How do I replace a basin waste?

A basin waste is a threaded tube through the basin, clamped by a backnut underneath, with washers making the seal. Nothing about it is complicated; the two things that go wrong are buying the wrong type and overtightening.

What you'll need

• A new waste, slotted or unslotted to match
• Sanitary silicone
• Grips or a basin wrench
• A bowl and cloths

Step by step

1. Check whether the basin has an overflow.Look for a slot or hole near the rim inside the basin. If there is one, you need a slotted waste — the slot lines up with the overflow channel in the ceramic.
2. Turn off the water and drain the trap.Close the isolating valves under the basin, open the tap to prove they are shut, and put a bowl under the trap.
3. Disconnect the trap.Unscrew the trap's top nut by hand. Keep the washer — you may reuse it, and it stops the nut going down the drain.
4. Free the backnut.It is often solidly limescaled. Penetrating spray and time helps; a basin wrench or long-handled grips gives the leverage. Support the waste body from above with a screwdriver in the slots so it does not spin.
5. Clean the ceramic seat thoroughly.Scrape off old putty, silicone and scale from the basin surface. New seals only work against a clean, flat face.
6. Fit the new waste with the slot aligned.On a slotted waste, the slot must face into the overflow channel — usually towards the back of the basin. Getting this wrong means the overflow does not work.
7. Seal the top flange.Either the supplied flat washer or a ring of sanitary silicone under the flange. Do not use plumber's putty on a modern acrylic or composite basin — it can stain and degrade it.
8. Build the underside washer stack correctly.Usually a thick rubber or fibre washer against the ceramic, then a plastic spacer if supplied, then the backnut. The rubber goes against the basin, not against the nut.
9. Tighten by hand tools, gently.Firm is enough. Ceramic basins crack around the waste hole, and a cracked basin is not repairable.
10. Trim the squeeze-out and refit the trap.Cut the excess silicone from around the flange inside the basin with a plastic scraper, refit the trap with a new washer, and test with a full basin.

Tips

• Take the old waste to the merchant. Slotted and unslotted look almost identical from below.
• Support the waste from inside the basin with a wide screwdriver while you turn the backnut — otherwise it spins and shreds the new seal.
• Test by filling the basin right up to the overflow, not just by running the tap.

Common mistakes

• Fitting an unslotted waste to a basin with an overflow — The overflow channel has nowhere to discharge, so an overfilled basin simply spills onto the floor, and water can sit in the channel and smell.
• Overtightening the backnut — The seal is made by a compressed washer, not by force. Ceramic cracks radially around the waste hole and the whole basin is then scrap.

If it doesn't work

Questions people ask