Can I Apply Waterproofing Membrane Over Existing Tiles?

In many situations, yes — you can apply a waterproofing membrane over existing tiles, but only if the tiles are in good condition, firmly bonded, and properly prepared. Skipping the preparation steps is the most common reason waterproofing fails, so it pays to understand what's involved before you start.

When Tiling Over Existing Tiles Is Acceptable

Applying a waterproofing membrane over existing tiles is generally acceptable when:

• All tiles are fully bonded with no hollow spots, cracks, or loose edges
• The grout lines are intact and not crumbling
• There is no evidence of existing water damage, mould, or substrate movement beneath the tiles
• The existing tile surface is clean, dry, and free of soap scum, sealers, or contaminants
• The added height (tiles + membrane + new tiles, if retiling) won't cause issues with door clearances, drainage falls, or step heights

If any tiles are hollow, cracked, or lifting, those areas must be removed and the substrate repaired before you proceed. Waterproofing over a compromised surface will simply trap moisture and accelerate damage.

NZ Building Code Requirements

In New Zealand, wet areas such as showers, bathrooms, and laundries must comply with NZ Building Code Clause E3 (Internal Moisture). This means waterproofing in these areas must be fit for purpose and durable. Whether you're doing a repair or a full renovation, the waterproofing system you choose should be appropriate for the substrate and use case.

For most residential wet areas, building consent is not required for like-for-like waterproofing repairs under Schedule 1 exemptions — but always confirm this with your local council before starting work, as exemptions can depend on the scope of the job. You can find guidance at MBIE Building Performance.

Surface Preparation: The Most Important Step

Proper prep is everything. Here's what you need to do before applying any membrane:

1. Check for hollow tiles — tap each tile with a coin or knuckle. A dull thud indicates a hollow spot. Remove and re-bond any hollow tiles.
2. Clean thoroughly — remove all soap scum, grease, and cleaning product residue. Use a suitable degreaser and rinse well.
3. Sand or abrade the surface — glazed tiles are non-porous, so lightly sanding or using a bonding primer helps the membrane adhere.
4. Fill grout lines — some membrane systems require grout lines to be filled flush before application. Check your product's technical data sheet.
5. Allow to dry completely — the surface must be bone dry before membrane application.

Choosing the Right Membrane Product

Not all waterproofing membranes are suitable for application over tiles. Look for products specifically rated for this use. In New Zealand, well-regarded options include liquid-applied membranes from Ardex, Mapei, and Sika. Always read the technical data sheet (TDS) and application guide for your chosen product — these will specify:

• Whether a primer is required over glazed tiles
• Minimum dry film thickness
• Number of coats needed
• Cure times between coats and before tiling
• Reinforcing fabric requirements at corners and junctions

Key Application Tips

• Reinforce all internal corners and junctions with membrane fabric tape — this is where movement and cracking most commonly occur
• Apply membrane in two or more coats, allowing each coat to cure as per the manufacturer's instructions
• Pay particular attention to penetrations (waste outlets, tap holes) — these are common leak points
• Don't rush — membrane applied too thickly in one coat can skin over while remaining wet underneath
• Keep the area well ventilated during application and curing

You can find membrane products and tools at Bunnings, Mitre 10, and PlaceMakers.

When NOT to Waterproof Over Existing Tiles

Avoid applying membrane over existing tiles if:

• There is active leaking or water damage to the substrate or framing below
• The tiles are on a timber substrate that shows signs of rot or movement
• More than a small number of tiles are hollow or damaged
• The existing waterproofing has failed completely and the substrate is compromised
• Adding height would create a trip hazard or affect drainage falls

In these situations, a full strip-out and re-waterproofing from the substrate up is the safer and more durable solution.

When to Call a Professional

While applying waterproofing membrane is not restricted work under NZ law (unlike plumbing or electrical), it is a skill-dependent task where mistakes can be costly. A failed waterproofing job can cause serious structural damage to framing and linings — damage that often isn't visible until it's extensive.

Consider hiring a qualified waterproofer or tiler if:

• You're unsure about the condition of the existing substrate
• The area has a history of leaks
• You're working in a rental property (see Tenancy Services for Healthy Homes obligations)
• The job involves any plumbing penetrations — plumbing work must be done by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board

You can find experienced, local waterproofing and tiling professionals on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – NZ Building Code
• BRANZ – Wet Area Waterproofing Guidance
• Ardex NZ – Waterproofing Systems
• Mapei NZ – Waterproofing Products
• Sika NZ – Construction Products
• WorkSafe NZ – Safe Work Practices