Basin wastes and bottle traps: slotted, unslotted and matching the parts

Under every bathroom basin there are two parts that get water away: the waste, the fitting in the plughole, and the trap, the bend below that holds water to keep drain smells out. Buying the wrong waste for a basin is one of the most common mistakes when changing a basin or tap, and it leads to slow leaks or water that never drains properly.

This guide explains how to match them. For clearing a blockage, see blocked sinks and basins. For baths, see bath panels and traps.

The waste

The basin waste is a threaded tube that passes through the plughole. A flange sits on top in the bowl, and a backnut underneath clamps it to the basin with seals or washers either side. The trap screws onto the bottom of it.

Slotted or unslotted

Most UK basins have a built-in overflow: a hole near the top of the bowl, connected by a channel inside the ceramic to the plughole. If the basin overfills, water runs into the overflow and down the waste.

• A slotted waste has a slot or holes in the side of its body. Water from the basin's overflow channel enters through the slot. Use a slotted waste on a basin with an overflow.
• An unslotted waste has a solid body. Use it on a basin without an overflow, which is common on countertop bowls and some modern designs.

Fit an unslotted waste to a basin with an overflow and the overflow does nothing. Fit a slotted waste to a basin without one and water can leak out of the slot under the basin.

Some wastes are sold as universal, meaning they suit both slotted and unslotted basins. Check how the maker says to fit it for your type.

Plug types

• Plug and chain. The traditional option. Simple and easy to repair.
• Click-clack (push-button) waste. Press the plug to close it, press again to open. No lever. Common on modern basins and sold with many taps.
• Pop-up waste. A lever behind the tap opens and closes the plug through a linkage under the basin. Often supplied with the tap.
• Freeflow (open) waste. No plug at all, just a grille. Often used on cloakroom basins.

If you are changing the tap, check whether it comes with a pop-up waste. If it does, you will need to fit that waste too, or buy a version of the tap without one. Our guide to mixer and bath taps covers choosing taps.

Length and fit

The waste body must be long enough to pass through the basin and leave thread for the trap, but not so long that it pushes the trap into the wall. Ceramic basins vary in thickness. Some wastes are adjustable or come with spacers. Measure before you buy.

The trap

The standard UK size for basin wastes and traps is 32mm. Kitchen sinks and baths usually use 40mm. Our blocked sink guide mentions the same split.

Bottle trap

A bottle trap is an upright cylinder. Water enters at the top, drops into the bottle and leaves through a side outlet to the waste pipe. The base unscrews for cleaning.

• Good for: basins on a pedestal or in a vanity unit where space is tight, and where you want to clear hair and soap easily.
• Chrome or designer bottle traps are made to be seen, under wall-hung or countertop basins.
• Telescopic versions adjust in height to line up with the waste pipe.

P-trap

A P-trap (or U-bend) is a curved trap, often with two nuts, where the outlet leaves horizontally into the wall.

• Good for: where there is room, and where the waste pipe runs straight back into the wall.
• Has no chamber at the bottom for debris to settle in, though it can still block, and it is harder to clean without taking it off.

There is also the S-trap, where the outlet points down through the floor. It is less common on basins.

Water seal

The water held in the trap is the seal. If it is lost, drain smells come into the room. Seals can be lost by evaporation in a basin that is rarely used, or by being siphoned out when water rushes down the pipe. A gurgling basin and smells after other appliances drain can point to a waste pipe problem that a plumber should look at.

Matching waste, trap and pipe

Before you buy:

1. Basin: does it have an overflow? That decides slotted or unslotted.
2. Plug type: chain, click-clack, pop-up from the tap, or freeflow?
3. Size: basin traps are usually 32mm. Check the trap says it suits a basin waste and matches your waste's thread.
4. Trap shape: bottle trap or P-trap, depending on space and where the pipe goes.
5. Pipe: check the trap's outlet suits the waste pipe material and size: compression, push-fit or solvent weld.

When you change a basin, it is a good moment to check the isolation valves on the tap supplies, so you can work without shutting off the house. See isolation valves.

Fitting and leaks

A waste is sealed to the basin with washers, or with a sealant the maker approves. The trap is usually hand-tight with its own washers. Common leak points:

• under the waste flange, from a missing or misplaced washer;
• at the backnut, if it was over-tightened and cracked the plastic, or under-tightened;
• at the trap nuts, from a missing or twisted washer;
• from the overflow slot, on a slotted waste fitted to a basin without an overflow.

Dry the area, run water, and watch with a torch to find the exact point. A kitchen sink works on the same principles with larger parts; our guide to kitchen sink installs covers that side. Finish the basin edge with sealant if needed; see sealant and silicone.

Examples at Wickes

We haven't tested these. They are examples, with what the retailer's pages say.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

What is the difference between a slotted and unslotted basin waste?

A slotted waste has an opening in its side for water from the basin's built-in overflow. Use it on basins with an overflow. An unslotted waste has a solid body for basins without an overflow.

Should I use a bottle trap or a P-trap under a basin?

A bottle trap suits tight spaces and is easy to clean. A P-trap suits a straight run back into the wall where there is room. Both are common at 32mm for basins.

Why does my new basin waste leak underneath?

Usually a washer is missing, twisted or on the wrong side, or the backnut is over- or under-tight. On a basin without an overflow, a slotted waste can leak through its slot. Check which waste you have.