If your tiler says "we'll just slap on a bit of waterproofing," that's your cue to start asking serious questions. What happens behind your tiles matters far more than what you see in front of them. Get it wrong, and you're looking at hidden moisture damage, mould, structural decay, and insurance claims that won't be covered because the work wasn't done properly.

The reality is this: bathroom waterproofing in New Zealand isn't optional, it's not negotiable, and it's not something to cut corners on. It's a legal requirement under the Building Code, it triggers specific compliance obligations, and it directly affects whether your home will be insurable and sellable down the track.

This guide explains what you actually need to do, when you need to do it, and what documentation you must keep.

Waterproofing is mandatory in any wet area where water is regularly splashed or sprayed. That includes showers, shower-over-bath setups, bathrooms with floor drains, and laundries with washing machines. It's not a suggestion. Waterproofing is a requirement under NZBC clause E3 (Internal Moisture).

The specific areas that must be waterproofed:

Shower walls must be waterproofed to at least 1800mm above the shower floor. Adjacent walls beside a bath or basin need protection to at least 150mm above the top edge. Entire shower floor areas require full waterproofing coverage. Laundry floors with water sources need protection.

If you're renovating a bathroom and installing new tiles over any substrate (plasterboard, concrete, timber), you must install a waterproof membrane system underneath. This isn't just best practice. It's a legal obligation under the Building Code. Councils inspect for it. Insurance companies ask about it. Future buyers will want evidence of it.

Water finds its way through grout and tile. Without a membrane underneath, that water soaks into the substrate, rots timber framing, causes mould growth, and damages the building structure. A proper membrane stops water at the source and directs it safely to drainage.

Two main membrane types are used in NZ bathrooms: liquid-applied and sheet membranes. Both work, but they have different installation requirements and performance characteristics.

Liquid membranes are painted or sprayed onto the substrate. They cure to form a seamless, flexible waterproof layer. The advantage is simplicity: no seams to worry about in most areas. The disadvantage is that application quality depends heavily on the installer's skill and attention to detail.

Liquid membranes must be applied in multiple coats to achieve the required thickness (typically 2-3mm). Each coat must cure properly before the next is applied. Corners, penetrations, and edges require careful detailing. If the installer rushes or misses spots, the membrane fails.

Sheet membranes are rolls of flexible material (usually PVC or similar) that are laid over the substrate. The critical detail here is the seams: they must be heat-welded together, not just overlapped. Welded seams create a continuous, reliable barrier. Overlapped seams without welding are a common failure point and won't meet Building Code requirements.

Sheet membranes are more forgiving of substrate imperfections because they're thicker and more durable. They're also easier to inspect: you can see the seams and verify they're properly welded.

Both membrane types must work with proper drainage and falls. This is where many DIY or budget jobs fail.

The substrate underneath the membrane must be sloped toward a drain outlet. Minimum fall is typically 1:80 (roughly 12mm drop per metre). Without adequate fall, water pools on the membrane, eventually finding its way through seams or penetrations.

Drainage outlets must be properly installed and sealed. The membrane must be heat-welded or sealed around the outlet flange. If water can get behind the membrane at the drain, the whole system fails.

The membrane must extend at least 25mm past the wet zone boundary. For floors, it covers the entire floor area. For walls, it extends to the required height (1800mm for shower walls, 150mm for adjacent walls).

This is the critical part that catches many homeowners off guard. Not all bathroom work requires building consent, but waterproofing work often does, and when it does, it triggers "Restricted Building Work" (RBW) requirements.

Restricted Building Work means you must use a Licensed Building Practitioner (LBP). You cannot do this work yourself, and you cannot hire an unlicensed tradesperson to do it. Fines for breaching this are up to $50,000.

Your bathroom work triggers RBW if it involves:

Not all bathroom work requires consent. Replacing tiles without touching the substrate underneath, for example, might not. But the moment you're installing a new membrane or renovating the substrate, you're likely in RBW territory.

Ask your local council before you start. A 10-minute phone call will clarify whether your specific project needs consent.

If your bathroom work requires building consent, you'll need a Producer Statement (PS3) for waterproofing. This is a formal document signed by the person who installed the waterproofing, confirming that the work complies with the Building Code and the manufacturer's specifications.

A PS3 is required for building consent approval, Code of Compliance Certificate (CCC) issuance, future insurance claims, and property sales. It's not optional.

What the PS3 confirms:

The waterproofing was installed according to the manufacturer's system and specifications. The work complies with NZBC clause E3 (Internal Moisture). All seams, penetrations, and edges were properly sealed or welded. The membrane extends to the required heights and coverage areas. Drainage and falls are adequate.

You must also keep:

Why this matters: If you ever have a water damage claim, your insurance company will ask for evidence that the waterproofing was done properly. Without documentation, they can refuse to pay. If you sell your home, buyers' inspectors will look for evidence of proper waterproofing. If the council comes knocking years later, you need proof the work was compliant.

NZBC clause E3 (Internal Moisture) governs bathroom waterproofing and requires wet areas be constructed to prevent moisture accumulation and fungal growth.

E3 requires:

E2 (External Moisture) is related but different. E2 governs how the building envelope resists external moisture (rain, wind-driven moisture). E3 governs internal moisture (splashing, condensation, drainage). Both matter for a healthy home, but they're separate requirements.

If your bathroom waterproofing fails and water gets into the building structure, it can compromise E2 compliance as well. Moisture in the walls can lead to rot, mould, and structural damage. That's why proper waterproofing isn't just about passing inspection. It's about protecting your home's long-term health.

Scenario 1: Hidden Damage

Water seeps through grout and tile, soaks into the substrate, and rots timber framing behind the walls. You don't notice for months or years. By the time you do, the damage is extensive and expensive to fix.

Scenario 2: Insurance Rejection

You have a water damage claim. The insurer investigates and finds the waterproofing wasn't done to code. They refuse to pay because the damage resulted from non-compliant work.

Scenario 3: Sale Complications

You sell your home. The buyer's inspector finds evidence of poor waterproofing or water damage. The sale falls through, or the price drops significantly.

Scenario 4: Council Action

The council discovers unconsented or non-compliant bathroom work. They issue a notice to fix, which can be expensive and disruptive.

The cost of doing it right the first time is far less than fixing these problems later.

Before you start any bathroom renovation:

Bathroom waterproofing isn't glamorous, but it's essential. It's the difference between a bathroom that lasts 20 years and one that causes structural damage within five. Do it properly, document it thoroughly, and you'll protect your home and your investment.