Can I Waterproof a Shower Tray Without Removing the Existing Tiles?

In most situations, you cannot properly waterproof a shower tray without removing the existing tiles. Waterproofing membranes are designed to be installed beneath the tile layer, bonded directly to the substrate. Applying a surface treatment over old tiles is rarely a compliant or long-lasting solution — but there are some limited options worth understanding before you make a decision.

Why Waterproofing Goes Under the Tiles

The NZ Building Code clause E3 (Internal Moisture) requires that wet areas like showers are waterproofed to prevent moisture from damaging the building structure. The standard approach — and the one recognised by BRANZ and the industry — is to apply a liquid-applied membrane or sheet membrane directly to the prepared substrate (typically fibre cement sheet, concrete, or GIB Aqualine plasterboard in wet areas), then tile over the top.

When tiles are already in place, the waterproofing layer is effectively buried. If that original membrane has failed — or was never installed correctly — moisture is already getting through, and painting something over the tile surface won't stop it.

What Happens When Shower Waterproofing Fails

Signs that your shower waterproofing has failed include:

• Grout cracking or discolouring
• Tiles becoming loose or hollow-sounding when tapped
• Musty smells or mould around the shower base
• Water staining or damage on the other side of the wall or floor
• Soft or spongy substrate when you press on tiles

If you're seeing any of these, surface-only treatments will not fix the underlying problem. The tiles will need to come off.

Can You Apply Waterproofing Over Existing Tiles?

There are surface-applied waterproofing products on the market — including some from Sika and Ardex — that can be applied over existing tiles in certain circumstances. However, there are important caveats:

• The existing tiles must be fully bonded, with no hollow spots, cracks, or loose tiles
• All grout joints must be sound and free of mould or contamination
• The surface must be thoroughly cleaned and prepared
• These products are generally considered a temporary or remedial measure, not a full Code-compliant solution
• They are typically not suitable as a primary waterproofing system for a shower floor (the tray), where water pooling and foot traffic create the most stress

Always check the manufacturer's technical data sheet to confirm whether their product is rated for use over existing tiles in a shower tray application. Do not assume a product is suitable without reading the documentation.

The Grout and Silicone Option

If your tiles are structurally sound and the issue is limited to failed grout or silicone joints, re-grouting and replacing the silicone at junctions (floor-to-wall, wall corners, around the drain) can help reduce water ingress. This is not waterproofing in the Building Code sense, but it can extend the life of an otherwise intact installation.

Use a mould-resistant silicone rated for wet areas, and make sure all old silicone is fully removed before applying new product. Products are available at Bunnings and Mitre 10.

The Right Fix: Strip and Re-Waterproof

If your shower tray waterproofing has genuinely failed, the correct and lasting solution is:

1. Remove all tiles from the affected area
2. Inspect and repair the substrate (replace any damaged sheet if needed)
3. Apply a compliant waterproofing membrane — liquid-applied systems from Ardex, Mapei, or Sika are commonly used by NZ waterproofers
4. Allow full cure time as specified by the manufacturer
5. Re-tile using appropriate adhesive and grout for wet areas

This is the approach that will meet NZ Building Code E3 requirements and give you a durable, watertight result.

Do You Need a Building Consent?

In many cases, replacing waterproofing and retiling a shower is considered maintenance or like-for-like repair and may fall under Schedule 1 exemptions — meaning no building consent is required. However, this depends on the scope of work and your local council's interpretation. Always check with your local council before starting work to confirm whether consent is needed. You can find general guidance at MBIE Building Performance.

When to Call a Professional

Waterproofing a shower tray is a job where getting it wrong is costly — failed waterproofing can cause serious structural damage that is far more expensive to fix than the original repair. Consider calling a qualified waterproofer or tiler if:

• You can see signs of substrate damage or rot
• The shower has leaked for some time
• You're unsure whether the original waterproofing was ever installed correctly
• You want the work to be compliant and defensible if you sell the property

A licensed waterproofer will know which membrane systems are appropriate for your substrate and situation, and can apply them correctly. You can find experienced local waterproofers and tilers on tradedirectory.co.nz.

Note: Plumbing work — such as moving or replacing the shower drain — is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the PGDB.

Sources & Further Reading

• MBIE Building Performance – Building Code clause E3
• BRANZ – Wet area construction guidance
• Ardex NZ – Waterproofing systems
• Sika NZ – Waterproofing products
• Mapei NZ – Tile and waterproofing systems
• PGDB – Licensed plumbers and drainlayers