Washing machines and dishwashers empty a lot of water quickly. If the waste pipe they share with your kitchen sink is partly blocked, that rush of water has nowhere to go and comes back up into the sink.
Common causes
- Grease and food build-up in the sink waste pipe, narrowing it over time.
- Lint and fibres from the washing machine combining with soap and grease.
- A blocked outside gully, where the kitchen waste discharges. See how to clean an outside drain gully.
- A partly blocked underground drain further along.
- The way the machine is plumbed, for example a standpipe that is too short or a connection that lets water flow back.
What to check
- Does the sink drain normally on its own? If it is slow, the sink waste or gully is partly blocked.
- Look at the gully outside. Is it full of grease or water when the machine empties?
- Check the sink trap under the sink. Unscrew it with a bowl underneath and clean it out.
- Check the washing machine filter (usually behind a small panel at the bottom front). A blocked filter does not cause the sink to back up, but cleaning it reduces lint going into the pipe.
- Check the inspection chamber if the gully is clear. A full chamber points to a blockage in the underground drain.
Simple fixes
- Clean the sink trap and the outside gully.
- Run hot (not boiling) water through the sink after cleaning.
- Avoid chemical products. See do chemical drain unblockers work?
When it keeps happening
If the sink backs up every time the machine empties, even after cleaning the trap and gully, grease has probably built up along the waste pipe or drain. Professional drain cleaning or drain jetting removes it from the pipe walls.
See also why does my sink keep blocking? and blocked sinks.