AS 3740 is the Australian Standard for waterproofing of domestic wet areas: bathrooms, laundries and similar internal wet areas in residential construction. It answers the questions supervisors actually have on site, such as how far a membrane must run up a shower wall and what a graded fall to waste means in a compliance sense. It is a Standards Australia document, so the clause values are not reproduced here, and anyone signing off wet-area work should be working from the current edition rather than a summary.
The short version
Once tiles are down, AS 3740 compliance cannot be verified again without demolition. That makes the pre-cover inspection the only compliance checkpoint that will ever exist for that wet area. Three things to check: membrane extent, overlap at every junction, and fall to the waste. Photograph it before the tiler starts.
Why this is a supervisor’s problem, not only the waterproofer’s
It is tempting to treat waterproofing as the subcontractor’s liability: they are licensed, they know the Standard. But a supervisor who cannot independently verify a wet area before it is covered is signing off work they will never be able to check again. That is not a criticism of waterproofers; it is a structural fact about build sequence. The membrane is visible for a matter of days, and then it is under screed and tile for the life of the building.
The liability position follows from that. Under most standard residential building contracts the head contractor carries responsibility for defects in the work regardless of which subcontractor installed what. When a defect surfaces inside the statutory warranty period, the question is what the builder verified and what they can evidence.
The three checks
- Membrane extent. The Standard sets minimum heights up walls, higher around showers than in splash zones, and full floor coverage in fully wet areas. Check the actual installation against the relevant clause before anything covers it, not against what looks about right.
- Overlap at junctions. Wall-to-floor junctions, pipe penetrations and door thresholds carry specified minimum overlaps. This is where nearly every real failure starts, because a junction is a judgement call made under time pressure rather than a flat repetitive application.
- Fall and drainage. Floors must fall to the waste so water drains rather than pools. Water still ponding twenty-four hours after a wet-area test is a failure you can find with a bucket and a timer, and it is one of the few checks that needs no specialist knowledge at all.
Two things that are not compliance
- “The waterproofer is licensed, so it is compliant.” A licence establishes competence to do the work. It is not evidence that a particular instance of work met the Standard.
- “It looks waterproofed.” A membrane can look complete and still have insufficient overlap at a junction that will fail within two years. The Standard exists precisely because the failure modes that matter are not the ones visual inspection catches.
The thirty seconds that decides a future dispute
A dated photograph of the completed membrane before tiling starts, covering the full extent with the junctions clearly visible, costs half a minute. If a defect claim arrives two years later it may be the most valuable document in the file. It does not replace understanding what the Standard requires, but it converts a supervisor’s judgement call into something documented rather than recalled.
The habit generalises. Verify before you cover, and do not rely solely on a subcontractor’s word for something that becomes uncheckable. The same logic applies to structural connections before cladding, and to electrical and plumbing rough-in before plasterboard. Waterproofing is where the discipline is most expensive to skip.
Where the failures actually happen
Junctions, not the flat field of the membrane, and usually because of substrate condition or time pressure rather than a knowledge gap. The companion guide on balcony and wet-area waterproofing defects covers the cost and failure patterns in detail.
For the CPD side, Technical issues is the NSW topic these sit under, and waterproofing is one of the regulator’s own examples for it. The NSW builder CPD guide explains how the points work.
Questions builders ask
What does AS 3740 cover?
Waterproofing of domestic wet areas: bathrooms, laundries, and similar internal wet areas in residential construction. It sets where membrane is required, how far it must extend up walls and across floors, the minimum overlaps at junctions and penetrations, and the fall and drainage requirements that stop water ponding.
Do I need to buy the Standard to comply with it?
AS 3740 is a copyright Standards Australia document, bought as a copy or accessed through a subscription. Any builder or supervisor signing off wet-area work should have access to the current edition rather than relying on a summary, including this one.
Is waterproofing compliance the waterproofer's responsibility or mine?
Both, differently. The waterproofer performs and warrants the work. Under most standard residential contracts the head contractor carries responsibility for defects in the work overall, so the builder's own verification before the membrane is covered is what determines their position if a defect surfaces later.
How long after handover do waterproofing failures usually appear?
Typically twelve to twenty-four months, when a stain appears on the ceiling below or mould develops around a shower recess. By then the water has usually been tracking for a while, which is why the repair cost escalates well beyond the membrane itself.
Sources: AS 3740 Waterproofing of domestic wet areas (Standards Australia, available for purchase or through a subscription; not reproduced here). Home Building Act 1989 (NSW) for the statutory warranty position.