Quick Answer:
If your sink stops draining only when a second fixture runs, the two fixtures are sharing a branch of piping and that branch is partially restricted or poorly vented. A quick test is to run the second fixture and watch the sink level response to confirm the shared behavior. If the sink rises, bubbles, or backs up, the problem is in the shared branch or venting rather than the sink trap alone.
Why This Happens
Plumbing in a home is usually arranged with multiple fixtures joining the same drain line. When that shared branch is narrowed by buildup, debris, or a venting issue, the flow from one fixture can block or push water back into the other. The effect shows up when the two fixtures are used at the same time: one’s discharge interferes with the other’s drainage path.
Common causes:
- Partial clog in the shared branch (hair, grease, soap scum).
- Poor venting that allows negative pressure or air pockets to form.
- Obstructions or a sagged pipe that traps water.
Step-by-Step What to Do
1. Observe the behavior
Run the second fixture and watch sink level response. Turn on the nearby toilet, shower, or another sink and watch the affected sink for rising water, gurgling, or slow draining. Note exactly what happens and how quickly it responds when the second fixture is used.
2. Check the sink trap
Place a bucket under the P-trap, loosen the slip nuts, and inspect for visible clogs. Clear any debris and flush with hot water. If the trap is clear but the problem returns only when the second fixture runs, the issue is beyond the trap.
3. Test other nearby fixtures
- Run the sink by itself, then run the other fixture alone, then run them together to compare.
- Listen for gurgling or air sounds—those point to venting or partial blockages further down the line.
4. Try a mechanical snake on the shared branch
If the trap is clear, use a hand auger through the sink drain to reach further into the branch. Do this gently; avoid aggressive use that could damage older pipes.
5. Re-check and document
After any clearing, repeat the run-and-watch test. If the sink no longer responds when the second fixture runs, you likely removed the restriction. If the same response persists, the clog or vent issue is farther down the line and will need more thorough inspection.
What Not to Do
- Do not blame the main line without verifying shared-branch behavior.
- Do not pour large amounts of chemical drain cleaners if you suspect a shared-branch clog; they rarely fix branch clogs and can make future repairs hazardous.
- Do not force plumbing parts or tight connections with excessive tools that can crack fittings or loosen seals.
When to Call a Professional
- Repeated backups or rising water from multiple fixtures at once.
- Strong sewer odors, water backing into fixtures you didn’t use, or sewage-level backups—these suggest a mainline or sewer issue that needs a pro.
- If you can’t clear the clog with a hand auger or if camera inspection is required to locate the obstruction.
If a technician is needed, explain that the sink reacts only when another fixture runs; this speeds diagnosis toward shared-branch or vent problems. If you saw bubble patterns or specific responses, mention those details.
Safety Notes
- Wear gloves and eye protection when working on drains.
- Have a bucket and rags ready before opening the trap; water will spill out.
- Avoid mixing chemical cleaners and do not use them if you plan to inspect or snake the drain soon after.
Common Homeowner Questions
- What does bubbling or gurgling mean?
It often means poor venting or partial blockage in the shared branch. - Can I fix this with a plunger?
A plunger can help small blockages but usually won’t clear deep shared-branch clogs. - Will clearing the sink trap solve it?
Only if the clog is at or before the trap; if the sink only backs up when the other fixture runs, the restriction is likely farther down.
For patterns where the sink drains then behaves oddly after another fixture runs, reading about related issues can help—see Sink drains then air pushes water back and Sink drains then backs up for more detailed scenarios and fixes.