What is the difference between a bottle trap, a P-trap and an S-trap?

The whole job of a trap is to hold water. The differences between the three types are about where the outlet points, how much they resist blocking, and how easily they lose that water seal.

Step by step

1. Understand what the water seal does.Every trap holds 50 to 75mm of standing water between the appliance and the drain. That water is the only thing stopping drain gases entering the room, and every trap failure is really a lost water seal.
2. Recognise a P-trap.A U-bend followed by a horizontal outlet into the wall. It has the fullest bore and the most reliable seal, and it is the default choice for sinks and baths.
3. Recognise an S-trap.A U-bend followed by a downward outlet through the floor. It works, but a fast discharge can pull the seal out with it, leaving the trap dry and the room smelling.
4. Recognise a bottle trap.A vertical cylinder with an internal tube. Water drops down the tube, rises around it and leaves through a side outlet. Compact and visually tidy, which is why it is used on pedestal and wall-hung basins.
5. Compare them on blockage.Bottle traps have the smallest effective bore and catch hair and soap most readily. P-traps swallow much more before they restrict. In a kitchen sink a bottle trap is a poor choice.
6. Compare them on smell.S-traps siphon most easily and go dry in unused rooms fastest. If a guest bathroom smells after a fortnight unused, an S-trap is a likely suspect.
7. Check the seal depth you need.75mm seal for most appliances on a standard system, 50mm where it discharges to a gully. Shallow-seal traps exist for tight spaces and lose their seal more readily.
8. Consider the low-profile options for baths and showers.Shower and bath traps are shallow by necessity. Look for one described as a fast-flow or high-flow trap so it can swallow a shower's discharge.
9. Match the size to the appliance.32mm for basins, 40mm for sinks, baths and showers. Fitting a 32mm trap on a bath makes it drain painfully slowly.

Tips

• If a basin blocks repeatedly and has a bottle trap, swapping to a P-trap usually ends the problem outright.
• A trap that has gone dry is cured in ten seconds by running the tap. Do that before diagnosing anything else about a smell.
• Keep the old trap's nuts and washers when replacing one — they are useful spares.

Common mistakes

• Fitting a bottle trap under a kitchen sink — Kitchen waste carries fat and food debris, and the narrow annular gap in a bottle trap catches both. It blocks far more often than a P-trap in the same position.
• Using a shallow-seal trap on an unvented branch — The shallower the seal, the more easily it siphons away. On a long or unvented branch that leaves the room smelling of drains within days.

Alternatives

• Waterless or membrane trap: Uses a flexible membrane instead of stored water, so it cannot dry out. Useful for rarely used appliances and cellar gullies, but the membranes eventually perish.
• Anti-siphon or resealing trap: A P- or S-trap with a small air inlet that lets air in rather than pulling the seal out. Sensible where siphoning is a known problem.
• Space-saver or shallow trap: For wall-hung basins and shower trays where there is no depth. Accept the shallower seal and check it regularly.

Questions people ask