A Homeowner's Guide to Bathroom Waterproofing Standards in Australia (AS 3740)

Waterproofing is the single most important part of any bathroom renovation, and the one homeowners are least equipped to check for themselves once tiles go down over the top. Here's a plain-English guide to AS 3740, the Australian Standard that governs bathroom waterproofing, and what it means for your project.

What Is AS 3740?

AS 3740 is the Australian Standard for waterproofing of domestic wet areas — bathrooms, laundries and similar spaces. It sets out minimum requirements for which surfaces must be waterproofed, how membranes should be applied, and how water is meant to be managed even if it does get past the tile surface, such as via floor falls directing water to the drain.

Which Areas Must Be Waterproofed

Under the standard, certain areas are always required to be fully waterproofed (membrane covering the entire floor and walls to a set height), while others require waterproofing only in specific zones. Requirements vary based on the flooring type below and whether the bathroom is on a ground floor or an upper level, but generally include:

• The entire shower recess floor and walls, typically to at least 1800mm height
• Walls adjoining a bath, generally to at least 150mm above the bath edge
• The full bathroom floor if it's a timber or particleboard substrate, or on an upper storey above a habitable room
• Floor wastes and any penetrations (pipes, drains) sealed as part of the membrane system

Why Floor Falls Matter as Much as the Membrane

A correctly waterproofed floor isn't just coated — it's also graded with a slight fall (typically 1:100) toward the floor waste, so any water that reaches the floor drains away rather than pooling. A membrane without correct falls can still lead to water sitting against skirting or door thresholds over time, even if the waterproofing itself hasn't failed.

How Waterproofing Actually Fails

Most failures we're called out to inspect come down to one of a handful of causes: membrane not extending high enough up the wall, corners and penetrations (like pipe entries) not properly sealed, insufficient cure time before tiling, or a membrane that was never installed correctly in the first place — often on older homes renovated decades before current practices were standard.

What a Compliant Job Should Include

• A licensed waterproofer (in Victoria, this requires specific registration for wet area waterproofing)
• A Certificate of Compliance for the waterproofing work, which you're entitled to request and keep
• Correct cure time between coats before tiling begins — rushing this is a common shortcut
• Floor falls tested and confirmed before tiling covers them permanently

What to Ask Your Renovator

Before work begins, it's reasonable to ask who is doing the waterproofing, whether they're separately licensed for it, and whether you'll receive a compliance certificate on completion. A renovator who can answer these clearly and without hesitation is one who takes this part of the job as seriously as they should — because unlike tiling or paint, a waterproofing failure is invisible until it's already caused expensive damage.

Why This Is Worth Getting Right the First Time

Because waterproofing sits underneath the finished surface, a failure is rarely visible until water has already reached the subfloor, wall framing, or the ceiling of the room below — at which point the repair involves ripping out tiles and fixtures to fix a problem that a correctly applied membrane would have prevented entirely. It's the one line item in a renovation budget that's genuinely false economy to skimp on.