This is the single most common ordering mistake with a toilet bowl bought online: getting the trap type wrong. Get it right before you order and installation is a straight swap. Get it wrong and you're either returning a bowl or paying a plumber to modify your waste pipe. Here's how to check it yourself, no site visit required.
S-trap vs P-trap: what's actually different
The trap type describes which direction the waste outlet points, based on where your existing plumbing already runs. An S-trap outlet points straight down into the floor, and it's the most common setup in older HDB flats. A P-trap outlet points out through the wall behind the bowl, and it's more common in newer builds and some renovated bathrooms.
You cannot swap a P-trap bowl onto an S-trap outlet, or the reverse, without re-routing the waste pipe. That's a plumbing job, not a toilet-installation job, so it's worth getting right at the ordering stage.
How to check which one you have
- Look at your existing toilet bowl's base. If the outlet disappears straight into the floor beneath the bowl, you almost certainly have an S-trap.
- If there's a pipe running from the back of the bowl into the wall, that's a P-trap.
- Measure the roughing-in distance. For an S-trap, that's the distance from the wall to the centre of the floor outlet. For a P-trap, it's the height from the floor to the centre of the wall outlet.
A real example from the catalog
The Aalto ATAL8010 one-piece closet (660x360x700mm) is listed with both configurations available: S-Trap at 150mm or 250mm roughing-in, and P-Trap at 180mm roughing-in. That range is fairly typical. Most bowls in the catalog list a small set of standard roughing-in distances rather than one fixed number, which is exactly why matching your own measurement to the listing matters before you order.
What to do if your measurement falls between listed sizes
Some bowls include an adjustable connector or offset pan collar that bridges a small gap between your existing outlet and the new bowl's roughing-in distance. Check the product listing or ask before assuming a mismatch is a dead end.
FAQ
Can I convert an S-trap toilet to a P-trap, or the reverse?
Not without re-routing the waste pipe, which is plumbing work, not toilet installation. Confirm your trap type before ordering rather than after.
What roughing-in distance is most common in HDB flats?
It varies by flat and renovation history. Measure your own outlet; the Aalto ATAL8010 example above shows the same model listing more than one roughing-in option for exactly this reason.
Not sure which trap type you have? The full Toilet Bowl collection lists S-trap/P-trap options and roughing-in distances on every model, or see Plumbing Fittings & Valves for pan collars and offset connectors.