Quick Answer:
If your kitchen sink floods while the washer pumps out, the most likely causes are a partial clog in a shared drain line or a venting problem that prevents air from moving through the waste line. Start by stopping water use, running targeted diagnostics on the sink and laundry drain, and checking connected fixtures and simple trap or hose issues. If the problem repeats after basic clearing, call a plumber.
Why This Happens
Many homes share drain lines for the kitchen and laundry. When the washing machine discharges a large volume quickly, that flow can meet resistance in a clogged or poorly vented section of the waste pipe. Pressure forces water to take the easiest exit — often up through the sink. You should also consider nearby appliance connections (dishwasher, garbage disposal) and fixture parts like the sink basket, P-trap, and the washer’s standpipe.
If you notice other symptoms such as a slimy residue around the drain or unusual gurgling sounds, those point to related problems: Kitchen Sink Has Slippery Black Film Around Drain or Kitchen Sink Gurgles When Dishwasher Drains.
Step-by-Step What to Do
1. Stop and contain the immediate overflow
- Turn off the washing machine mid-cycle if it’s safe to do so and it has a pause or cancel feature.
- Catch and remove standing water with a bucket and towels to limit water damage.
- Shut off the water to fixtures if leaking continues or if you have repeated overflows.
2. Reproduce the problem safely to diagnose
- Run the washer on a drain/spin-only setting while someone watches the sink drain to confirm timing and behavior.
- Note whether the sink backs up only when the washer pumps or also during normal sink use.
3. Check visible, connected appliances and fittings
- Inspect the washer drain hose to ensure it’s inserted correctly into the standpipe and not kinked or pushed too far down.
- Look under the sink at the tailpiece, P-trap, and any tee or branch where a dishwasher or disposal connects for obvious clogs or leaks.
- Make sure dishwasher drain hoses and air gaps are secure and not routed downhill where debris can collect.
4. Clear the sink trap and strainers
- Put a bucket under the P-trap, loosen the slip nuts by hand (use a wrench only if needed), and clear trapped debris.
- Clean the sink strainer and run a small amount of water to see if flow improves.
5. Use a plunger and a short hand snake for the sink
- A cup plunger can dislodge partial clogs in the sink drain. Seal the other sink bowl (if double) before plunging.
- Use a hand auger (plumber’s snake) designed for sinks to reach blockages in the tailpiece and trap arm.
6. Test and clear the laundry standpipe or branch
- If the sink clears but backup still happens when the washer drains, the clog may be in the laundry standpipe or the common drain farther down.
- Run water from a hose into the standpipe or use a longer auger from the laundry side to push or pull the obstruction free.
7. Check venting behavior
- Observe if sinks or toilets gurgle when the washer drains; gurgling suggests an obstructed vent or failing air admittance valve.
- Do not attempt roof vent work unless you are experienced and it is safe — call a pro to clear roof vents.
8. Re-test and monitor
- After clearing, run the washer through a drain cycle while watching the sink. If the overflow stops, monitor for recurrence over several wash cycles.
- Keep a note of when it happens (only with large loads, always, after heavy rain) — details help a plumber diagnose recurring issues.
What Not to Do
- Do not force fittings apart with excessive torque — you can break plastic pipes and fittings.
- Do not use caustic drain chemicals — they can damage pipes, harm you, and make later repairs harder.
- Do not ignore early warning signs such as slow drainage, gurgling, or odors — small problems usually get worse if left alone.
- Do not reach blindly into a garbage disposal or turn it on while hands or tools are inside; always disconnect power first.
- Do not climb on the roof to clear vents unless you are trained and equipped for safe ladder work.
When to Call a Professional
- Multiple fixtures back up at once or you smell sewage — call a plumber promptly.
- You cannot clear the clog with a plunger or a short auger, or the clog is deep in the main or branch drain.
- Signs of damaged or broken vent stacks, recurring backups after repairs, or if roof access is required.
- If your washer or disposal appears to be failing mechanically and contributing to the problem.
Safety Notes
- Wear gloves and eye protection when handling dirty water or removing trap debris. Sewage can carry bacteria.
- Turn off electrical power to disposals or nearby outlets before working under the sink.
- Use a bucket and towels to manage spills and avoid slipping on wet floors.
- Avoid chemical drain cleaners. If you suspect hazardous materials or a sewer line break, keep everyone away and call a professional.
Common Homeowner Questions
- Q: Can I use a shop vac to remove the clog? A: A shop vac can help with loose water and small debris but is not a safe substitute for proper drain clearing on deeper clogs.
- Q: Will a washing machine filter prevent this? A: A washer’s lint filter helps reduce lint but won’t stop clogs in shared waste lines; proper drain maintenance is still needed.
- Q: Is this covered by home insurance? A: Sudden accidental damage may be covered, but gradual maintenance issues and sewer backups often require specific endorsements—check your policy.