Overflow when dishwasher runs

Quick Answer:

If a floor drain bubbles while the dishwasher runs, it usually means a partial clog or restricted flow in the shared kitchen branch drain or a problem with the disposal/air gap. Start with simple checks you can do safely: run a short dishwasher cycle and watch the sink and floor drain, inspect the disposal and sink trap for food clogs, and open the nearest cleanout to flush the line. If you detect persistent sewage odor or repeated backups, stop and call a pro.

Why This Happens

Kitchen drains from the sink, dishwasher and disposal often share the same branch line. When that line is partially blocked, dishwasher discharge can push air and water back through the nearest low point — often the floor drain — which causes bubbling. Common causes include:

• Food, grease or small debris stuck in the sink trap or disposal.
• A blocked dishwasher air gap or clogged disposal inlet from the dishwasher hose.
• A partial clog further down the branch or main line that slows flow.
• Recent work that left construction debris in the drain (see overflow after remodel).
• Multiple appliances discharging together, which can overwhelm a partially restricted line (see overflow when multiple appliances run).

Step-by-Step What to Do

1. Run a short/delicate dishwasher cycle and watch

• Select a short or delicate cycle so the machine discharges a small amount of water. Watch the sink drain and the floor drain as the cycle runs.
• Note where bubbling or backup appears and whether it happens only during dishwasher discharge.

2. Check the garbage disposal and dishwasher air gap

• Look into the air gap (if present) on the sink top for debris and clear it with a brush or pick. A blocked air gap can cause slow drainage.
• Inspect the disposal inlet where the dishwasher hose connects for food buildup. Remove visible debris carefully.

3. Inspect the sink trap and disposal for clogs

• Put on gloves and remove the sink trap (P‑trap) if you can do so safely. Check for and remove trapped food or grease.
• Never put your hand into the disposal. Use tongs or pliers for visible debris and ensure the disposal power is off at the switch or breaker before working on it.

4. Run the disposal with a small amount of water to test flow

• With the disposal power on, run a steady stream of cold water and briefly turn on the disposal to help clear soft food buildup at the inlet.
• If the flow out of the sink is quick and the floor drain stops bubbling, the problem may have been a local blockage.

5. Open the nearest cleanout and flush the line

• Locate the nearest cleanout fitting (often a capped pipe with a square or round plug) and carefully open it to relieve pressure before flushing.
• Flush with several gallons of water from a hose or buckets to try to move debris downstream. If debris clears and bubbling stops, you’ve likely removed the blockage.
• If you’re uncomfortable opening a cleanout, or if water is under pressure and won’t drain, stop and call a professional.

What Not to Do

• Do not run multiple heavy dishwasher cycles back-to-back trying to force a clog through — this can push sewage or debris into other fixtures and make the problem worse.
• Do not pour grease, oil or food solids down the sink; these make clogs worse.
• Do not delay calling a professional when the sink or gutters back up or you detect a persistent sewage odor; those are signs of a more serious blockage or sewer issue.

When to Call a Professional

• Multiple fixtures back up at once (sinks, floor drains, toilets) — possible main sewer or branch clog.
• Persistent sewage smell or repeat backups after you’ve tried basic clearing steps.
• You can’t locate or safely open the cleanout, or water remains backed up under pressure.
• Visible leaks, damaged pipes, or you’re unsure about turning off power to the disposal.

Safety Notes

• Turn off power to the disposal at the breaker before inspecting or working on it.
• Wear gloves and eye protection when handling traps or opening cleanouts.
• Don’t stick your hand into the disposal or reach into moving parts.
• Avoid chemical drain cleaners for kitchen lines — they can damage pipes and don’t reliably clear solid obstructions.

Common Homeowner Questions

• Why does the floor drain only bubble when the dishwasher runs?
  A partial blockage in the shared branch allows air and water from the dishwasher to push back into the nearest low point.
• Will running the garbage disposal clear the problem?
  Sometimes — running the disposal with water can clear food caught at the inlet, but it won’t fix a clog further down the line.
• Can I use a chemical drain cleaner to fix this?
  No — chemical cleaners are not recommended for kitchen branch clogs and can harm pipes; try mechanical clearing and call a pro if it persists.

More in this topic

For more related fixes and similar symptoms, see Floor Drain Overflows During Appliance Use.