Waterproofing wet areas to AS/NZS 3740

How to waterproof residential wet areas to AS/NZS 3740 — membrane types, where it extends, falls and drains, penetrations, and the flood test that catches leaks before you tile.

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AS/NZS 3740:2010 is the cited standard for waterproofing residential wet areas — bathrooms, ensuites, laundries and kitchens — and it’s cited by NZBC E3/AS1. The membrane is your primary line of defence: if it leaks, the whole building gets wet, not just the bathroom.

Membrane types

Four membrane systems turn up on residential wet-area jobs, each brushed, rolled or set differently:

Liquid-applied acrylic (Mapelastic, Bostik Membrane, Ardex WPM, Sika MaxBond) — the most common. Brushed or rolled on in multiple coats, with reinforcement at corners and penetrations.
Polyurethane (Equus, Viking) — premium, more flexible, longer life.
Sheet membrane (bitumen, Kerdi) — pre-formed sheets bonded to the substrate. Less common in residential, common in commercial.
Tile-and-set membrane (Lanko Sealflex) — applied with a tile adhesive system, so there’s no separate membrane coat.

Where the membrane must extend

Under AS/NZS 3740 and E3/AS1, the membrane reaches different heights depending on the zone:

Shower: tank the floor and go 1800mm up the walls in the cubicle — or 150mm above the shower rose if that’s higher.
Outside shower walls: 150mm up the wall in the wet area.
Floor: membrane the entire wet area (bathroom), sloped 1:60 to the drain.
Vanity zone: 100mm up the wall behind the basin.
Laundry: tank the floor and 150mm up the walls in the tub zone.
Kitchen: 100mm up the wall behind the sink and behind the dishwasher.

Falls and drains

Water has to run to the drain, so build the fall into the screed before the membrane goes down — don’t try to slope it with membrane thickness:

Shower floor: 1:60 (16.7mm/m) minimum fall to the drain.
Bathroom floor: 1:80 minimum fall to the drain, where required.
Linear drain in a shower: at least 80mm wide, 70mm deep.
Round drain (60–80mm): compatible with the membrane upturn.

Substrate and penetrations

The membrane is only as good as what it sits on and what pokes through it. Suitable substrates include Aqualine 10mm or 13mm green-paper GIB (BRANZ Appraisal 427), Villaboard 6mm James Hardie fibre cement, HardieBacker for premium tile work, and concrete with a screed for fall. Leave a 5–10mm gap at the floor for the membrane upturn (don’t butt sheets to the floor), and use a 32×32×0.55mm galvanised metal angle in the shower internal corners (BRANZ Appraisal 427 detail).

Handle each penetration on its own detail:

Tap penetrations: pre-form the opening, trim the membrane around the pipe, and seal with PU sealant plus a ring.
Shower drain: rebate it into the floor, lap the membrane OVER the flange, so the drain body sits ON the membrane.
Mixer trim: behind the tile, seal the membrane to the body using the manufacturer-specified detail.
Wall outlets (basin): copper-to-plastic transition, sealed with PU.

Test before you tile

The flood test is your last chance to catch a leak while it’s still cheap to fix. If it leaks now, it leaks forever — and most failures show up at the drain-to-membrane junction or the pipe penetrations.

Plug the drain.
Fill the wet area to the top of the membrane.
Wait 24 hours.
Check for any drop in level and any visible leakage below.

Common failures

Internal corner cracking — needs a reinforcement fillet of fabric plus sealant.
Pinholes from inadequate thickness — apply multiple coats and check the wet film thickness.
Sub-fall pooling — the fall isn’t steep enough, so water sits.

Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.

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Common questions

Which standard covers waterproofing residential wet areas in NZ?

How high does the membrane go up the walls in a shower?

In the shower cubicle, tank the floor and take the membrane 1800mm up the walls — or 150mm above the shower rose if that’s higher. Outside the shower walls it goes 150mm up the wall in the wet area.

What fall do I need to the drain?

A shower floor needs a minimum fall of 1:60 (16.7mm/m) to the drain, and a bathroom floor needs a minimum 1:80 fall where required. Pre-form the fall in the screed before the membrane — don’t try to slope it with membrane thickness.

How do I test the waterproofing before tiling?

Flood-test the wet area before tiling: plug the drain, fill to the top of the membrane, wait 24 hours, then check for any drop in level or visible leakage below. If it leaks now, it leaks forever — most failures are at the drain-to-membrane junction or pipe penetrations.

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