S Trap Definition: What It Means for Aussie Toilets

Replacing a toilet and suddenly confronted with a pipe poking straight up through the floor? That's an S trap — and knowing exactly what you're dealing with will save you from buying the wrong suite entirely.

An S trap in plumbing is a drain fitting where the pipes form a double-bend — like the letter "S" — with the pipe exiting the toilet pan vertically downward through the floor. The critical measurement is the setout: the distance between the wall and the centre of the floor waste pipe should be 100–120mm. Get that measurement wrong and nothing fits. I've seen it happen more times than I'd like to admit.

What Is an S Trap? The Core Definition

The s trap definition is easy to understand when you look at the shape. An s trap in plumbing is a drain fitting where the pipes form a double-bend — like the letter "S" — with the pipe exiting the toilet pan vertically downward through the floor. A floor outlet is the defining characteristic. If your waste pipe goes down to the ground below the toilet, there's a good chance you have an s trap toilet.

For an s trap toilet setout, the distance between the wall and the centre of the pipe should be 100–120mm. It is important to remember that it is the centre of the pipe you measure, not the outside edge — many people make this mistake and purchase a toilet suite that doesn't fit.

An s trap is designed to maintain a water level in the bottom portion of the S-bend. This water acts as a barrier and prevents sewer gases, such as hydrogen sulphide, from entering the bathroom through the toilet pan.

How S Traps Work in Australian Toilet Plumbing

Now that we have a better idea of the shape and geometry, let's examine exactly what that double-bend does to facilitate the flow of water in a working Australian plumbing system.

Every time you push the flush button, the water in the cistern is released into the bowl, pushed through the S-bend and then down the waste pipe. The lower curve immediately refills with standing water as flow subsides, re-establishing the gas seal. S trap toilets are susceptible to siphonage, which occurs when the flow of water creates enough suction to pull the water seal out of the trap. Australian plumbing standards address this through venting requirements — vent pipes relieve negative pressure in the waste line so the water seal isn't sucked out (AS/NZS 3500.2:2025). Typical waste pipe diameter in Australian residential installations is DN100 (100mm) in PVC-U, which is the material found under virtually every Australian home built in the last four decades.

S Trap vs P Trap: Key Differences for Aussie Bathrooms

Since s trap toilets are more prone to siphonage, you may be wondering whether an s trap or a p trap toilet is the right fit for your bathroom.

The core difference is exit direction. An s trap exits through the floor; a p trap toilet exits through the wall. That single distinction drives everything else — which toilet suite you can buy, what your licensed plumber needs to do, and what type of floor construction you're working with.

S traps are more commonly found in older Australian homes, particularly those built on slab-on-ground foundations where the waste pipe was cast into the concrete. P traps are the standard in new builds and renovations where wall cavities are accessible and drainage can be routed horizontally. Suspended timber floors can accommodate either configuration, giving renovators more flexibility.

Siphonage susceptibility is genuinely higher with S traps — correct venting manages it effectively. P traps are inherently more resistant to siphonage by virtue of their horizontal outlet. The setout for a p trap toilet is 185mm from the wall to the pipe centre, compared to 100–120mm for an S trap, so they are not interchangeable without relocating pipework.

Choosing and Installing an S Trap Toilet in Australia

Now that we've established whether an S trap suits your floor type and drainage layout, you need to select a suitable suite and ensure it's installed correctly.

First, measure the setout distance from the finished wall to the centre of the floor waste pipe. This determines which s trap toilet suites will fit. Brands like Poseidon and Fienza offer back-to-wall S trap suite ranges with setouts across the 100–120mm span. Ensure you select a model whose pan collar aligns with your measured distance before purchasing.

All toilet suites sold in Australia must carry WaterMark certification (AS/NZS 6400:2016) (WaterMark Certification Scheme) and be WELS registered to be legally sold or installed. Reputable suppliers make this information easy to find. If a toilet suite lacks these credentials, don't buy it.

When discussing s trap in plumbing and the installation process, you should understand what is involved. A licensed plumber will disconnect your old suite, attach the new pan collar to the waste pipe, then secure and test the whole suite. A typical supply-and-install cost for a standard close-coupled S trap suite ranges from AU$400–$900, depending on the suite and your location. Entry-level suites can be as low as $185–$400 for the pan. Mid-range rimless models from brands such as Fienza or Kohler cost between $400–$800.

Always have an isolation valve installed — around $30 for the fitting — so you can shut the water supply at the toilet itself rather than at the mains when future maintenance is required.

S Trap Compliance and Practical Considerations

Beyond the hardware, there are some important legal boxes to tick before any toilet goes in. Your licensed plumber is required to complete and sign the compliance certificate. Installing an S trap yourself is unlawful regardless of DIY competence. That compliance certificate also matters for insurance — water-related damage caused by unlicensed plumbing work is likely to invalidate a home insurance claim.

Regarding maintenance, two things are worth noting. If a property is unoccupied for several weeks, the water in the trap may evaporate; flushing will replenish it, but the odour in the meantime can be unpleasant. A flush that doesn't clear the pan completely, or produces a gurgling sound, could indicate partial siphonage or a venting issue — contact a licensed plumber to assess and rectify rather than attempting a fix yourself.

The s trap definition is best summarised as a floor-draining double-bend pipe fitting that forms a water seal to prevent sewer gases from entering your bathroom. Due to specific setout requirements and compliance obligations, professional installation is required across Australia.

References

AS/NZS 3500.2:2025 Plumbing and Drainage — Sanitary Plumbing and Drainage, Standards Australia

WaterMark Certification Scheme, Australian Building Codes Board

AS/NZS 6400:2016 Water efficient products — Rating and labelling (incorporating Amendment No. 1:2022 and Amendment No. 2:2022), Standards Australia

State and territory plumbing licensing authorities (Building and Plumbing Commission Victoria; Building Commission NSW / NSW Fair Trading; Queensland Building and Construction Commission; Plumbers Licensing Board Western Australia (administered by Building and Energy); Consumer, Building and Occupational Services Tasmania; Access Canberra ACT; Office of the Technical Regulator / Consumer and Business Services South Australia; Plumbers and Drainers Licensing Board Northern Territory)