● Wet areas

Bathroom floor falls. 1:100 or 1:80?

11 Nov 20262 min readFORSPEC Technical Team

The short version

AS 3740: at least 1:100 in the floor under the membrane where a floor waste is required, and at least 1:80 on the finished shower floor.
The NCC: any floor with a floor waste falls between 1:80 and 1:50.
Both apply on the same job. Build to the stricter figure and check the finished floor.

Three fall figures, three layers of the same floor.

Ask three trades what fall a bathroom floor needs and you will hear three numbers. Each of them is in a rule book. They apply to different layers of the same floor.

The three numbers

Under the membrane (AS 3740): 1:100 or more, where a floor waste is required
Finished shower floor (AS 3740): 1:80 or more
Any floor with a floor waste (NCC): Between 1:80 and 1:50

Building to AS 3740 does not switch off the NCC figure. For houses, and for apartments and commercial buildings, the code's own fall rule sits on top of the standard.

How they fit together

The NCC does not say whether its fall is measured on the finished tiles or on the floor underneath. FORSPEC reads it this way, and it satisfies both documents:

Finished floor: between 1:80 and 1:50 to the waste.
Under the membrane: at least 1:100, where the membrane sits below a screed or tile bed.

Where no fall is needed

A bathroom floor with no floor waste outside an enclosed shower needs no fall under AS 3740, as long as the shower is enclosed and water stays in the room. Once a floor waste is installed, the fall rules apply.

On site

Find out before the screed: is there a floor waste, and is the membrane above or below the screed?
Set the fall in the floor the membrane will sit on.
Check the finished floor with a level before the tiler leaves. Water should run to the waste and none should sit.