Look inside your basin for the small overflow hole near the rim. If it is there, you need a slotted waste; if it is not, you need unslotted. That is the whole decision, and it is the commonest expensive mistake in this family.
Why it matters
An overflow is a channel moulded into the ceramic, running from that little hole down to the waste. A slotted waste has an opening in its side that lets the channel drain into it. An unslotted waste is sealed all the way round.
What happens if you get it wrong
Fit a slotted waste to a basin with no overflow and water finds the slot on its way past the seal, into the cupboard. Fit an unslotted waste to a basin with an overflow and the channel has nowhere to drain — it fills, stagnates, and smells.
Why it catches people
The two are visually near-identical in listings, they are often photographed from the top, and the word is buried in the specification rather than the title.
While you are there
Note the thread — 1”1/4 on a basin, 1”1/2 on a sink — and whether you want a click-clack or a rod-operated pop-up.
Questions people actually ask
Can I use a slotted waste on a basin without an overflow if I seal the slot?
It is not what the part is for and the seal is exactly where the water arrives. Unslotted wastes cost the same and are sold beside them.
What is the difference between click-clack and pop-up?
Click-clack opens and closes by pushing the plug itself, with nothing under the basin. A pop-up is driven by a rod through the tap and needs a tap that has one. Most modern basins use click-clack.
Last reviewed 5 September 2026