If your sink doesn't have an overflow, you will need to install an unslotted basin waste. An unslotted basin waste is watertight, so it won't allow any water which has gone into the overflow back into the waste pipe. Slotted wastes will allow overflow water to be drained, whether the waste is open or not.

Why would you use a slotted waste?

Whichever basin design you choose, there will invariably be a waste hole for water to drain through. An unslotted waste will be completely watertight while closed, while a slotted waste allows water from the overflow to be drained, whether the waste is open or closed. ...

Are pop up wastes any good?

The pop up waste is the most popular choice in basin wastes. It can come in a variety of finishes to match with your chosen tapware. The pop up mechanism allows for ease of draining and plugging down to seal the water in the basin when needed.