Quick Answer:

When your bathroom sink overflows while the washing machine drains, the most likely cause is a shared drain that’s partially blocked or has poor venting. Check the shared drain line, clear the branch (sink) line and clean or inspect the vent stack. These checks often stop backups and slow drainage.

Why This Happens

Most homes have the washer and a bathroom sink tied into the same waste system. When the washer pumps a high volume of water quickly into a shared line, any partial clog or poor venting can force water back up through the lowest open fixture — the bathroom sink. A clog in the sink’s branch or the main shared line reduces flow. A blocked vent stack can also slow drainage and cause gurgling or backups, increasing the chance of overflow.

Related problems you may have seen include Bathroom Sink Gurgles When Toilet Is Flushed and Bathroom Sink Fills Then Drains Slowly, which point to the same shared-drain or venting issues.

Step-by-Step What to Do

Step 1: Stop the washer and reduce the immediate risk

• Pause or cancel the washer cycle to stop more water entering the drain.
• Remove standing water from the sink with towels or a small bucket to prevent floor damage.
• Keep the washer off until you confirm the drain is clear.

Step 2: Check the shared drain

• Identify where the sink’s drain ties into the main waste line (often under the sink or in a nearby wall or floor cleanout).
• Open the sink drain and watch for slow flow, bubbles, or backflow when you run water. These are signs the shared line is partially blocked.
• If your home has a cleanout plug near the washer or main stack, you can open it carefully to see if water backs up there (have a bucket and rags ready).

Step 3: Clear the branch line (sink P‑trap and tailpiece)

• Place a bucket under the P‑trap, then loosen the slip nuts and remove the trap to clear accumulated gunk. Clean the trap and reassemble.
• If the trap is clear but the sink still backs up, use a hand drain snake or auger into the tailpiece and then the branch toward the shared line to dislodge a clog.
• After clearing, run water to confirm flow. If water still backs up when the washer drains, the blockage is farther down the shared line.

Step 4: Inspect the vent stack

• Roof vents can become clogged with debris, leaves, or bird nests. A blocked vent reduces air flow and slows drainage, making backups more likely when the washer pumps fast.
• If you’re comfortable and it’s safe to access the roof, visually inspect the vent opening. Do not climb on a wet or steep roof; safety is paramount.
• If you can’t safely check the vent, or if you see evidence of a blockage, call a plumber to clear the vent stack.

Step 5: Test and monitor

• Run a moderate flow of water from the sink while operating a small wash or rinse cycle and watch for backups.
• If the sink no longer overflows, continue testing a few cycles to be sure the problem is resolved.
• If the issue returns, the clog is likely in the main sewer or the venting needs professional attention.

What Not to Do

• Do not use chemical cleaners in shared drains. These products can damage pipes, harm septic systems, and may not clear large blockages.
• Don’t ignore repeated overflows — ongoing backups can damage cabinets, floors and create mold.
• Do not attempt roof vent work if you are uncomfortable with heights or if conditions are unsafe.
• Don’t force large drain snakes or power augers into pipes without knowing the pipe material — you can cause damage.

When to Call a Professional

• Call a plumber if the sink still overflows after you’ve cleared the P‑trap and branch line, or if water backs up at the main cleanout.
• Ask for professional help if you suspect the vent stack is clogged and you can’t safely inspect or clear it.
• If multiple fixtures back up (toilet, tubs, sinks) or you detect sewage odors, contact a plumber promptly — this often means a main sewer blockage.

Safety Notes

• Turn off or unplug the washer before working near it to avoid electrical hazards.
• Use gloves and eye protection when removing a P‑trap; debris can be messy and unpleasant.
• Never enter a sewer line or confined space. Leave confined-space work to trained professionals with the right equipment.
• Prioritize safe access: don’t climb on roofs or ladders in bad weather or without proper fall protection.

Common Homeowner Questions

• Why does the sink only overflow when the washer runs?
  Because the washer pumps a large volume of water quickly into a shared line and any partial blockage or venting problem forces water back through the sink.
• Will a plunger fix this?
  A plunger can help with minor branch clogs but often won’t clear blockages farther down the shared line.
• Is this a sign of a main sewer problem?
  It can be — if multiple fixtures back up or you have sewage odors, the main sewer should be inspected by a professional.