How Do I Prepare a Wet Area So a Waterproofer Can Start Immediately?

Getting your wet area properly prepared before a waterproofer arrives is one of the best things you can do to keep your project on time and on budget. A well-prepped surface means the waterproofer can apply their membrane straight away rather than spending billable hours doing work you could have handled yourself. Here's a practical, step-by-step guide tailored to New Zealand homes and the NZ Building Code.

Understand What the Waterproofer Needs

Waterproofing in wet areas — bathrooms, ensuites, laundries, and shower enclosures — is governed by NZ Building Code clause E3 (Internal Moisture). The waterproofing membrane must be applied to a surface that is clean, dry, structurally sound, and free of contamination. Any shortcuts at this stage can cause membrane failure, leaks, and costly remediation later. Check MBIE Building Performance for the full requirements around internal moisture control.

Step 1: Strip Out the Old Fitout

If you're renovating an existing wet area, remove everything that's coming out:

• Vanities, toilets, and tapware — turn off the water supply first at the isolation valve or toby. Note that disconnecting and reconnecting plumbing fixtures is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be done by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board. You can remove a vanity cabinet itself, but leave the pipe connections to a licensed plumber.
• Old tiles and adhesive — chip off existing tiles and grind or scrape adhesive residue back to a flat, clean substrate. Hire an angle grinder or floor grinder from a local hire shop if needed.
• Old sealant and waterproofing membrane — remove all traces of silicone, old membrane, and paint. Waterproofing products won't bond properly over existing coatings.

Safety note: Tiles in homes built before the 1990s may contain asbestos in the adhesive or backing. If you're unsure, don't grind or sand — get a sample tested by a certified asbestos assessor before proceeding. See WorkSafe NZ for guidance.

Step 2: Check and Fix the Substrate

The substrate (the surface being waterproofed) must be solid and stable. Common substrates in NZ wet areas include:

Replace any damaged sheets before the waterproofer arrives. Fill screw holes and joints with an appropriate filler — your waterproofer may have a preferred product, so ask them first. Products from Ardex or Mapei are commonly used for substrate preparation in NZ.

Step 3: Ensure the Surface Is Bone Dry

This is critical. Most waterproofing membranes — whether liquid-applied or sheet — require a dry substrate to bond correctly. In New Zealand's humid climate, especially in winter or in poorly ventilated bathrooms, this can take longer than expected.

• Run a dehumidifier or fan heater in the room for at least 24–48 hours before the waterproofer arrives.
• Check moisture levels with a moisture meter if you have one. Most membranes require substrate moisture content below 5%.
• Do not waterproof over a freshly poured concrete slab — new concrete typically needs at least 28 days to cure, though your waterproofer will advise based on the specific product.

Step 4: Pre-Install Penetrations and Backing

Before waterproofing begins, all pipe penetrations, floor wastes, and backing plates should already be in place. The waterproofer needs to membrane around these, not cut through a finished membrane later.

• Floor waste drains should be set at the correct finished floor height (accounting for tile and adhesive thickness).
• Backing noggings for grab rails, towel rails, and shower niches should be framed in.
• Any recessed shower niches should be lined and ready.

Again, any plumbing work here must be done by a licensed plumber.

Step 5: Clean the Surface Thoroughly

Vacuum or sweep away all dust and debris. Wipe surfaces with a damp cloth and allow to dry. Remove any grease, soap residue, or paint with an appropriate cleaner. Even a thin layer of dust can prevent a membrane from bonding properly.

Step 6: Confirm Access and Clear the Space

Practical but often overlooked:

• Clear the room completely — no tools, materials, or personal items.
• Ensure the waterproofer has easy access to power and water.
• Protect adjacent flooring and surfaces from splashes.
• If the job is in a tenanted property, coordinate access carefully and be aware of your obligations under Tenancy Services rules.

When to Call a Professional

Waterproofing itself is not legally restricted work in NZ, but it is high-consequence work — a failed membrane can cause significant structural damage and mould issues. Always use a qualified, experienced waterproofer. If your project involves a building consent (check with your local council — many bathroom renovations are Schedule 1 exempt, but confirm this), the waterproofing may need to be inspected before tiling. See MBIE Building Performance for consent guidance.

For any plumbing disconnections or reconnections, you must use a licensed plumber registered with the Plumbers Gasfitters & Drainlayers Board. For electrical work such as moving a heat lamp or extractor fan, use a registered electrician — see the Electrical Workers Registration Board.

You can find qualified local waterproofers and tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – NZ Building Code
• WorkSafe NZ – Asbestos guidance
• BRANZ – Wet area construction guidance
• Plumbers Gasfitters & Drainlayers Board
• James Hardie – Villaboard substrate information
• Ardex NZ – Waterproofing and substrate products