Most people working in residential construction know the requirement.
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 be between 1:80 and 1:50.
But I wanted to understand something that isn't immediately obvious from reading the NCC.
Why those two numbers?
We know fall is needed to get water to the waste. But if more fall helps water drain, why does the NCC also give us a maximum?
So I started digging.
First, put the ratios into something useful
The numbers make more sense when you convert them into millimetres of fall per metre.
Fall ratio | Fall over 1 m |
|---|---|
1:100 | 10 mm |
1:80 | 12.5 mm |
1:50 | 20 mm |
That means NCC 2022 gives us a relatively narrow window:
12.5 mm to 20 mm of fall per metre.
The difference between the minimum and maximum is only 7.5 mm per metre.
And that tells us something.
This requirement isn't simply about making water run downhill.
1:80 isn't actually new
Looking back through AS 3740, the history of 1:80 is reasonably clear.
AS 3740:2010 used a minimum fall of 1:100 for general bathroom floors and 1:80 for certain shower configurations.
AS 3740:2021 changed the way wet areas were categorised, but 1:100 and 1:80 remained part of the requirements.
So 1:80 already had an established place in Australian wet area construction before NCC 2022.
The more interesting number is 1:50.
So where does 1:50 fit?
The AS 3740 provisions I reviewed don't establish the 1:50 maximum now contained in NCC 2022.
That led me to look further.
AS 3740 tells us that determining floor falls involves more than drainage. It identifies things such as tile dimensions, doorway heights, fixtures, the area being drained and requirements for people with disabilities.
Then AS 1428.1:2021 becomes interesting.
The accessibility Standard contains a number of prescribed gradients for floors in accessible sanitary facilities, including 1:50 in a particular drainage configuration.
That doesn't prove the ABCB directly took the NCC's 1:50 maximum from AS 1428.1.
And I don't think we should pretend that it does.
What it does show is that 1:50 wasn't an unfamiliar number in Australian sanitary floor design. It was already being considered in a technical framework where drainage, accessibility and usability all have to work together.
More fall isn't automatically better
This is probably the biggest takeaway.
A bathroom floor isn't just a drainage surface.
It has to work with:
Drainage. Waterproofing. Tiles. Thresholds. Fixtures. Accessibility. Adjoining floor levels.
And somebody still has to use it.
The steeper we make the floor, the greater the difference in finished levels across the room.
Take a point 2.4 metres from the waste.
Fall ratio | Fall over 2.4 m |
1:100 | 24 mm |
1:80 | 30 mm |
1:50 | 48 mm |
Between 1:80 and 1:50, that's an 18 mm difference across one bathroom floor.
Now put a doorway, adjoining floor finish, shower screen, vanity, floor waste and 600 × 600 tiles into that room.
Suddenly 18 mm matters.
That's why wet area falls need to be considered as part of the floor design, rather than something left for the tiler to work out at the end.
There is more behind the numbers
The deeper I went into this requirement, the clearer it became that 1:80 to 1:50 isn't just a waterproofing number.
It sits at the point where drainage, waterproofing, tiling, accessibility and constructability all meet.
I've put together the full breakdown, including the history through AS 3740, the accessibility connection through AS 1428.1, ponding and surface tension, tile geometry and the practical calculations.
The NCC gives us the limits.
Understanding what sits behind them is where the real knowledge starts.
Better knowledge. Better construction.