What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Close, not identicaltrap
also U-bend, bottle trap, P-trap, gully
U-bend is used colloquially for any trap regardless of shape, including bottle traps that contain no U at all.
Australia
Close, not identicaltrap
also P-trap, S-trap, water seal, gully trap
As the UK, with the external gully trap a standard part of the drainage arrangement rather than an occasional fitting.
United Statesyours
Same word, different thingP-trap
also trap, trap seal, drum trap
The shape is named rather than generic: the S-trap is prohibited by plumbing code in most of the country because it siphons dry, so trap and P-trap are used almost interchangeably.
Canada
Same word, different thingP-trap
also trap, trap arm, running trap
As the United States, and the trap ARM — the pipe between the trap and the vent — is a separately dimensioned component that British documents do not name.
The governing standard, by market
United Kingdom
BS EN 12056-2 / Approved Document H
Sanitary pipework inside buildings, including trap seal depths and the conditions that break them.
United States
IPC Chapter 10 / UPC Chapter 10
Traps, trap seals, prohibited trap types, and the maximum distance from a trap to its vent.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why is an S-trap a problem when a P-trap is not?
- Because of what the outgoing pipe does immediately after the seal. A P-trap turns and runs roughly HORIZONTALLY into the wall, so water leaving the fitting slows and breaks contact with the pipe, and the seal behind it is left full. An S-trap drops VERTICALLY straight down, so a full basin discharging gives the falling column enough momentum to pull the seal out behind it — the trap self-siphons and is left dry, and a dry trap is an open pipe to the drain. That is why American and Canadian codes prohibit the S-trap outright and why British guidance limits the conditions in which a vertical leg is acceptable. The failure is silent: the fitting looks correct and only smells wrong days later, when the remaining water has evaporated as well.
- What does trap seal depth mean, and when does it matter?
- It is the vertical depth of water the trap holds — typically 50 mm or 75 mm — and it is the budget the seal has to survive pressure changes in the drain. Every discharge elsewhere in the system pushes and pulls air past the trap, and each event takes a little water with it. A deeper seal survives more of that, which is why a shallow seal is specified only where the run is short and well vented, and why a shower trap that had to be shallow to fit under the tray is the one most likely to smell. Evaporation spends the same budget: an unused trap loses its seal over weeks, which is why a guest bathroom, a floor gully or a rarely used utility sink smells and a daily-use basin does not.
- Is a grease trap the same kind of thing?
- No, and the shared word causes real specification errors. A sanitary trap exists to hold a water seal against gas, and it is sized in millimetres of seal depth. A grease trap — or grease interceptor — exists to separate fats from wastewater so they cool and solidify before they reach the drain, and it is sized in litres or gallons of retention volume against a flow rate and a retention time. A petrol or silt interceptor does the same job for a different contaminant. They are all called traps, they all sit in a drainage line, and only one of them is about smell. Sizing one with the other's rules produces either a fitting that cannot hold a seal or a chamber far too small for the kitchen it serves.