Look underneath almost any bathroom basin and you will find a curved or bottle-shaped fitting between the plughole and the wall. That fitting is the basin trap. It is a small part, but without it your bathroom would quickly fill with unpleasant smells from the drains.
What Does a Basin Trap Do?
A trap does two jobs. It connects the basin waste to the waste pipe, and it holds a small amount of water in its lowest point. This water is called the water seal. Because air cannot pass through water, the seal blocks drain gases from travelling back up the pipe and out through the plughole.
Every time you use the basin, fresh water flows through the trap and replaces the water in the seal, so it stays topped up.
Why Every Basin Needs One
A basin connected straight to the drain without a trap would let sewer air into the room. As well as smelling bad, that air can carry moisture and bacteria. Building regulations in the UK expect sanitary fittings such as basins to have a trap with a suitable water seal, so a trap is a standard part of every basin installation.
The Main Types of Basin Trap
- Bottle trap – a compact, upright trap with a removable base. Popular where the pipework is on show, such as under a pedestal or wall-hung basin.
- P trap – a curved trap shaped like a sideways P, with the outlet going into the wall.
- S trap – a curved trap with the outlet going down into the floor.
- Straight-through trap – a slim design that keeps the outlet in line with the inlet, useful in tight spaces.
- Telescopic or adjustable trap – lets you change the height or length during fitting when the pipes do not line up perfectly.
- Compact or furniture traps – shaped to fit inside vanity units where space is limited.
Plastic or Metal?
If the trap is hidden inside a cabinet or behind a pedestal, a plastic trap is practical, light and easy to fit. If the trap is visible, a metal bottle trap in a finish such as chrome, brushed brass or matt black becomes part of the design and can match your tap and waste.
Sizes
Most UK basins use a 1¼" (32mm) waste and trap. Baths and showers use a larger 1½" (40mm) size, so make sure you choose a trap made for a basin. Check the distance from the bottom of the waste to the waste pipe as well, since many bottle traps come with an extension tube to bridge the gap.
Signs Your Trap Needs Attention
- A smell from the plughole, which can mean the seal has dried out or the trap is dirty.
- Water collecting under the basin, which can point to a loose nut or worn washer.
- Slow draining, often caused by hair and soap building up inside the trap.
Many bottle traps have a removable base, which makes cleaning simple. If a trap is cracked or the seals have perished, replacing it is usually straightforward.
How to Choose a Basin Trap
- Decide whether the trap will be on show or hidden.
- Check where your waste pipe goes: into the wall or into the floor.
- Measure the height and distance between the waste and the pipe.
- Check how much space there is, especially inside a vanity unit.
- If the trap is visible, pick a finish that matches your tap and waste.
Find the Right Trap
Browse our basin waste traps to compare bottle traps, plastic traps and compact designs. For the bigger picture, see our full waste and traps range, or read our guide on what a basin waste is and how it works.