Bathroom Waste Traps
When a bathroom waste trap fails through age, freezing or a poor fitting, you smell it before you see it. Replacement traps come in a small number of standard shapes and connection sizes that cover every UK basin, sink, bath and wet-room install.
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BATHROOM WASTE TRAPS | BUYING GUIDE
Types of Waste Trap
Trap shape is determined by how the waste pipe runs from the appliance. The four most common shapes in UK bathroom installs are below.
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P-trap
The most common trap in UK bathrooms. The waste pipe leaves the basin or sink horizontally, curves down and back up through the trap, then exits horizontally into the wall. Used wherever the soil pipe runs through the wall behind the basin.
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S-trap
Used where the waste pipe runs straight down through the floor rather than into the wall. Same internal water seal as a P-trap; the difference is the orientation of the outlet. Less common in modern bathrooms because building regulations favour wall-routed pipework.
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U-bend
A traditional brass or chrome trap, often visible under pedestal basins and Victorian-style baths. Functionally identical to a P-trap but built as a single curved tube. Usually paired with an adjustable arm for fine-tuning the outlet position.
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Low-profile and anti-siphon traps
Designed for shallow installs (under-vanity, wet-room, walk-in shower). The trap body is compressed vertically, with anti-siphon valves on some designs to prevent the water seal being pulled out by negative pressure when nearby fixtures drain.
Common Connection Sizes
Inlet size matches the appliance outlet; the trap outlet matches the soil pipe behind the wall or under the floor. The two are often different sizes, which step-down adapters resolve.
| Connection | Common application | Typical inlet x outlet |
|---|---|---|
| 32 mm (1.25 inch) | Most UK basins and small sinks | 32 x 32 mm or 32 x 40 mm |
| 40 mm (1.5 inch) | Baths, showers, kitchen sinks | 40 x 40 mm or 40 x 50 mm |
| 50 mm | Soil pipe extension or larger drainage | 40 x 50 mm or 50 x 50 mm |
Step-down adapters cover the case where appliance and pipe are different sizes. Cylindrical bottle-style alternatives are listed separately in our bottle trap range, with the rest of the wastes catalogue grouped together in our bathroom wastes hub.
Identifying Your Existing Trap
Photograph the existing trap from the side and from below before you remove it. Note the orientation of the outlet (horizontal into wall, or vertical down through floor) and the inlet and outlet pipe diameters. A ruler placed against the pipe gives you the dimensions.
If the trap has a brand name printed on it (often on the side of the bowl or near the union nut), make a note. Most modern UK traps are interchangeable across brands as long as the connection sizes match, but brand-matched parts often share visual finishes that matter on exposed installs. Bath and shower waste assemblies are covered separately in our bath wastes and shower wastes ranges.
Waste Trap FAQs
What is a bathroom waste trap?
A waste trap is the curved section of pipe between the waste outlet and the soil pipe. It holds a small volume of water that seals the drain, stopping odours and gases from coming back up into the room. Traps come in several shapes (P-trap, S-trap, U-bend, bottle trap, low-profile) chosen to suit how the waste pipe runs from the basin or sink to the wall or floor.
Are bathroom waste traps a standard size?
Connection sizes are mostly standard. UK basins use 32mm (1.25 inch) inlet, baths and showers use 40mm (1.5 inch). The trap shape and depth vary considerably to suit different pipework runs and clearances. Always match the inlet size and the outlet pipe size to the existing trap before ordering.
How do I know which trap shape to buy?
Look at how the existing pipe runs from the waste outlet. If the pipe runs into the wall horizontally, you need a P-trap. If it runs straight down through the floor, you need an S-trap. If you need both options or are converting between layouts, a U-bend with adjustable arms is more flexible. For wet rooms or low-clearance installs (like under-bath or shallow vanity units), a low-profile or anti-siphon trap fits where standard traps will not.
Can I replace a bathroom waste trap myself?
Yes, in most cases. Turn the water off, place a bucket below the trap, unscrew the union nuts at the inlet and outlet, remove the trap, fit the replacement with hand-tight unions and check for leaks by running water through. PTFE tape on the threaded ends seats the seals. Compression and push-fit traps are easier than threaded; check which type you have before ordering.
Do you supply traps for older or non-standard installations?
Yes. Older British plumbing sometimes uses imperial or proprietary fittings that no longer match modern 32mm or 40mm standards. Adapters and step-down fittings cover most of these, and we hold stock for older bathroom and trade fittings, with brand-matched parts catalogued in our manufacturer directory.
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