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Most people never think about the shape of the pipe under their toilet until the day they go to replace the toilet and discover that the shiny new one they bought will not connect. The difference between an S-trap toilet and a P-trap toilet is the single most important measurement when you buy a new pan, and getting it wrong means an expensive return or a plumber's bill to adapt it.
Here is how to tell which one you have, and why it matters more than almost any other spec on the box.
Every toilet has a trap: a curved section of pipe that holds a small amount of water to seal sewer gas out of your bathroom. On a toilet, the trap is built into the pan, and the difference between the two types comes down to one thing, the direction the waste exits.
An S-trap exits straight down through the floor. A P-trap exits horizontally out the back, through the wall. That is the whole distinction, and it dictates which toilet will physically fit your existing plumbing.
You do not need to lift anything. Look at where the waste pipe disappears.
If the pipe goes down through the floor behind or beneath the pan, you have an S-trap. This is the most common set-up in older Sydney homes, including most heritage terraces and anything built on a raised floor.
If the pipe runs out the back of the pan and into the wall, you have a P-trap. These are common in newer builds, apartments and concrete slab construction where running waste through the floor is not practical.
Knowing S or P is only half the job. The number that actually matters when you buy a toilet is the set-out, the distance from the finished wall to the centre of the waste pipe.
For an S-trap, measure from the finished wall to the centre of the floor outlet. For a P-trap, measure from the floor to the centre of the wall outlet. Toilets are sold to suit specific set-out distances, and a pan designed for a 140mm set-out will not sit correctly over a 100mm outlet. This single measurement causes more failed toilet swaps than anything else, so write it down before you go shopping.
If you are simply replacing a like-for-like toilet, you want to match the existing trap type and set-out as closely as possible to avoid moving any pipework. If the numbers do not match, you are not stuck, but you are now into either an adjustable pan connector or relocating the waste, which is a bigger job.
This is also why buying a toilet online without checking your set-out is risky. The photos rarely make the trap type obvious, and a beautiful wall-faced pan is useless if your outlet is in the floor and the model only suits a wall outlet.
Yes, within reason. A licensed plumber can fit a pan connector or adjust the waste to convert between trap types or to shift the set-out by a modest amount. It is routine work, but it is plumbing work: cutting and resealing a waste connection has to be watertight and gas-tight, and a poor seal lets sewer odour into the room. If your dream toilet does not match your existing outlet, this is the path, and it is worth getting a quote before you commit to the pan.
Neither is better. They suit different building types. S-traps suit floor-waste plumbing common in older homes; P-traps suit wall-waste plumbing common in newer builds and apartments. The right one is whichever matches your existing outlet.
Some modern pans are sold with an adjustable connector that can accommodate a range of set-out distances and, in some cases, both floor and wall outlets. They cost a little more but reduce the risk of buying the wrong model. Always check the stated set-out range before you rely on one.
Replacing a toilet involves disconnecting and resealing the waste and water supply. A faulty seal can leak water or let sewer gas into the room, and in New South Wales this is licensed plumbing work. It is a job worth handing to a licensed plumber to get the seal right the first time.
For a floor outlet (S-trap), measure from the finished wall to the centre of the waste pipe. For a wall outlet (P-trap), measure from the floor to the centre of the outlet. Take the measurement with you when you buy.
Replacing a toilet and not sure what will fit? Send us a photo or call Plumberoo on (02) 9191 8787. We are licensed (289252c) and across Sydney, and we will tell you exactly what set-out you need.