AS 3740 wet area waterproofing compliance

What it is

AS 3740 Waterproofing of domestic wet areas is the Australian Standard that sets the construction detail rules for membrane waterproofing in bathrooms ensuites laundries and water closets in Class 1 and Class 10 buildings. The current edition is AS 3740-2021. It is the deemed to satisfy pathway through Part H4D2 of NCC Volume Two and Part 10.2 of the ABCB Housing Provisions.

A builder can comply with the wet area performance requirement H4P1 by following AS 3740 or by following the equivalent construction details now embedded directly in Part 10.2 of the Housing Provisions. The two are aligned. Most product data sheets and trade documentation still reference AS 3740.

What the standard actually requires

AS 3740-2021 sets minimums that are easy to measure on site. The construction failures that cause leaks almost always trace back to a breach of one of these clauses.

Fall to drain

Shower areas need a minimum fall of 1:80 to the floor waste. The remainder of the wet area floor needs a minimum fall of 1:100 where the floor itself is the waterproof layer. A spirit level on a 1 metre straight edge gives a quick site check.

Wall to floor junctions

Internal corners and the wall to floor junction must have a bond breaker. This is usually a polyethylene strip or backing rod placed in the corner before the membrane is applied. Without it the membrane bridges the corner and tears as the building moves.

Upstands and hobs

Membrane must turn up at the perimeter of the wet area. Minimum heights vary by element. Shower hobs need a turn up of at least 25 mm above the finished tile level. Wall membrane in shower zones needs to extend at least 1800 mm above the floor.

Penetrations

Every penetration (tap body floor waste shower outlet) must be sealed with a compatible flange collar or proprietary detail from the membrane manufacturer. Smearing silicone over the tap body is the most common defect on a complete site inspection.

Substrate

The substrate must be sound clean and dry before priming. Compressed fibre cement sheet joints need to be set with the recommended fibreglass scrim. Plasterboard for shower walls must be the moisture resistant grade.

Compliance pathway through the NCC

NCC Volume Two H4D2 sets the wet area performance requirement. The deemed to satisfy options are:

• AS 3740-2021 directly.
• Part 10.2 of the ABCB Housing Provisions which reproduces the equivalent details.

Either route satisfies the performance requirement. The performance solution pathway is also open if a designer can demonstrate equivalence by evidence, but in practice almost all residential wet areas use the deemed to satisfy route.

Who is licensed

Waterproofing is a separately regulated class of work in NSW QLD and Victoria. The waterproofer must hold the relevant licence or registration on the day the work is done. The principal contractor is liable for arranging unlicensed work even where the head builder did not perform it personally.

What an auditor looks for

A TradeLens audit assessing AS 3740 compliance checks for:

• Membrane product data sheet matched to the substrate and location.
• Photo records of bond breaker installation before membrane.
• Photo records of upstand heights at hobs and door thresholds.
• Penetration detail photos showing compatible flanges or collars.
• Fall test photos with a level on a 1 metre straight edge.
• Flood test record for showers and balconies.
• Membrane curing time logged against tile bedding start.

Gaps in any of these create the technical exposure that surfaces years later as a major defect claim under jurisdictional warranty regimes.