CONSTRUCTION PROFESSIONALS WITH OVER TWO DECADES OF INDUSTRY EXPERIENCE
Waterproofing Falls in Wet Areas — Bathrooms, Showers & Laundries (AS 3740, the NCC & the Home Building Act)
How floor falls are regulated, when they are required, and when a failure becomes a major defect
Of all the items examined in a bathroom dispute, floor falls are among the most measurable. A floor either drains or it doesn’t, and the gradient can be checked on site with a digital level against a clear requirement. But understanding whether a fall is non-compliant — and why that matters legally — means following the chain from the Performance Requirement of the National Construction Code down to the standard that satisfies it, and back up to the Home Building Act. This page sets out that chain for falls in wet areas.
The Performance Requirement Comes First
The National Construction Code (NCC) is performance-based. At the top sits a Performance Requirement — the outcome the building must achieve. For wet areas in houses (Class 1 and 10 buildings) under NCC 2022, that Performance Requirement is H4P1, which requires wet areas to be protected so that water does not penetrate and cause unhealthy or dangerous conditions, or damage to building elements. (In the earlier NCC 2019, the equivalent Performance Requirement was numbered P2.4.1 — the requirement is similar, but the reference changed when the Code was restructured.)
The Performance Requirement does not, by itself, tell a builder how to build. It states the outcome. The “how” is satisfied through a Deemed-to-Satisfy pathway — and this is where the standards come in.
How the Standards Are Given Power
A Performance Requirement is satisfied if the relevant Deemed-to-Satisfy provisions are followed. In plain terms:
Performance Requirement H4P1 is satisfied if the wet area is constructed in accordance with AS 3740, or with Part 10.2 of the ABCB Housing Provisions (depending on the edition of the NCC that applies to the matter).
This is the mechanism that gives an Australian Standard its legal force. AS 3740 on its own is just a standard. But because the NCC calls it up as a way of satisfying the Performance Requirement, a failure to meet AS 3740 becomes a failure to satisfy the Deemed-to-Satisfy provisions — and therefore a breach of the Performance Requirement of the NCC.
Which Document Applies Depends on the Edition
This is a critical and frequently-missed point. The document that governs the falls in your matter depends on the edition of the NCC in force at the relevant date (generally the date of the Construction Certificate application):
- NCC 2016 and NCC 2019 call up AS 3740 (the 2010 edition) to satisfy the wet-area Performance Requirement.
- NCC 2022 references AS 3740:2021, and introduced Part 10.2 of the ABCB Housing Provisions, which now contains the Deemed-to-Satisfy construction provisions for wet areas in houses.
An important note on the ABCB Housing Provisions: this document only came into effect with NCC 2022. Matters governed by NCC 2016 or 2019 do not call up the Housing Provisions — citing it against an older building is an error. For those matters, AS 3740:2010 is the operative reference.
Getting the edition right is not a technicality. Citing the wrong edition’s clause — or the wrong document entirely — weakens a report and invites it to be argued.
When Are Falls Required? The Floor Waste Distinction
This is where a precise reading matters, and where many assessments go wrong.
Falls are required so that water drains to a floor waste. Under AS 3740, a floor waste is defined as a grated inlet within a graded floor intended to drain the floor surface. The requirement to grade the floor is tied to the presence of a floor waste — and crucially, not every drainage point in a wet area is a floor waste.
A shower may drain to a waste that serves the shower only; a bathroom may have a waste that is, in fact, a floor waste intended to drain the whole floor. The distinction governs where falls are required and to what. Establishing whether a given waste is genuinely a floor waste — drained surface against defined gradient — is the first step in assessing whether a fall is non-compliant. An assessment that assumes every waste is a floor waste, or overlooks one that is, will reach the wrong conclusion.
Bathroom floor: where a floor waste is installed, the general bathroom floor must grade to it.
Shower floor: the shower floor must fall sufficiently to drain to the waste without ponding.
Laundry: where a floor waste is installed in a laundry, the floor must likewise grade to it.
In each case, the trigger is the floor waste. Identify the floor waste correctly, and the fall requirement follows.
The Minimum Falls
The required gradients differ between the editions — another reason the applicable document matters.
Under AS 3740:2010 (NCC 2016 / 2019 matters):
- General bathroom floor: minimum fall to the waste of 1:100.
- Shower floor with a vertical separation (a shower screen, hob, step-down or waterstop): 1:100.
- Shower floor without a vertical separation: a minimum of 1:80.
Under Part 10.2 of the ABCB Housing Provisions (NCC 2022 matters):
- Where a floor waste is installed, the floor plane to the waste must have a minimum continuous fall of 1:80 and a maximum continuous fall of 1:50.
Note the 2022 provisions set both a minimum and a maximum — a floor can, in principle, be too steep as well as too flat. And a floor that ponds — water remaining on the surface rather than draining — is the visible, measurable symptom of a fall that does not comply.
How a Falls Defect Becomes a Major Defect
Establishing the breach is one thing; its consequence under the Home Building Act 1989 is another.
Section 18B implies the statutory warranties into every residential building contract — including that the work will be performed with due care and skill, in accordance with the plans and specifications, and in compliance with the law (which includes the NCC). A wet area that fails to satisfy the Performance Requirement of the NCC is, on its face, a breach of the section 18B warranties.
Section 18E sets the warranty periods, and draws the distinction between a major defect and other defects. A defect in a major element of the building — which includes the waterproofing of the building — may be a major defect where it is attributable to defective design, defective or faulty workmanship, or a failure to comply with the Performance Requirements of the NCC, and where it causes (or is likely to cause) an inability to use the building for its intended purpose, the destruction of the building or part of it, or a threat of collapse.
A waterproofing failure caused by inadequate falls is therefore frequently characterised as a major defect: it goes to the waterproofing — a major element — it fails to comply with the NCC, and water that is not drained will, over time, deteriorate building elements and create health risks. Whether any particular defect meets the section 18E threshold is ultimately a matter for the Tribunal — but the pathway from a flat floor to a major defect is a direct one.
What This Means in Practice
A floor fall is a small thing to measure and a significant thing to prove. The measurement is straightforward; the value of a report lies in tracing it properly — identifying the correct floor waste, applying the document and gradient that govern the matter by its edition, and following the chain from the breach of the standard, to the Deemed-to-Satisfy provisions, to the Performance Requirement of the NCC, to the statutory warranties under the Home Building Act.
If you would like to understand more about how these defects are assessed and reported, see our overview of waterproofing and weatherproofing defects and our page on bathroom and wet area waterproofing defects. To understand the role of the report itself, see building expert witness reports.
It is a mistake to treat the tiling and the waterproofing as separate trades with separate consequences. The tiling of the floor forms part of the waterproofing system, and the standard of the tiling is often the clearest visible signal of what lies beneath. Wall and floor tiles are required to achieve a minimum adhesive coverage — 90% in wet areas under AS 3958.1 — and where tiles have instead been “spot fixed” (dabs of adhesive achieving only partial contact), the consequences are twofold. First, inadequate adhesion can itself be a serious defect, exposing the tiling to debonding. Second, and more importantly, spot-fixing is a strong indicator of workmanship that did not take the underlying membrane detailing seriously either.
What This Means for Your Matter
If you have been told your bathroom leak is “normal,” or a builder is proposing a quick patch, the real question is whether the work complies with AS 3740 as called up by the NCC. That is a question for inspection and measurement — not assertion. An expert report documents each non-compliance, traces the breach pathway, and sets out the rectification methodology and indicative costs in a form the Tribunal can rely on.
Click on the links below to learn more about expert reports:
Expert reports
Discuss Your Matter
Speak directly with a building expert about the specifics of your dispute — the defects, the standards, and where you sit in the NCAT process.