PERTHFloor wastes: the fall that decides whether a bathroom leaks
Wet-area floor wastes and grading under AS 3740 and the NCC: the minimum falls, why membrane errors around the waste cause damage
The single most common reason a new bathroom leaks is not the membrane failing. It is the floor not sloping properly to the drain, so water sits where it should run away. Building and Energy issued a bulletin on exactly this, because floor wastes and the grading to them go wrong often enough to cause real damage.
Why the fall matters
A wet-area floor has to slope to the floor waste so water drains rather than pools. Water that ponds sits on the waterproof membrane and works at every weakness in it, and it finds the one imperfect joint eventually. The Australian Standard for waterproofing of domestic wet areas, AS 3740, sets minimum falls: a general wet-area floor grades to the waste at a minimum of about 1:100, and a hobless shower area needs a steeper fall, around 1:80, to move water off the shower floor quickly.
Those numbers sound small. A 1:80 fall is 12.5 millimetres of drop over a metre. Get it flat, or worse, get it falling the wrong way, and the water has nowhere to go.
Where installers get it wrong
Building and Energy's bulletin makes the key point directly: incorrect installation of the waterproof membrane in critical locations, particularly around the floor waste, often leads to building damage from water ingress into adjacent areas. The floor waste is the lowest point and the most heavily worked joint in the whole wet area, so it is exactly where a membrane error concentrates.
The common failures are a floor graded flat or to a high point instead of the waste, a membrane not properly dressed into the floor waste flange, and a screed that ponds in one corner because the fall was set by eye rather than measured. On a completed floor these are invisible until the water below finds its way into the next room.
What the Code requires, and where
Waterproofing sits under the National Construction Code, with the Building Code of Australia as Volume One and Two and the Plumbing Code of Australia as Volume Three. The NCC is performance-based: a builder can follow the deemed-to-satisfy provisions, which call up AS 3740, or use a certified alternative solution. Whether a floor waste is even required depends on the building class — they are generally required in the Class 2, 3 and 4 buildings that include apartments, and not mandated in a standard Class 1 house, though they remain good practice anywhere a fixture could overflow.
How we find it
On a completed floor we cannot see the fall directly, so we read its effects: water left standing after the shower is run, staining on the ceiling of the room below, damp readings at the base of wet-area walls, and where a spirit level shows the floor is not moving water toward the waste. The absence of ponding after use is one of the quiet signs a wet area was built properly.
This is one of the checks that is far easier before the tiler covers everything, which is why it is on the list at every practical completion inspection. On an existing home a pre-purchase inspection reports the evidence, and it sits alongside the broader wet-area waterproofing picture, because the fall and the membrane are two halves of the same job.
What a wet area is actually trying to do
To understand why the fall to the floor waste matters so much, it is worth seeing what a wet area is fundamentally designed to do: contain water and get it to the drain, every time, for the life of the building. A bathroom floor is not a dry surface that occasionally gets wet; it is a surface that is deliberately soaked, day after day, and the whole assembly beneath it, the waterproofing membrane, the screed, the fall, the drainage, exists to make sure that water goes where it is meant to and nowhere else. When the fall is right, water runs to the floor waste and disappears. When it is not, water sits, and standing water in a wet area is the beginning of most of the problems that wet areas develop.
The consequences of getting it wrong are slow and expensive rather than immediate and obvious, which is part of why the defect is so common and so under-appreciated. Water that ponds instead of draining sits on the membrane, works at the sealed junctions, and over time finds any weakness. If it gets past the membrane, it is into the structure, and a wet area that leaks into the surrounding floor and walls is one of the more costly failures a house can develop, because putting it right usually means stripping the wet area back and rebuilding it. All of that traces back to whether the water was made to drain in the first place.
The falls the Code actually asks for
The requirements are specific about fall because the fall is what makes water move. Broadly, the floor of a wet area has to be graded so that water runs to the floor waste rather than pooling, with particular attention to the areas that get the most water. In a shower, the grade within the showered area has to be enough to carry water to the waste briskly, and the whole floor of the wet area has to be set so that water does not run toward the door or sit in low spots away from the drain. The membrane has to be turned up at the walls and at the hob or the shower recess so that water cannot escape at the edges, and the floor waste itself has to be set at the low point so that it actually receives the water the fall delivers to it.
Where installers get into trouble is usually in the execution rather than the intent. A floor screeded flat, or with the fall running the wrong way, or with a dip that holds water short of the waste. A floor waste set slightly high, so that a shallow film of water sits around it and never quite drains. A membrane not turned up far enough at a junction. These are not exotic errors; they are the ordinary results of a job done in a hurry or without enough care, and they are exactly the kind of thing that a pre-tile inspection can catch while it is still cheap to fix and the finished house cannot.
Why it is nearly impossible to fix later
The reason wet-area fall is worth getting right the first time is that it is one of the hardest things to correct after the fact. The fall is built into the screed and the membrane, beneath the tiles, and changing it means removing the tiles, the membrane, and the screed and starting again. There is no adjustment you can make from the surface; a wet area that ponds because the fall is wrong is a wet area that has to be rebuilt to fix properly. This is the whole argument for checking it before the tiles go on, at the waterproofing stage of a build, because that is the one moment when the fall and the membrane are visible and a problem can be corrected for the cost of a re-do rather than a demolition.
For a buyer of an existing house, ponding in a wet area is worth taking seriously as a sign, because it points at a fall problem that cannot be cheaply fixed and may already be letting water where it should not go. A shower floor that holds water after use, staining or damage at the base of the walls around a wet area, or evidence of moisture in the adjacent floor are the signals, and they are worth having assessed rather than assuming they are cosmetic, because the wet area is one of the places where a small visible symptom can sit over a large concealed problem.
Common questions
What fall does a wet-area floor need?
AS 3740 sets minimum falls to the floor waste — around 1:100 for a general wet-area floor and about 1:80 for a hobless shower — so water drains rather than ponds on the membrane.
Why do bathrooms leak around the floor waste?
Building and Energy notes that incorrect membrane installation in critical locations, particularly around the floor waste, often leads to water ingress into adjacent areas. The waste is the lowest, most-worked joint in the wet area.
Are floor wastes required in a house?
Floor wastes are generally required in Class 2, 3 and 4 buildings such as apartments, and not mandated in a standard Class 1 house, though they remain good practice.
This comes up on every practical completion (pci) inspection and pre-purchase building inspection we carry out.
Read next: wet areas are the most commonly failed part of a new build and balcony waterproofing: the membrane over your living room.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.