What is a drain trap?
A drain trap is a bent or cup-shaped fitting in a waste pipe that holds back a small amount of water after every discharge: the water seal. That water closes off the pipe, so sewer gases and smells cannot come back up through the basin, shower or sink. Every appliance connected to the drainage system needs one.
Types
- Tubular trap: a bent pipe in U, P or S shape (P-trap, S-trap). Flows well and blocks less easily.
- Bottle trap: a cylinder under the basin in which the water has to rise before it can drain away. Compact and easy to clean through the unscrewing base.
- Floor gully or shower channel with integral trap: the water seal sits in the drain itself.
- Integral trap: a WC pan has the trap built into the ceramic.
The European standard EN 12056-2 requires a water seal of at least 50 mm. Some national rules ask for more for certain appliances; in the UK, 75 mm seals are common, so check local requirements.
Common problems
- Evaporation: in a guest bathroom or floor drain that is rarely used, the water dries out. A smell is the warning sign. Fix: run water regularly.
- Siphonage or blow-out: if the pipework is poorly ventilated, water flowing elsewhere in the system sucks the trap empty. A gurgling sound is the giveaway. Fix: an air admittance valve or a working vent stack.
- Blockage: hair, soap residue and grease collect in the bend. Unscrew the access cap or bottle (bucket underneath) and clean it, rather than using aggressive drain chemicals.
| Evaporation | Cause: the water dries out in a rarely used guest bathroom or floor drain. Sign: a smell. Fix: run water regularly. |
|---|---|
| Siphonage or blow-out | Cause: poorly ventilated pipework lets water flowing elsewhere suck the trap empty. Sign: a gurgling sound. Fix: an air admittance valve or a working vent stack. |
| Blockage | Cause: hair, soap residue and grease collect in the bend. Fix: unscrew the access cap or bottle (bucket underneath) and clean it, rather than using aggressive drain chemicals. |
Application: tips for fitting
- Never fit two traps in series. The air trapped between them cannot escape, so the waste runs slowly and gurgles.
- Keep a fall. The pipe after the trap must slope gently towards the stack.
- Match the diameter. Basins usually have a smaller waste connection than sinks and showers; follow the size of the appliance and pipe.
- Test for leaks after fitting by filling the appliance and letting it empty in one go.
Related terms
- Air admittance valve
- Soil vent pipe
- Floor drain
- Air vent
- Sewer
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