Bathroom Waterproofing: Why It Matters (and What Goes Wrong)

Waterproofing is the single most important step in a bathroom — and the one you'll never see. It's a continuous membrane under your tiles and behind your walls that stops water reaching the floor, framing and the rooms beyond. Done to Australian Standard AS 3740 by a licensed waterproofer, it protects the whole job for decades. Done badly, it's the reason bathrooms rot, leak and have to be torn out years early.

What the waterproofing actually does

Tiles and grout are not waterproof — water passes straight through them over time. The membrane underneath is what actually holds water in the wet area and channels it to the floor waste. In the shower it runs up the walls, across the floor and over every junction; across the rest of the bathroom it protects the floor and the base of the walls. Without it, every shower slowly soaks your subfloor and framing.

The Australian Standard: AS 3740

AS 3740 is the national standard for waterproofing wet areas, and it exists because water damage is one of the most common and expensive building defects in the country. It dictates where the membrane goes, how high it runs in and around the shower, how it turns up the walls, and how floor wastes and junctions must be sealed. A bathroom built to the standard and certified by a licensed waterproofer is one you can forget about — as it should be.

Where waterproofing goes wrong

When a bathroom leaks, it's rarely the tiles — it's the detailing underneath, and almost always one of these:

• Poor detailing at junctions. Wall-to-floor corners, pipe penetrations and the floor waste are where movement concentrates; skimped bond breakers and reinforcing here are the number-one failure point.
• No membrane behind the bath. Hidden walls behind a bath or a hob get skipped because 'no one sees them' — until water finds the gap.
• Skipping the falls. If the floor isn't graded correctly to the waste, water pools instead of draining and eventually works its way through.
• One coat, or the wrong product. A single thin coat, or a membrane that isn't compatible with the substrate, cracks and lets go far too early.
• Tiling too soon. Covering a membrane before it's fully cured compromises the seal from day one.

Signs your waterproofing has failed

Failures announce themselves slowly, so catching them early saves thousands. Look and smell for:

• A musty, damp smell that won't clear no matter how well you ventilate.
• Drummy, lifting or cracked tiles, especially in the shower base.
• Discoloured, soft or crumbling grout and silicone around the floor and corners.
• Bubbling paint or damp patches on the far side of a shower wall or in the next room.
• Water stains on the ceiling of the room directly below the bathroom.

The cost of getting it wrong versus doing it right

This is why waterproofing is the last place to save money. Redoing a failed membrane means stripping the tiles, screed and often the vanity, shower and toilet to get back to the slab — so a re-waterproof frequently costs as much as a brand-new bathroom, commonly $20,000 or more, plus repairs to whatever the water damaged on its way through. Doing it properly the first time is a tiny fraction of that. Our bathroom renovation cost guide shows where it sits in a full quote.

The Reno Build way

We've waterproofed thousands of bathrooms across the Northern Beaches since 2009, and we don't leave the most important step to chance. Every wet area gets certified, dual-layer waterproofing installed by our own licensed waterproofers — not a sub-contractor squeezed in between jobs — and it's all covered by our fixed price and 21-day guarantee. It's the part of the job you'll never see, and the part we're most particular about. Get your fixed-price quote.

Frequently asked questions