Sizes in the UK
- 32mm — basins.
- 40mm — kitchen sinks, baths and showers.
- Deep seal (75mm) and shallow seal (38–50mm) versions exist.
The main types
- P trap: outlet goes horizontally out through the wall.
- S trap: outlet goes down through the floor.
- Bottle trap: compact cylinder, popular under basins and pedestal basins.
- Bath trap: low and shallow to fit under the bath.
- Shower tray trap: often cleanable from above.
- Anti-siphon trap: includes a valve to protect the water seal.
- Washing machine trap: has a spigot for the hose.
How a trap joint seals
Plastic traps use compression nuts with cone washers or flat washers. Push the pipe in, the washer sits between pipe and trap body, and the nut squeezes it. Hand tight is usually enough — that is your Solid – Seal – Solid joint.
Pro tip: Cone washers have a direction: the thin end points into the fitting. Fit it backwards and it leaks.
Choosing the right trap
Match the size to the waste, the outlet direction to the pipe, and the seal depth to the regulations for that pipe run. Use an anti-siphon trap where the pipe is long or drops steeply.
