What Is an S Trap?

If you live in an older house, peek under your sink — you might spot a pipe that curves like a roller-coaster, looping down and then back up before disappearing into the floor. That’s an S trap, and it was once a common solution for preventing sewer odours from creeping back into homes.

Traps are curved pipes connecting fixtures like sinks, basins and toilets to the underlying sewage system near your home. Every pipe is a two-way channel; if your water drains out, your sewage gases can leak in.

Plumbers developed the S-shaped pipe design to prevent smells and harmful gases from entering your home. The idea behind the S-shaped piping is simple: it holds a small amount of water in the bend. The water acts as a seal, stopping sewer gases from travelling back up the pipe and into your home, and the seal refreshes every time you use the tap.

Water flowing quickly through an S trap can create a siphoning effect. Basically, the rushing water pulls the water seal out of the trap, leaving the pipe dry and open to sewage. That’s when you get the telltale sulphuric smell.

Another issue? S traps rely on a vertical drop into the floor, making them harder to vent properly than newer designs. Pressure can build up in the pipes, and further increases the risk of that water seal getting sucked in.

That’s why the Australian Plumbing Code now requires updated, vented trap designs like the P trap.

P traps look like an upside-down letter P or the letter J. The curved section connects horizontally to the wall rather than dropping straight to the floor like the S trap. This small design change makes a big difference — it allows for proper venting, which helps equalise pressure and prevents siphoning. That means no dry pipes and sewage smells in your bathroom.

P traps are also far easier to maintain and upgrade. Their horizontal orientation gives you more flexibility with pipe routing. As they comply with modern plumbing regulations, you won’t run into headaches with inspectors.

S traps are non-compliant in new Australian construction and renovation projects because of the siphoning risks. Something as simple as filling the sink with water and pulling the drain plug can dry the seal. This increases the risk of sewer gases like hydrogen sulphide, methane and even carbon monoxide entering your home.

So, if you’ve got an S trap lurking under your basin or toilet, it might be time for an upgrade.

You can replace S traps with P traps under high-volume toilets and kitchen sinks. However, a compact bottle trap is much better for bathroom basins. It looks like a horizontal letter T with a bottle-like component that can be screwed open for easy cleaning. The water seal forms in the horizontal section, so there are no siphoning risks.

Just make sure that you don’t try to do it yourself! Get a plumber in so you do it safely and properly.

If you’re in the middle of a home build or reno, take the opportunity to get your basin trap right the first time.