetting the drain flange | Seal around drain carefully |
Suitable Floor Substrates
Paint-on membranes work on concrete slabs, fibre cement sheet, and some plywood substrates. If your laundry has a timber subfloor, make sure it's rigid enough and use a membrane specifically rated for timber. Products like James Hardie fibre cement sheet are commonly used as a stable tile substrate over timber frames before membrane application.
For plasterboard walls in the laundry, GIB offers moisture-resistant board options suitable for wet areas.
When to Call a Professional
While a paint-on membrane is one of the more DIY-friendly waterproofing options, there are situations where you should bring in a qualified waterproofer or tiler:
- The subfloor is damaged, rotten, or structurally compromised
- You're unsure about the correct product for your substrate
- The laundry is on an upper floor (a leak could cause serious damage below)
- You want the work to carry a manufacturer's warranty (many require professional application)
- You're not confident about achieving the correct film thickness or junction details
A licensed professional will also be familiar with BRANZ good practice guidelines and can ensure the work meets the NZ Building Code. You can find qualified local waterproofers and tilers on tradedirectory.co.nz.
Note: Plumbing connections to your laundry (taps, waste pipes, washing machine connections) are restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board.
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