Leaking showers and balconies are among the most common and most expensive defects in new Australian homes. The fix usually means ripping out tiles, and sometimes framing and plasterboard with them. Yet most inclusions lists sum up the whole subject in two words: "fully waterproofed".
That phrase tells you nothing about what product is going in, whether it suits the surface it's going on, or which standard it's meant to meet. Here's what you should be asking for instead.
Which standards apply where
Two Australian Standards do most of the work, both called up by the National Construction Code (NCC):
- AS 3740:2021, Waterproofing of domestic wet areas covers inside the house: bathrooms, showers, laundries, toilets and the floor around baths. The NCC 2022 Housing Provisions set out which parts of each room must be waterproof and which only need to be water resistant.
- AS 4654.1 and AS 4654.2, Waterproofing membranes for external above-ground use cover balconies, decks over living areas, podiums and planter boxes. Part 1 deals with the membrane materials, Part 2 with design and installation.
Wet area membranes themselves are tested and classified under AS/NZS 4858. That's where the classes come in.
What Class I, II and III actually mean
A common misunderstanding, and one I've repeated in the past, is that the classes describe how wet an area gets. They don't. Membrane classes describe extensibility: how far the membrane can stretch before it tears.
- Class I, low extensibility (under about 60% elongation): some cement-based and epoxy systems.
- Class II, medium extensibility (about 60 to 300%): many acrylic and bitumen emulsion membranes.
- Class III, high extensibility (over about 300%): polyurethane and some rubber-based membranes.
Why it matters: buildings move. Timber shrinks, sheets expand and contract, slabs creep, and big tiled areas move with temperature. If the surface under the tiles moves more than the membrane can stretch, the membrane cracks and the leak starts, usually somewhere you can't see until the damage is done.
Matching the membrane to the surface
The right class depends mainly on how much the substrate (the surface the membrane is applied to) will move:
- Concrete slab on ground (low movement): Class I, II or III.
- Fibre cement or wet area plasterboard walls (moderate movement at sheet joints): Class II or III.
- Timber or particleboard flooring (high movement): Class III, or as the manufacturer specifies.
- Large tiled areas, or areas exposed to heat and sun such as balconies (high movement): Class III, and a product rated for external use under AS 4654.
Joints, corners and junctions between different materials also need bond breakers, tapes or fillets that suit the membrane class. This is where many failures start, so the detail matters as much as the product.
The membrane manufacturer's technical data sheet will tell you the class, the substrates it's approved for and the system components it needs. Follow that, not whatever's left in the ute.
What to ask your builder
Before you sign, ask for your specification to state, for every wet area and balcony:
- The membrane product name and manufacturer.
- Its class under AS/NZS 4858 (or AS 4654.1 for external areas).
- The substrate it's going onto.
- The standard it will be installed to: AS 3740 inside, AS 4654.2 outside.
- Who installs it, and that they'll provide a certificate of installation at completion.
If the answer is still "it's fully waterproofed, don't worry", that's your cue to worry a little.
C13A - Waterproofing Design Checklist
$59.00 AUD
Last updated 28 Oct 2026 · Revision 3 · Free updates for life
Checklist Type: Minor
Form Factor: Fillable, print-ready PDF (A4, 23 pages)
When to use: Design stage: before waterproofing starts, while products and details can still change
A 59-point waterproofing design checklist. Settle shower types, set-downs, falls and one compatible set of products on paper, before anyone picks up a brush.
Our C13A Waterproofing Design checklist walks you through these questions at the design and contract stage, and C13B Waterproofing Installation covers what to check on site before the tiler covers it up. Both are at constructor.net.au/checklists.
Making sure the right product goes in
Specifying the right membrane is half the job. The other half is confirming it's the one actually installed, applied at the right thickness, and left to cure before tiling.
An Inspection and Test Plan (ITP) does exactly that. It lists each check, the acceptance criteria and who signs it off, so the specified product, primer, bond breakers and flood test are recorded rather than assumed. I've written more about ITPs here:
Anthony Painter
The bottom line
Waterproofing classes are about flexibility, not wetness. Ask for the product, class and standard in writing for every wet area and balcony, match the membrane to how much the surface will move, and check it before it disappears under tiles.
Further reading
A Introductory Guide to Waterproofing Wet Areas in Your HomeWaterproofing wet areas like bathrooms and laundry is crucial. This overview introduces to waterproofing concepts, from preparation to material selection and application. Learn basic waterproofing terms and methods.End of Free Australian Standards: Impact AnalysisThis article discusses the end of free access to Australian Standards in public libraries since 2018, its effects on small businesses, safety compliance, and the balance between public and corporate interests. Options for future access and public mobilization strategies are also explored.
Stress-Free Home Building with a Quality Management PlanTired of horror stories about botched home builds? The secret to a stress-free home construction lies in a Quality Management Plan (QMP). This plan outlines how quality will be achieved, monitored, and improved in your project.
Demand Better: Accountability in Australian Home BuildingWe’ve grown accustomed to flashy kitchens and sleek “hamptons style” façades, but what about the hidden quality issues? In this article we discuss why Australian builders and their directors must be held accountable for delivering healthier, more durable homes beyond mere compliance.
