Quick Answer:
If the sink begins to fill when the shower runs, a shared drain or a partial clog downstream is the most likely cause. Run the shower and observe sink behavior: note how fast the sink rises, whether water backs into other fixtures, and whether you hear gurgling. These observations guide the next steps and help decide if you can fix it yourself or need a plumber.
Why This Happens
Most homes have multiple fixtures tied to the same drain line. When the shower discharges water faster than the shared pipe can carry it away, the slower path — often a sink — fills up. Common causes:
- Partial clog in the branch or main drain (hair, soap scum, grease).
- Blocked or slow venting causing slow drainage and gurgling.
- A clog further down the line or a problem in the sewer main affecting several fixtures.
- Issues that affect how fixtures share flow — for example, similar symptoms appear when a washing machine or a neighbor’s plumbing sends a surge of water, like Sink backs up when washer drains or Sink backs up when neighbor uses water.
Step-by-Step What to Do
1. Run a simple test
Turn on the shower to a normal flow and watch the sink. Note these details:
- Does the sink begin to rise immediately or after a delay?
- How much does it fill — a little slow rise, to the overflow, or just slight pooling?
- Do other fixtures (toilet, tub, basement drain) show problems at the same time?
- Any gurgling or bubbling noises?
2. Check nearby fixtures
Turn on and off other nearby fixtures to see if the behavior changes. If multiple fixtures back up, the issue is likely farther down the drain line or in the main sewer.
3. Clear the sink trap
Place a bucket under the P-trap, remove the trap nut, and clean out hair and debris. Reassemble and retest the shower. Many partial clogs are in the trap and clearing it often improves flow.
4. Use a plunger
With the sink stopper removed (or overflow covered), use a sink plunger while the shower runs to try to dislodge a clog. Plunge the sink first, then the shower drain if accessible.
5. Try a manual snake
If plunging doesn’t help, run a hand auger or plumber’s snake into the sink drain and, if accessible, the shower drain. Feed the snake until you feel resistance, then rotate and pull back to remove the clog. Retest after cleaning.
6. Avoid harsh chemicals and flush with hot water
After mechanical clearing, flush the drain with several minutes of hot water to wash away soap scum. Avoid pouring strong chemical drain cleaners — they rarely fix shared-line problems and can damage pipes.
7. Log and retest over time
If the sink only fills during high-flow events, keep notes about when it happens (time of day, which fixtures were used). This helps a professional pinpoint shared-line issues if you need to call one.
What Not to Do
- Do not ignore slow drainage between events. A slow sink is a warning the clog is worsening and can lead to larger backups.
- Do not pour strong acids or drain-opening chemicals into the drain; they can harm pipes and create hazardous fumes.
- Do not remove sewer cleanout covers or attempt deep mainline work unless you have proper training and equipment.
When to Call a Professional
Contact a licensed plumber if any of the following apply:
- Multiple fixtures back up at once or you smell sewage.
- Clearing the trap and snaking accessible lines don’t help.
- Frequent or worsening backups, or water reaches living spaces.
- You suspect a main sewer or venting problem beyond the easily accessible pipes.
Safety Notes
- Wear gloves and eye protection when working on drains.
- If you use a snake, avoid forcing it; excessive force can damage pipes.
- Avoid mixing any chemical cleaners. If there’s been chemical use, ventilate the area and consider a professional.
- If you see sewage, stop and call a pro — raw sewage carries health risks.
Common Homeowner Questions
- Why does the sink only fill when the shower runs? Because the shower sends a pulse of water into a shared drain that a partial clog can’t handle, so the sink is the low-resistance overflow point.
- Will pouring hot water fix it? Hot water can help clear soap and grease clogs in the short term, but it won’t fix a significant or deep blockage.
- Can I fix this myself? You can try clearing the trap, plunging, and using a hand auger. If those fail or multiple fixtures are affected, hire a plumber.
For more related articles, see the Sink Backs Up Only When Other Fixtures Run hub.