Waterproofing failures cause $20,000+ in damage. Learn what AS 3740 requires, what your builder must do, and how to spot shortcuts before it’s too late.
If your builder cuts one corner during a bathroom renovation, you want it to be anywhere except the waterproofing. A bad tile job looks ugly. Bad waterproofing destroys your floor, your walls, your subfloor, and potentially the ceiling of the room below. We’ve seen leak repairs cost $$20,000 to$$40,000 on bathrooms that were “renovated” just a few years earlier.
Waterproofing is invisible once the tiles go on. You can’t see it, you can’t inspect it after the fact, and you won’t know it failed until water starts showing up where it shouldn’t. That’s why it has to be done right the first time.
What waterproofing actually is
Waterproofing is a liquid membrane applied to the floor and walls of your bathroom before tiling. The membrane cures into a flexible, impervious layer that prevents water from reaching the structural elements behind and beneath the tiles.
In a shower, water doesn’t just hit the tiles. It seeps through grout lines, it pools at junctions, and it condenses on surfaces. Without a membrane, that moisture works its way into the substrate (the wall sheeting and floor), where it causes rot, mould, and structural deterioration.
The membrane is applied after the plumbing rough-in and floor preparation, but before any tiling begins. It needs to cure fully (usually 24 to 48 hours depending on the product) before tiles are laid on top.
What the Australian Standard requires
In Victoria, all bathroom waterproofing must comply with AS 3740:2021 (Waterproofing of domestic wet areas). This isn’t a guideline or a recommendation. It’s a legal requirement.
Here’s what AS 3740 specifies for a standard bathroom.
Shower floor and hob: The entire shower floor must be waterproofed, including the step-up (hob) at the shower entry. The membrane must extend at least 1500mm from the shower rose in all directions.
Shower walls: Waterproofing must extend to at least 1800mm above the finished floor level on all shower walls. If the shower is frameless (no screen at the top), the entire wall should be waterproofed to ceiling height.
Bathroom floor: The entire bathroom floor must be waterproofed, with the membrane turning up the walls by at least 150mm (the “turn-up”). This prevents water that pools on the floor from wicking into the wall framing.
Junctions and penetrations: Every junction between floor and wall, every pipe penetration, every floor waste, and every corner must be reinforced with waterproofing tape or bandage before the membrane is applied. These junctions are where most failures occur.
Fall to waste: The shower floor must fall towards the floor waste at a gradient of between 1:80 and 1:60. This ensures water drains rather than pooling.
The inspection is mandatory in Victoria
Here’s something many homeowners don’t know: in Victoria, an independent waterproofing inspection is required before tiling can begin. A licensed building inspector must verify that the membrane has been correctly applied, that all junctions are sealed, and that the installation complies with AS 3740.
The inspector issues a waterproofing compliance certificate. Without this certificate, the work is technically non-compliant, which can affect your insurance coverage and create problems if you sell the property.
At Hexon, we arrange this inspection as part of every bathroom renovation. The inspector comes to site, checks the membrane, and issues the certificate before our tilers start work. It’s built into our process and our quote.
How to spot waterproofing shortcuts
You can’t inspect waterproofing after the tiles are on. But you can ask the right questions before work starts.
Ask who applies the waterproofing. It should be a qualified waterproofer or a licensed builder, not the tiler doing a quick coat before laying tiles. Waterproofing and tiling are separate trades with separate skills.
Ask which product they use. The membrane should be a product that meets AS 4858 (Wet area membranes). Common products in Melbourne include Ardex WPM 300, Davco K10 Plus, and Gripset 38. Ask for the product name and check the data sheet.
Ask about curing time. A single coat of membrane needs 4 to 6 hours to cure before the second coat. The second coat needs 24 to 48 hours before tiling. If your builder is applying membrane and tiling on the same day, the curing time hasn’t been respected.
Ask about the junction treatment. Before the membrane goes on, all internal corners, floor-wall junctions, and pipe penetrations should be bandaged with waterproofing tape embedded in a coat of membrane. This reinforcement prevents cracking at stress points. If the builder just brushes membrane over a corner with no tape, it will crack.
Ask for the compliance certificate. If the builder says they “don’t bother” with the inspection, or that “it’s not required for small jobs,” they’re wrong. The inspection is required regardless of project size in Victoria. Walk away.
What bad waterproofing looks like (when it's too late)
Waterproofing failures usually show up 1 to 3 years after a renovation. The signs include: water stains on the ceiling below the bathroom, bubbling or lifting paint on walls adjacent to the bathroom, musty smell that doesn’t go away with cleaning, grout that keeps cracking or falling out (indicating movement in the substrate), and soft or spongy flooring near the bathroom entry.
By the time you see these signs, the damage is already significant. The repair involves removing all tiles, stripping the failed membrane, drying the structure (which can take weeks), replacing any damaged framing or sheeting, re-waterproofing, re-tiling, and re-installing fixtures. It’s essentially a second renovation on top of the first one.
What proper waterproofing costs
On a standard Melbourne bathroom renovation, waterproofing (including membrane, application, junction treatment, and the independent inspection) costs between $$1,500 and$$3,000. That’s roughly 8% to 12% of a mid-range renovation budget.
Compare that to the $$20,000 to$$40,000 cost of a leak repair, and it’s the best investment in the entire project. There’s no argument for cutting this cost.
The one thing you can't afford to get wrong
Every other part of a bathroom renovation is fixable after the fact. You can replace a vanity. You can re-grout tiles. You can swap tapware. But you can’t fix waterproofing without ripping everything out and starting again.
This is why we use qualified waterproofers on every project, apply the membrane in two full coats with proper curing time, reinforce every junction and penetration, arrange an independent inspection, and issue a compliance certificate before tiling begins.
If you’re planning a bathroom renovation in Melbourne, ask your builder about their waterproofing process. If the answer is vague, that tells you everything you need to know.
Talk to Hexon Renovations about your bathroom project, or call 0426 992 464.
