Quick Answer:
If your sink backs up every time the toilet is flushed, it usually means a shared drain or vent is partially blocked or overwhelmed. Quick checks can tell you whether this is a simple trap issue you can fix yourself or a sign of a deeper mainline or vent problem.
Why This Happens
Fixtures in the same bathroom often share a drain line or connect to the same stack. When the toilet discharges a large volume of water, that flow can push against a partial blockage further down the line or pull air through a blocked vent. The result: water in the sink will rise or even flow backward while the toilet empties.
Repeated backups, the presence of gurgling sounds in other drains, or related symptoms like a smelly drain point to a mainline or vent issue rather than a single fixture. If you also notice issues like Sink fills when shower runs or Sink overflows only during peak hours, that strengthens the case for a shared line problem.
Step-by-Step What to Do
1. Stop using the fixtures
- Avoid running water, flushing other toilets, or using appliances that drain until you find the cause. This reduces the chance of an overflow.
2. Clear the sink trap
- Place a bucket under the P-trap, loosen the slip nuts, and remove the trap to clean out hair and debris. Reassemble and test the sink.
- Tools: adjustable wrench, bucket, gloves, and rags.
3. Check the sink drain and overflow
- Remove the pop-up stopper or strainer and clear any visible blockages. Clean the overflow slot—clogs there can affect drainage behavior.
4. Observe while flushing
- Flush the toilet while watching the sink drain. Note whether the sink water rises, gurgles, or flows backward. This observation helps identify a shared drain or vent problem.
5. Try a plunger
- Use a sink plunger on the sink (seal the overflow with a wet rag) or a toilet plunger on the toilet to try to dislodge a partial clog. Work gently and check results after several attempts.
6. Use a drain snake for the sink or tailpiece
- A handheld auger can reach clogs in the sink drain and trap arm. If you can reach and remove the blockage, reassemble and test again by flushing the toilet.
7. Inspect other fixtures
- Test the tub, shower, and other sinks. Multiple slow drains or backups usually mean a mainline, vent, or sewer issue rather than a single clogged fixture.
8. Check the cleanout
- If you have a basement or exterior mainline cleanout, open it (cautiously) and see if water backs up there when the toilet is flushed. If it does, the blockage is in the main sewer line.
What Not to Do
- Do not assume the toilet itself is clogged.
- Do not pour large amounts of chemical drain cleaner into clogged drains—these can damage pipes and create hazardous conditions if mixed with other substances.
- Do not keep running water or flushing if you see sewage backing up or a toilet that won’t stop overflowing.
When to Call a Professional
- Multiple fixtures are slow or backing up at once.
- Water or sewage is coming up into a basement or lower-level fixtures.
- Plunging and cleaning the trap don’t fix the problem, or you can’t access the cleanout safely.
- There is a strong sewer smell, or you suspect a collapsed or tree-root intrusion in the main line.
Safety Notes
- Wear gloves and eye protection when handling drain water or removing traps—sewage can carry pathogens.
- If you open a cleanout and expect sewage, have towels, a bucket, and a disinfectant ready and avoid skin contact.
- Avoid mixing chemical cleaners; if you’ve used one, wait and flush thoroughly before trying another method.
Common Homeowner Questions
- Why does this only happen when I flush? A large flush sends a sudden surge of water; a partial blockage or poor venting makes that surge push water back into nearby fixtures.
- Can I fix this myself? Often yes for sink-trap clogs or surface blockages, but mainline or vent problems usually need a plumber.
- Is this an emergency? Not always—if there’s no sewage overflow, you can do basic checks; call a pro if backups continue or sewage appears.
For more related articles, see the Sink Backs Up Only When Other Fixtures Run hub.