Waterproofing compliance for bathroom renovations (AS 3740)

Waterproofing is the single most important part of a bathroom renovation that nobody sees once the job is finished — tiling and fixtures get all the attention in before-and-after photos, but a poorly waterproofed bathroom is the renovation mistake most likely to cause expensive, hidden damage years down the track. Understanding what the relevant Australian Standard actually requires makes it much easier to check whether your own renovation is being done properly.

The standard in question is AS 3740, formally titled Waterproofing of domestic wet areas, with the current version dated 2021. It sets out the minimum requirements for the materials, design and installation of waterproofing in wet areas of a home — primarily bathrooms and ensuites, laundries and toilets — and it treats different zones within those rooms differently depending on how much water exposure each area typically gets. Critically, AS 3740 is referenced directly by the National Construction Code as a Deemed-to-Satisfy provision, which means it is not an optional industry guideline — it carries legal force through state building law, and work that does not meet it is not compliant with the building code, regardless of how good the tiling looks on top.

The specific requirements vary by zone within the bathroom. Inside the shower itself, the entire floor commonly needs to be fully waterproof, not just the area directly under the shower rose, and the shower walls commonly need to be waterproofed to a height of at least 1800 millimetres. Elsewhere in the bathroom, walls generally need a lower minimum, commonly at least 150 millimetres, reflecting the lower water exposure outside the shower zone itself. Floors need particular attention in two specific situations: where the bathroom floor has a timber substrate, or where the bathroom is on an upper storey rather than a ground-floor slab, the entire floor commonly needs to be waterproofed, generally with an upturn at the perimeter of at least 100 millimetres, since a leak in either of these situations has much more potential to cause structural damage below than a slab-on-ground bathroom would.

The reason this matters so much in a renovation specifically, rather than new construction, is that waterproofing has to be reapplied properly any time a bathroom is stripped back to the studs — an old membrane cannot simply be tiled over and assumed to still be doing its job, and disturbing the substrate during demolition generally means the waterproofing needs to start fresh. This is also why waterproofing needs proper curing time before tiling begins, which our separate article on renovation timelines covers in more detail — rushing tiling onto a membrane that has not cured properly is one of the more common causes of waterproofing failure in an otherwise well-executed renovation.

Checking that this part of the job has actually been done properly is harder than checking tiling or fixtures, simply because the membrane is invisible once tiling covers it. A few practical steps help regardless: ask whether the renovator or a licensed waterproofer specifically will be carrying out this stage, since waterproofing is treated as a distinct, sometimes separately licensed trade in several states rather than something any renovator can simply do as part of the broader job. Ask for photos of the completed waterproofing before tiling starts — a reputable renovator will have these as standard practice, precisely because the work becomes invisible afterward. And ask whether a compliance certificate or equivalent documentation will be provided once the work is finished, which becomes valuable evidence if a leak or dispute arises later, much like the compliance certificates required for licensed gas or electrical work.

This article explains the general requirements of AS 3740 as commonly described across Australian building and waterproofing industry sources, but it is not a substitute for a site-specific assessment of your own bathroom. Standards and state administration of them can change over time. If you are planning a bathroom renovation, confirming how waterproofing will be handled and documented before work starts is worth raising directly — our directory lists Australian bathroom and kitchen renovators by area if you are ready to start comparing.

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