A waterproofing failure is invisible for years — until it isn't, and the fix means demolishing tiles, membrane, and sometimes the subfloor underneath. AS 3740 exists specifically to prevent this, and it's non-negotiable, not a nice-to-have.
What AS 3740 actually is
AS 3740:2021 "Waterproofing of domestic wet areas" sets minimum requirements for the materials, design, and installation of waterproofing in bathrooms, showers, laundries, and toilets in Australian homes. It's referenced directly in the National Construction Code (NCC), and in states like NSW it's legally enforced through the NCC whenever a bathroom is newly built or significantly renovated with council approval or private certification required.
The specific numbers that matter
Shower areas must be waterproofed over the entire floor and up the walls to a minimum height of 1800mm above the finished floor level, including the full height of any hob or enclosure — and the 2021 revision raised this requirement to the full water-resistant wall lining in the shower area, to whichever is higher: 1800mm or 50mm above the shower rose. Outside the shower enclosure, waterproofing must still extend at least 100mm up adjoining walls, and this 100mm minimum becomes mandatory for bathrooms on upper floors or in apartments specifically.
Why the details you can't see matter most
The standard requires a "bond breaker" — a special tape or sealant — at internal corners and junctions, such as where a wall meets the floor, allowing slight structural movement without tearing the waterproof membrane. Waterproof membranes must also be graded correctly toward the drain, and all wall junctions, floor-wall junctions, and penetrations (for pipes, fittings) must be fully waterproofed. None of this is visible once tiling is complete — which is exactly why it needs to be checked before that happens, not after.
Why AS 3740 is a "Deemed-to-Satisfy" solution
Under the NCC, AS 3740 is one recognised way to demonstrate compliance with the underlying performance requirement for wet area waterproofing (a "Deemed-to-Satisfy" solution). This means a bathroom waterproofed in line with AS 3740 is automatically treated as meeting the code — but it also means work that doesn't follow the standard hasn't met the code, regardless of how it looks once tiled.
How to protect yourself before it's covered up
- Ask your contractor to notify you before tiling begins, specifically so you (or an independent inspector) can view the membrane while it's still visible.
- Check the waterproofing extends to at least 1800mm in the shower area and 100mm on other walls, as a minimum.
- Confirm waterproofing must be performed by a specifically licensed tradesperson in your state — this isn't DIY-legal work in most of Australia.
- Always request a Certificate of Compliance for the waterproofing specifically, separate from the general building completion documentation — this is essential for insurance and warranty purposes later.
- If a leak appears after the bathroom is finished, don't assume a full rebuild is automatically required — targeted repairs like epoxy regrouting can sometimes resolve an issue caught early, before it's spread to the subfloor.
What this means practically
- AS 3740 sets specific, checkable minimums — 1800mm in showers, 100mm elsewhere — not vague "best practice" guidance.
- The waterproofing membrane is only inspectable before tiling — arrange your own check at that stage if you have any doubt about the contractor's work.
- A Certificate of Compliance for waterproofing specifically should be requested and kept — it matters for both insurance and any future warranty claim.
- Waterproofing must be done by a licensed tradesperson in most states — it is not legal DIY work in a renovation context.
Sources
- HIA — AS 3740 Waterproofing Standard, wet areas in buildings: hia.com.au
- Sydney Sealed — NSW Waterproofing Standards (AS 3740) Explained: sydneysealed.com.au
Related Kibbo Tools
- Final Inspection Checklist — includes bathroom-specific checks to run before signing off on completed work.
- Defective Work & Contractor Dispute Evidence Pack — document a waterproofing failure if one appears after completion.