P Trap vs S Trap Toilet: What's the Difference and Which to Choose?

When it comes to p trap vs s trap for a toilet, this is one of those little things that can cause a big headache later on. I've seen it time and time again: a homeowner chooses the wrong trap, and it's not compatible with their current plumbing. That's a nightmare to correct.
Let me explain. The trap is the U-bend or curved piece of pipe under your toilet, that contains water. The water acts as a barrier to prevent sewer gases from rising back up through the drain, into your bathroom. Both p trap and s trap accomplish this, but they do it in different ways. And there's a reason why one of these is becoming less popular in Australia.

What's the Actual Difference Between a P Trap and an S Trap?

The p trap connects to the waste pipe through the wall. Viewed from the side, the pipe resembles a P lying on its side. Waste exits the toilet horizontally, out through the wall either behind or beside it.
An s trap, however, connects to the waste pipe through the floor. The pipe curves down and then back up, before heading straight down into the floor. When viewed from the side, the pipe looks like an S. Simple, really.
So what is an s trap toilet? Essentially, it's a toilet that has the waste pipe exiting straight down through the floor. In most older Australian homes, this is how the trap was set up. The p trap vs s trap toilet distinction? It's all about whether your waste pipe exits into the wall or floor.

Why S Traps Are Being Phased Out in Australian Homes

Here's the thing: many homeowners don't realise that s traps have a major flaw that p traps don't — they can lose their water seal.
As water flows out through an s trap, it creates a siphoning effect. This can suck the water out of the trap. Once that happens, you have an open channel for sewer gases to rise back up into your bathroom. Not good.
The National Construction Code and Australian standards now favour the p trap, for this very reason. In many councils, it's no longer acceptable to install an s trap without additional venting. I've been on jobs where the plumber has had to fit an air admittance valve, just to bring an s trap up to code. It's just extra hassle and expense that a p trap doesn't have.
That being said, there are still millions of homes in Australia with s traps. If yours is working fine and is properly vented, there's no real need to tear it out. But if you're renovating, you might want to consider making the switch.

How to Tell Which Trap Your Toilet Has

This is easier than you might think. Take a peek at the back of your toilet, and look where the waste pipe goes.
If it leads into the wall behind the toilet, you have a p trap. The distance from the wall to the centre of the waste outlet is known as the set-out, and in Australia it's usually somewhere between 140mm and 220mm for a p trap.
If it leads straight down into the floor, you have an s trap. For an s trap, the set-out is measured from the wall to the centre of the floor outlet, and it's usually somewhere between 60mm and 200mm.
You need to measure your set-out before purchasing a new toilet. The types of toilets available come in specific set-out ranges, and getting this measurement wrong means the toilet won't fit. I've seen it happen more times than I'd like to admit.

Replacing or Converting Your Toilet Trap

If you're replacing your toilet suite and you want to keep the same trap, it's a no-brainer. Measure up your set-out, purchase your new toilet suite and a licensed plumber will have it fitted in under two hours.
If you're swapping from one trap to the other, it's a big deal. To go from s trap to p trap you need to run the waste pipe out through the wall instead of the floor. That means running a new pipe and ensuring you have the correct fall. Going from p trap to s trap, you need to core the floor. Either way, this is not a DIY job.
If you're sticking with an s trap, make sure you purchase a good quality toilet with s trap configuration and ensure the suite has a vent connection point. A well-made s trap toilet suite, with proper venting, will serve you just as well as any other toilet suite.
The cost of converting from one trap to the other runs between $800 and $2,000, depending on access and the amount of pipe that needs to be run. Factor this into your renovation budget if you're planning the switch.

Choosing the Right Trap for a New Bathroom

If you're building a new home or doing a full renovation and have the option of either, every time I would recommend going with a p trap. There's less chance of the water seal being lost, the venting is easier and it complies with modern standards.
When purchasing a new toilet suite, don't just think about whether it's a p trap vs s trap — also consider the height of the cistern, the comfort of the seat, the WELS rating and the performance of the flush. Deciding on your trap type is important, but it's just one part of the process.
If you're renovating, you may also want to consider installing a bidet on the toilet as well. There are some fantastic smart bidet seats that can be fitted to most modern toilet suites which will add enormous value to your bathroom. If you're updating all your plumbing, adding a bidet for the toilet at the same time makes sense — the water connection is already there.