What "waterproofing to code" actually means

AS 3740 is the line between a bathroom that lasts twenty years and one that leaks within two. Most waterproofing failures aren't bad luck, they're shortcuts taken where no one can see them. Here's what genuine compliance looks like, and what to ask any builder before they tile.

What AS 3740 actually requires

AS 3740 is the Australian Standard for waterproofing of domestic wet areas. It sets out where membranes must go, how high they must rise, and how junctions and penetrations must be treated. It isn't a suggestion, compliant waterproofing of wet areas is a requirement, and on insured building work it's part of what protects the homeowner.

In practice, the parts that matter most are the ones you'll never see once the tiles are on:

• The shower floor and walls, the membrane must extend up the walls to a defined height, not just across the floor.
• The hob or step-down, water must be contained to the wet area and drained, not allowed to wick into adjoining rooms.
• Penetrations, every tap, drain and outlet is a potential leak point and must be sealed into the membrane, not around it.
• Junctions and corners, wall-to-floor junctions move over time and need a bond breaker so the membrane can flex without tearing.

A bathroom doesn't leak because of the tile. It leaks because of what's under the tile, and by then it's a demolition job, not a repair.

Where the shortcuts happen

The fastest way to save a day on site is to skimp on the membrane: one coat instead of two, no bond breaker in the corners, the membrane stopped too low, or tiling before it has cured. None of it is visible at handover. All of it surfaces eighteen months later as a stain on the ceiling below.

What to ask your builder

• Who is doing the waterproofing, and are they certified for it?
• How many coats, and is there a cure time before tiling?
• Will there be documented photos of the membrane before tiling?
• Is the waterproofing covered under the building warranty?

At PR&D, waterproofing is photographed at the membrane stage and logged to your client portal before a single tile goes down, so the part you can't see is the part you can prove.

Common questions