Why 1:80 to 1:50? Understanding the NCC’s Wet Area Floor Falls
1:80 minimum. 1:50 maximum.
If you work in residential construction, you've probably seen those numbers countless times since NCC 2022.
But why those two numbers?
Why did 1:80 become the minimum? And if more fall helps water drain, why does the NCC stop us at 1:50?
The answer isn't sitting neatly underneath NCC 2022 Housing Provisions 10.2.12. To understand it, we need to look beyond the NCC and follow the requirement through the Australian Standards dealing with waterproofing, drainage, tiling and accessibility.
And when we do, it becomes clear that wet area falls are about much more than simply getting water to run downhill.
First, what do the numbers actually mean?
Under NCC 2022 Housing Provisions 10.2.12, where a floor waste is installed, the continuous fall of the floor plane to the waste must have:
a minimum fall of 1:80
a maximum fall of 1:50
On site, those ratios are easier to understand when converted into millimetres per metre.
Fall ratio
Gradient
Fall over 1 m
Fall over 2 m
1:100
1.00%
10 mm
20 mm
1:80
1.25%
12.5 mm
25 mm
1:50
2.00%
20 mm
40 mm
NCC 2022 therefore gives us a surprisingly narrow window:
12.5 mm to 20 mm of fall per metre.
That tells us something important straight away.
If the only objective was getting water to run downhill, why would we need a maximum?
To answer that, we first need to look at where we came from.
It wasn't always 1:80 to 1:50
The current requirement represents a change in the way wet area falls have traditionally been dealt with.
Looking back at AS 3740:2010, Waterproofing of domestic wet areas, the general bathroom floor requirement was a minimum fall of 1:100 towards the waste.
Shower floors were treated separately.
Where there was vertical separation between the shower and the remainder of the wet area, such as a shower screen, hob, step-down or water stop, the minimum fall was also 1:100.
For other shower configurations, the minimum increased to 1:80.
AS 3740:2021 reorganised wet areas into different categories, but 1:100 and 1:80 remained important figures.
A simplified comparison shows the progression:
Document
General wet area
Shower area
Maximum fall
AS 3740:2010
Min. 1:100
Min. 1:80 in applicable shower areas
Not specified in provisions reviewed
AS 3740:2021
Min. 1:100 in applicable wet areas
Min. 1:80 in applicable shower categories
Not specified in provisions reviewed
NCC 2022 HP 10.2.12
Min. 1:80 where 10.2.12 applies
Min. 1:80 where 10.2.12 applies
Max. 1:50
So 1:80 isn't particularly new.
The 1:50 maximum is where things become more interesting.
So why wasn't 1:100 enough?
The obvious reason for putting fall into a wet area floor is drainage.
But drainage is more complicated than simply making water flow in the right direction.
Appendix B of AS 3740 discusses the purpose of falls in finished wet area floors. The primary consideration is preventing water from remaining on the finished surface where it could adversely affect occupants or contribute to the deterioration of building elements.
Put more simply:
Water needs somewhere to go, and the floor needs to help it get there.
But there is another important detail.
AS 3740 recognises that some residual water can remain on a finished tiled surface because of surface tension.
AS 3740:2021 provides further guidance around determining ponding and distinguishes accumulated water from residual water retained by surface tension.
That means a few droplets left on a tile do not automatically mean the floor has failed.
Persistent accumulation of water after the surrounding surface has dried is a different issue.
The purpose of fall is therefore not to create an instantly dry floor. It is to create a floor geometry that effectively drains surface water towards its intended drainage point.
Where does 1:80 come from?
This part has a much clearer history.
A minimum fall of 1:80 was already being used in Australian wet area construction before NCC 2022.
AS 3740:2010 used it for particular shower configurations, and AS 3740:2021 continued to use 1:80 for applicable shower categories.
That gives 1:80 an established connection with areas where effective surface drainage is particularly important.
But AS 3740 also tells us that designing falls isn't just about drainage.
It identifies several factors that can affect the falls achievable in a finished wet area, including:
finished heights at doorways
heights of fixtures and fittings
tile dimensions
the area being drained
requirements for people with disabilities
This is where the requirement starts becoming a construction issue rather than simply a waterproofing number.
Tile size changes the equation
Anyone who has tried to create multiple falls towards a point waste using large-format tiles will understand the problem.
As tile dimensions increase, creating the required floor geometry becomes increasingly difficult without cutting the tiles.
AS 3740 acknowledges this and discusses diagonal cutting around wastes where necessary to achieve drainage while controlling tile lipping.
So when we talk about achieving a particular fall, we're not dealing with an isolated line on a drawing.
The substrate, waterproofing system, screed, adhesive, tiles and floor waste all need to come together to create the finished floor plane.
Then that floor still needs to coordinate with the doorway, adjoining floor finish, shower screen, vanity and other fixtures.
The fall is part of the floor design. It shouldn't be something left for the tiler to solve at the end.
Then where does 1:50 come from?
This is the more difficult question.
The AS 3740 provisions reviewed for this article contain the familiar 1:100 and 1:80 minimum gradients, but they do not establish the 1:50 maximum now contained in NCC 2022 Housing Provisions 10.2.12.
So we need to look elsewhere.
One useful piece of the puzzle appears in AS 1428.1:2021, Design for access and mobility.
AS 1428.1 contains specific floor gradients for accessible sanitary facilities.
Depending on the area and drainage arrangement, these include ranges such as:
1:60 to 1:80
1:80 to 1:100
1:50 to 1:90 for a particular whole-sanitary-facility arrangement draining to a linear drain
That matters.
It demonstrates that 1:50 was already an established floor gradient within Australian Standards dealing with sanitary facilities where accessibility and usability are important considerations.
There is another connection.
AS 3740 specifically identifies the requirements of people with disabilities as one of the factors that need to be considered when determining wet area floor falls.
Does that prove that the ABCB took the 1:50 maximum directly from AS 1428.1?
No.
The material reviewed for this article does not establish that direct regulatory link.
But it does provide important technical context for why a maximum gradient makes sense.
Why not just make the floor steeper?
This is where the requirement starts to make more sense.
If drainage were our only concern, increasing the gradient might seem like the obvious solution.
But a bathroom floor isn't just a drainage surface.
People have to use it.
Increasing the gradient increases the difference in finished floor level across the room. That affects:
Drainage.
Accessibility.
Thresholds.
Fixtures.
Tile geometry.
Adjoining floor levels.
Constructability.
All of those requirements eventually meet at the same finished floor.
A maximum gradient therefore creates a limit on how steep that floor can become while still achieving the drainage objective.
The surrounding Australian Standards show us that drainage performance has to be balanced against the practical and functional requirements of the finished space.
Where does the tiling standard fit?
There is another part of the technical framework worth noting.
Both the 2010 and 2021 editions of AS 3740 refer to AS 3958.1 in relation to ceramic tile installation, drainage and controlling lipping.
That relationship makes sense.
Waterproofing, tiling and drainage may be addressed through different technical documents, but once construction starts they become parts of the same system.
The tile installation needs to accommodate the designed falls while still producing an acceptable finished surface.
For that reason, the 1:80 to 1:50 requirement shouldn't be viewed purely as a waterproofing provision.
It sits within a broader technical framework involving waterproofing, drainage, tiling, accessibility and constructability.
It is important, however, not to go further than the available evidence.
The material reviewed for this article does not establish that the ABCB directly derived the 1:50 maximum from AS 1428.1 or AS 3958.1.
What it does show is that the NCC requirement sits within an existing technical framework where these issues, and similar gradients, were already being considered.
This is where 18 mm starts to matter
Let's put the requirement into an actual bathroom.
Assume the furthest point of the floor is 2.4 metres from the floor waste.
Fall ratio
Fall over 2.4 m
1:100
24 mm
1:80
30 mm
1:50
48 mm
Between the NCC minimum and maximum, there is an 18 mm difference across that one floor.
On paper, 18 mm might not sound significant.
Now put a doorway, adjoining floor finish, shower screen, vanity, floor waste and 600 × 600 tiles into the same room.
Suddenly 18 mm isn't insignificant.
And that's before allowing for the tolerances involved in actually constructing the floor.
This is why falls need to be resolved early.
Waiting until waterproofing is complete and asking the tiler to "get the falls right" isn't floor design.
More fall isn't automatically better
The current NCC requirement starts making much more sense when we stop looking at it as two isolated ratios.
At 1:80, we have:
12.5 mm per metre.
At 1:50:
20 mm per metre.
The NCC therefore gives us a 7.5 mm-per-metre window in which to create the finished floor.
Within that relatively narrow range, the floor needs to drain effectively, minimise problematic ponding, accommodate the tile layout, coordinate with thresholds and fixtures and remain a practical surface for the person using the room.
That's a lot being asked of 7.5 mm.
The bigger lesson
Construction is full of numbers like these.
1:80. 1:50. 900 mm. 150 mm.
We remember them because we need to comply with them.
But remembering the number and understanding the requirement are two different things.
The next time you see 1:80 to 1:50 on a wet area floor, don't just think about whether the tiler has hit the numbers.
Think about what those numbers are trying to achieve.
Drainage. Waterproofing. Tile geometry. Accessibility. Finished floor levels. Usability.
They all meet at the same floor.
The NCC gives us the limits.
Understanding what sits behind them is where the real knowledge starts.
Better knowledge. Better construction.
References
Australian Building Codes Board (ABCB), NCC 2022 Housing Provisions, Part 10.2, particularly 10.2.12, Construction of wet area floors.
Standards Australia, AS 3740:2010 Waterproofing of domestic wet areas, particularly clauses 3.3, 3.4 and Appendix B.
Standards Australia, AS 3740:2021 Waterproofing of domestic wet areas, particularly clause 2.3 and Appendix B.
Standards Australia, AS 1428.1:2021 Design for access and mobility, Part 1: General requirements for access: New building work, particularly clause 12.5.2.
AS 3958.1 is referenced by AS 3740 in relation to ceramic tile installation and drainage. No specific gradient from AS 3958.1 has been attributed in this article without verification of the relevant provision.
