AS 3740 is the Australian Standard governing how the wet areas of a home are waterproofed. It is one of the most important standards for any new build, because the consequences of getting waterproofing wrong are among the most expensive to fix once a home is finished.
What AS 3740 actually means
AS 3740 sets out the minimum requirements for waterproofing showers, bathrooms, laundries, ensuites and other internal wet areas. It defines which surfaces must be waterproofed, the minimum heights a membrane must reach, how the membrane must be carried over junctions and penetrations, and how it must connect to floor wastes. In short, it describes how to build a continuous, unbroken barrier that keeps water where it belongs and out of the building structure.
The standard works hand in hand with the National Construction Code. The NCC states the performance outcome — water must not penetrate the building elements behind a wet area — while AS 3740 provides the accepted method for achieving that outcome.
Where it applies in your new home
Anywhere water is regularly present, AS 3740 is relevant. The most scrutinised area is the shower, where the membrane must extend up the walls and across the floor without interruption. But the standard also covers bathroom and ensuite floors, the area around baths, laundry tubs and floors, and the junctions where these meet walls, doorways and penetrations such as tap and waste outlets.
These junctions are where most failures occur. A membrane that performs perfectly across a flat floor can still fail if a single corner or pipe penetration is not properly dressed and sealed.
What VG Inspect checks against AS 3740
The most valuable time to verify waterproofing is at the waterproofing stage — after the membrane is applied but before tiling conceals it. At this point an inspection confirms the membrane reaches the required heights, that junctions and penetrations are properly treated, that falls direct water to the waste, and that the substrate was prepared correctly.
Where a home has already been tiled, the assessment shifts to the available evidence: the falls to the floor waste, the quality of junction detailing, the presence of compliant waterproofing certificates, and any early signs of moisture movement. Every finding is documented in writing with photographs.
What can go wrong
The classic failures are a membrane applied below the required height, a junction or corner left unsealed, a penetration around a tap or waste that was not dressed correctly, and physical damage to the membrane by a following trade before tiling. Any one of these allows water to bypass the barrier.
Because the membrane is hidden the instant tiling starts, these defects are invisible to the homeowner. The symptoms — efflorescence, swelling skirtings, peeling paint in adjacent rooms, musty smells, or structural timber decay — can take months or years to surface, by which point rectification means removing finished tiling and sometimes cabinetry.
What AS 3740 does and doesn't cover
AS 3740 covers internal domestic wet areas. It is not the standard for external waterproofing such as balconies and podiums above habitable spaces, which fall under separate provisions, nor does it govern roof or below-ground tanking. Within its scope, though, it is the benchmark — and an independent check against it at the right moment is one of the highest-value inspections in a new home.
A VG Inspect QBCC-licensed inspector (QBCC Licence 1318443) carries out every wet-area inspection personally. To verify your waterproofing before it disappears behind the tiles, call 07 3180 8041 or book online.