Quick Answer:
If one bowl of a double kitchen sink is blocked and water rises in the other, the shared drain and trap are redirecting the flow. Start by clearing the blockage in the blocked bowl and test drainage again. If that doesn’t fix it, follow the steps below to inspect the trap, tailpieces and venting before calling a plumber.
Why This Happens
Many double sinks share a common trap and waste arm. When one bowl is blocked, water has to go somewhere; it can back up into the other bowl if the shared drain is partially restricted or the venting is poor. Air pressure and partial clogs can force water to take the path of least resistance and appear to “spill” into the open bowl instead of flowing past the blockage.
This issue can be related to other household plumbing patterns, such as pressure changes after a municipal event—see Cross-flow after city outage—or surges during busy water use times that cause weird bowl-to-bowl movement, similar to Water jumps bowls during peak hours.
Step-by-Step What to Do
1. Stop adding water and clear the visible blockage
- Turn off the faucet and avoid using either bowl until you inspect the situation.
- Remove any visible debris from the blocked bowl’s strainer or stopper.
2. Remove the blockage from one bowl and test drainage again.
- With the visible obstruction removed, run a small stream of water in the formerly blocked bowl to see whether it drains normally and whether water still rises in the other bowl.
- If draining returns to normal, run both bowls briefly to confirm the shared trap is handling flow.
3. Check and clean the sink stoppers and strainers
- Remove stoppers, clean hair, grease and food buildup, then replace them and retest.
4. Inspect and clean the P-trap
- Place a bucket under the trap, loosen the slip nuts, and remove the trap to clear debris. Clean the trap and rinse before reinstalling.
- Retest drainage after reassembly.
5. Use a hand auger for deeper clogs
- If the trap was clear but the problem persists, use a small drain snake in each tailpiece and into the shared drain to break up or pull clear any deeper clog.
6. Test flow and watch for patterns
- Run water in each bowl separately and together. Note whether the issue is intermittent or continuous, and whether it changes with other household water use.
7. If you still see backup, check venting and main line
- Poor venting or a clog farther down the line can cause water to back up between bowls. If local tests don’t fix it, it’s time to escalate (see When to Call a Professional).
What Not to Do
- Do not plunge aggressively with the other bowl blocked. Forcing pressure can push a clog farther into the drain or damage seals.
- Avoid repeatedly pouring strong chemical drain cleaners; they can damage pipes and are unsafe if you later need to work on the trap.
- Don’t dismantle parts of the plumbing you’re not comfortable with—improper reassembly can cause leaks or sewer gas to enter the home.
When to Call a Professional
- Call a licensed plumber if the sink still backs up after you clear the trap and run a snake, or if multiple fixtures are slow/blocked—this can indicate a main line or venting issue.
- If you find signs of sewage odors, persistent gurgling, or sewage backing into multiple drains, call immediately.
- Contact a pro if you’re not comfortable disassembling the trap or using a snake safely.
Safety Notes
- Wear gloves and eye protection when working under the sink or handling debris.
- Have a bucket and towels ready to catch water when you open the trap.
- If you used any chemical cleaners, ventilate the area well and avoid mixing chemicals. If chemicals were used recently, consider calling a professional rather than handling the trap yourself.
Common Homeowner Questions
- Why does water rise in the other bowl? A shared trap or partial clog redirects water into the open bowl when one side is blocked.
- Will a drain cleaner fix this? Not always—chemical cleaners may not remove deeper obstructions and can cause damage; mechanical clearing is often better.
- How long before I need a plumber? If basic cleaning and a hand auger don’t fix it in one or two attempts, call a plumber to check the main line and venting.