Where to put the floor waste: how location decides how long a shower floor lasts

In our article on floor wastes, substrate and membrane failure we looked at what happens when the bed around a drain moves and tears the membrane. This article asks an earlier question: where should the waste be in the first place?

The reason it matters is simple. In most showers, people stand in the same spot every day. If the floor waste is in that spot, the weakest detail in the floor sits under the heaviest, hottest and wettest use. Other things being equal, it will fail sooner.

A note on what is code and what is reasoning. NCC 2022 sets the falls to a waste and requires a waste in a shower, but as far as we can find it does not say where the waste must sit. What follows on position is engineering reasoning and site experience, and we label it as such.

What the code does and does not say

Under NCC 2022 Housing Provisions Part 10.2:

• a shower area must include a floor waste, with falls that comply with clause 10.2.12 (clause 10.2.14);
• the minimum continuous fall of the floor plane to the waste is 1:80 and the maximum is 1:50 (clause 10.2.12);
• the membrane must terminate at or in a drainage flange to form a waterproof connection (clause 10.2.29); and
• the waste must be tall enough to suit the thickness of the tile and tile bed at the outlet position (clause 10.2.29).

For an unenclosed shower, clause 10.2.18 requires the whole wet area floor to be waterproofed and drained to a floor waste, with the shower waterstop at least 1500 mm horizontally from the shower rose.

None of those clauses says the waste must be central, at the edge, near the door or away from where you stand. That choice sits with the designer, and it is rarely made with durability in mind. It is usually made by where the plumbing happens to run, or by what looks good.

Why a waste in the standing zone ages faster

These are the mechanisms we think matter most. Each is reasoning from how the floor is built, not a measured failure rate.

• Repeated point loading. A person's weight lands on a small area, every day. The flange, the bed and the tiles immediately around a waste are cut, thin or built up in layers, so they are the least uniform part of the floor.
• The longest wet time. The standing zone is wet for the whole shower and stays damp afterwards. A bed that is wet and dried again and again shrinks, softens and effloresces sooner.
• Hot water straight onto the fitting. Shower water is hottest where it lands, and it runs directly into the waste. The grate, flange and pipe expand and contract more there than at the edge of the room.
• Soap, hair and cleaning chemicals collect at the lowest point, right where the membrane terminates. Chemicals and residue are hardest on sealants and membrane edges. See cleaning chemicals and building materials.
• Thin screed near the waste. The falls are built into the bed. A waste in the middle of a small shower has to create its fall in a short run, which pushes the bed thin towards the outlet. A thin, feathered bed is the weak bed we described in the previous article.
• Damage is felt sooner. A cracked grate, a lifted tile or a spongy bed under the feet is noticed straight away, but by then the membrane may already be damaged.
• Repairs mean closing the shower. The waste you stand on is also the hardest one to rework without stopping use of the room.

The layout options

Trade and supplier guidance, which is partly marketing, describes the same trade-off. A centre waste needs the floor to slope to it from all directions, while a linear drain can sit along one edge and needs a simpler one-way or two-way grade. A centre waste is the shortest drainage path, and a correctly tiled recess can make any type work. We read that as: the layout that is easiest to build correctly is usually the one that survives longest.

Trade-offs to be straight about

• Moving the waste is a design decision. It changes the plumbing, the falls and sometimes the structure. It needs to be settled before the slab is poured or the floor frame is set, not after the walls are lined.
• A linear drain is not automatically better. It replaces one point of risk with a long junction. A channel that is poorly sized, set too high or poorly sealed along its length fails along its length.
• Tile size and pattern interact with falls. Large-format tiles need flatter planes, which makes four-way falls hard. Edge or linear drains often suit large tiles better.
• Position relative to the structure matters. A waste over a joist, near a beam or close to a slab edge may sit on a different movement zone to the surrounding floor. Our reasoning is that any waste should sit where the floor is stiff and stable, and be fixed to the structure it relies on.
• Access matters. A waste that can be reached for cleaning, and in an apartment where the plumbing can be repaired without opening a neighbour's ceiling, is more likely to be maintained.

How we think about location at design stage

1. Mark the standing zone. Show where a person stands on the plan and keep the waste out of it where the design allows.
2. Choose the layout for buildability. Edge or linear drains for larger tiles and level-entry showers. A centre waste where the room and tile size genuinely suit it.
3. Settle position with the plumber early. Waste position, pipe fall and flange height are fixed before the screed or the floor sheets.
4. Check the falls on paper. Work out the screed thickness at the waste and at the farthest point, so the bed is not thin at the outlet.
5. Check the structure. Confirm what the waste sits on and that it will not be crossing a movement joint or an unsupported span.
6. Record the decision. The position and the reason go in the design file, so a later renovator does not move it.

Questions worth asking before the plumbing is set

1. Where is the waste in relation to where I will stand in the shower?
2. Why was this position chosen, and what are the alternatives?
3. What screed thickness is needed at the waste to achieve a fall of at least 1:80 across the whole shower floor?
4. If a linear drain is proposed, which product is it, and does the installer follow that maker's flange and membrane details?
5. What is the waste sitting on, and is it fixed to the structure?
6. Who confirms waste position and height before the screed goes in, and where is it recorded?

How we approach it

We treat waste position as a design hold point, not a trade detail. It is checked against the plan before the slab or floor frame is closed, and again with the plumber before the bed goes in. The membrane termination at the flange is then inspected and photographed, as part of the hold points on every wet area. If you are planning a bathroom or ensuite and want the layout checked for durability as well as appearance, get in touch.

Sources

• NCC 2022 Housing Provisions, Part 10.2 Wet area waterproofing, Australian Building Codes Board (clauses 10.2.12, 10.2.14, 10.2.18 and 10.2.29)
• NCC Navigator: Waterproofing in houses, Australian Building Codes Board
• AS 3740:2021, Waterproofing of domestic wet areas, Standards Australia
• How Australian bathroom designs are changing and what that means for drainage, The Good Builder (trade commentary on centre and linear drains)

This article is general information, not technical or legal advice for your project. Waste position, falls and flange details depend on the design, the membrane system and the manufacturer's instructions, and plumbing work must be done by a licensed plumber. Confirm the details with your designer, your certifier and the relevant standards. Full website disclaimer