Quick Answer:
Most often a sink that backs up when the washing machine drains means the washer and sink share a drain or the shared drain line has a partial clog. The washer pump moves a lot of water quickly, and if the line beyond the sink can’t pass that volume fast enough, water is pushed back into the sink.
Why This Happens
Two plumbing fixtures commonly share the same branch drain. When the washer empties, it dumps a high flow over a short time. If the pipe has grease, lint, soap scum, or a partial blockage, that surge has nowhere to go and backs up into the nearest open fixture—the sink. You might see similar behavior in related problems like Sink overflows when dishwasher runs, where one fixture’s discharge shows up in another.
Step-by-Step What to Do
1. Stop the washer and remove immediate water
- Pause the washer or turn it off. Remove standing water from the sink with a bucket or cup so you can watch what happens when you run tests.
2. Listen as the washer drains and note sink response
- Run a short drain cycle or start a spin-only cycle with a small load. Listen as the washer drains and note sink response. Watch whether water rises slowly, surges, or gurgles.
3. Check visible traps and vents
- Look under the sink at the P-trap for obvious clogs or buildup and remove debris if accessible. If the washer connects to a standpipe near the sink, inspect the standpipe area for lint or blockages.
- If multiple fixtures gurgle, a venting issue may be present (vent clogged or blocked). That usually needs a professional.
4. Try basic clearing steps
- Use a plunger on the sink (cover overflow if present) and give a few firm plunges. Don’t overdo it; you only need a few tries to see if the blockage is near the trap.
- A hand drain snake (3/8″ to 1/2″) can clear the P-trap or the first few feet of the line. If you’re comfortable, remove the trap and clean it out.
- Use an enzyme-based drain cleaner (safe for pipes) if you suspect organic buildup. Avoid repeated strong chemical treatments.
5. Test carefully
- After clearing, run a small washer drain/test cycle and watch the sink. Do this one small step at a time so you know what fixed or did not fix the issue.
What Not to Do
- Do not overload drains during testing.
- Do not pour large amounts of harsh chemical drain cleaner down the drain repeatedly—this can damage pipes and fixtures.
- Avoid dismantling the washer plumbing beyond its hose connections unless you know how; improper reassembly can cause leaks or drainage problems.
When to Call a Professional
Call a plumber if:
- The sink still backs up after you’ve cleaned the trap and tried basic clearing.
- Multiple fixtures back up or you detect a sewage smell—those signs often mean a deeper or main-line clog.
- You notice other patterns, such as the sink filling when other bathrooms are used; for example, Sink fills when upstairs bathroom used suggests a shared stack or main-line problem that needs professional equipment.
Safety Notes
- Turn off the washer and unplug it before working on nearby plumbing to avoid contact with moving parts or electricity.
- Wear gloves and eye protection when reaching into traps or using tools. Lint and wastewater can be unsanitary.
- If you smell sewage or suspect a main sewer blockage, stop using water and call a plumber right away—continued use can cause raw sewage backups into the home.
Common Homeowner Questions
- Why does this happen only when the washer runs? — The washer sends a fast, large volume of water that exposes partial clogs or weak venting in the shared drain.
- Can I keep using the washer until it’s fixed? — It’s best to avoid running the washer until you test and reduce the backup risk; repeated backups can damage cabinets and flooring.
- Will a snake or auger fix it? — A hand snake can often clear traps and near-line clogs, but deeper main-line blockages usually need a plumber’s power auger or camera inspection.
For more related articles, see the Sink Backs Up Only When Other Fixtures Run hub.