Quick Answer:
When you run the sink and see bubbles in the toilet, air is being pulled through the toilet trap. This is usually a sign of a partial clog or a venting problem in the shared drain system. It’s worth a few basic checks you can do yourself; if the bubbling is repeatable, call a plumber.
Why This Happens
Plumbing fixtures in the same bathroom often share a drain line and a vent. Traps under each fixture hold water and block sewer gases. If the drain line is partially blocked or the vent is restricted, draining water from one fixture can pull air through another fixture’s trap. That airflow shows up as bubbling in the toilet bowl when the sink is emptied.
Pressure changes from city main work or a recent repair can also cause bubbling; see Toilet gurgles after city outage for related situations.
Step-by-Step What to Do
1. Observe the problem
- Drain the sink while observing the toilet; bubbling during sink drain points to air being pulled through the trap. Note whether it happens every time or only occasionally.
2. Test other fixtures
- Run other nearby fixtures (shower, bathtub) and watch the toilet. If multiple fixtures cause bubbling, the issue is likely farther down the line or with the vent.
3. Check for slow drains and backing
- If the sink drains slowly or you see water backing up, try clearing the sink trap and the P-trap under the sink to remove hair and debris.
4. Clear the sink trap and run a test
- Remove and clean the sink trap (put a bucket under it first). Reassemble and drain the sink again while watching the toilet.
5. Look for vent problems
- If clearing traps doesn’t help, there may be a blocked roof vent. Birds, leaves, or ice can restrict airflow. A roof check is best left to someone comfortable working at height.
6. Consider what changed recently
- If you recently had plumbing work, or if the city had outages or maintenance, the change could explain new bubbling. For recent repairs, check advice like Gurgling after plumbing repair.
What Not to Do
- Don’t ignore bubbling during sink drains—this indicates airflow issues and can lead to sewer odors or worse problems if it keeps happening.
- Don’t pour large amounts of caustic drain chemicals down the sink to fix a suspected venting problem; they can damage pipes and harm you.
- Don’t attempt roof vent work if you’re not safe on ladders or unsure about fall protection—call a pro instead.
- If the bubbling is repeatable or you smell sewage, don’t wait: a plumber is appropriate.
When to Call a Professional
- Repeatable bubbling after you’ve cleared traps and tested other fixtures.
- Multiple fixtures gurgling or slow drains throughout the house.
- Foul sewer odors, sewage backup, or water coming up in unexpected places.
- If a roof vent is blocked and you’re not able to safely clear it.
Safety Notes
- Use gloves and eye protection when cleaning traps. Place a bucket under the trap to catch water.
- Be cautious with chemicals; many drain cleaners are hazardous and not recommended for vent or main-line issues.
- Don’t climb on the roof without proper safety equipment. A professional can inspect and clear roof vents safely.
Common Homeowner Questions
- Q: Why does this happen only when I run the sink?
A: The sink’s draining flow can create a small vacuum if the vent or drain is restricted, pulling air through the toilet trap and making bubbles. - Q: Can I fix it myself?
A: You can clear the sink trap and test other fixtures; if bubbling continues, you’ll likely need a professional to inspect vents or the main drain. - Q: Is it dangerous?
A: It’s not usually an immediate hazard, but it can let sewer gases into the home and indicate a clog that could get worse—so don’t ignore it.