Australian Standard AS 3740. Waterproofing of Domestic Wet Areas
AS 3740 Waterproofing of Domestic Wet Areas. Reference Guide
The Australian Standard behind every bathroom waterproofing dispute. Falls, junctions, hobs, floor wastes, what compliant work looks like and where it most often fails.
Failed waterproofing in domestic wet areas is one of the most expensive defects to rectify, and one of the most disputed in Australian residential construction. AS 3740 is the standard that defines what compliant waterproofing in showers, bathrooms, laundries, and toilets looks like. Building inspectors who understand the standard can spot the recurring defect modes long before they become structural damage.
What AS 3740 covers
The scope and focus of the standard in plain language.
Wet area definitions
AS 3740 defines what counts as a wet area within a Class 1 or Class 2 building. Showers, baths, laundries, kitchens, and toilet rooms all fall within scope, with different waterproofing extent rules.
Required waterproof areas
The standard specifies how high walls must be waterproofed (e.g. full shower walls to 1800mm), how far floors must extend, and how junctions between different surfaces must be sealed.
Falls and drainage
Floors in wet areas must fall to floor wastes at minimum gradients. Showers without enclosures must have falls that prevent water flowing into adjacent dry areas.
Membrane types and application
AS 3740 references AS 4858 for waterproofing membrane materials. Liquid membranes, sheet membranes, and proprietary systems all have specific application requirements.
Key obligations
What the standard actually requires of inspectors, operators, or duty holders.
Use a licensed waterproofer
Most states require waterproofing work over a certain value to be performed by a licensed waterproofer. Documentation of the licensee should be on file with the build.
Apply membrane to AS 4858 standard
The membrane material itself must meet AS 4858 (liquid-applied) or AS 4654 (sheet, used for above-ground external) as applicable.
Treat junctions and penetrations
Wall-to-floor junctions, hob-to-floor junctions, and penetrations (waste, taps, drains) all require specific detailing with reinforcing tape or fillets.
Maintain falls to floor waste
Minimum 1:100 fall to the floor waste over the whole wet area. Showers typically require 1:80 to ensure no ponding.
Wait for cure before tiling
Liquid membranes have specified cure times. Tiling over uncured membrane is one of the most common causes of subsequent failure.
Common pitfalls
Where inspectors and duty holders most often get caught out.
- !Tiling over an uncured membrane, the membrane never reaches design strength and fails within months.
- !Inadequate falls, water ponds against the membrane and finds the smallest defect.
- !Untreated wall-to-floor junctions, the most common rectification cause.
- !Penetrations through the membrane (taps, wastes) without proper sealing detail.
- !Missing or inadequate hob waterproofing on shower steps.
- !Failure to extend membrane up walls to the required height (typically 1800mm in shower areas).
How InspectAndGo helps with AS 3740
- Inspection templates with wet area items for kitchens, bathrooms, ensuites, laundries, and toilets
- Photo capture of suspect junctions, falls, hobs, and floor wastes with GPS metadata
- Comment library starter set includes common AS 3740 defect descriptions and rectification recommendations
- Defect tracking with severity classification (major/minor) for owners corporation reports
- Audit-ready inspection records that support warranty and SBBIS claims
Frequently asked questions
How is AS 3740 different from AS 4654?
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AS 3740 covers waterproofing of domestic wet areas, bathrooms, laundries, kitchens, toilets, inside the building envelope. AS 4654 covers external above-ground waterproofing membranes, balconies, planter boxes, roof terraces. They use different membrane types and different installation requirements.
Is a waterproofing certificate required?
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Most states require a Form 16 (QLD), Compliance Certificate (NSW), or equivalent licensed waterproofer certification for new wet areas. The certificate should be retained as part of the building documentation and produced on request.
How long does waterproofing last?
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A correctly installed membrane should last 15 to 25 years. Failed waterproofing usually shows up within the first 5 years if there are detailing or curing defects. The Strata Building Bond and Inspections Scheme (NSW) specifically targets the 15-24 month window for this reason.
What does rectification of failed wet area waterproofing involve?
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Typically: lifting tiles, removing the failed membrane, re-tanking the substrate, applying new membrane, allowing cure time, and re-tiling. It is disruptive and expensive, often $15k to $40k for a single bathroom, and is one of the strongest arguments for inspecting waterproofing while it is still under warranty.
Can a building inspector identify failed waterproofing?
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Inspectors can identify the symptoms, moisture stains on adjacent walls or ceilings, efflorescence on masonry, lifted tiles, soft skirtings, mould growth. The actual diagnosis usually requires a moisture meter, sometimes thermal imaging, and occasionally invasive investigation by a waterproofing specialist.
Document waterproofing inspections defensibly
GPS-verified photos, structured wet area templates, and defect tracking that holds up to warranty claims and OC disputes.