Symptoms
- Water rises on the opposite sink basin while disposal runs
- Gurgling from the other drain
- Both sides drain slowly together
- Smell of food waste from the clean side
Likely causes
- CommonClog in the shared trap or drain teeGrease and food particles build up where the two sink lines join.
- PossibleImproperly sloped or looped drain line (a P-trap installed backwards or too high)Common on DIY installs and can cause chronic backup.
- PossibleVent stack blockagePoor venting can cause one side to gurgle or fill when the other drains.
- Less commonMain line clog further downstreamA bigger blockage can show up first as cross-sink backup.
What to do
- Run the disposal with plenty of cold water and watch both basins closely.
- Plunge the non-disposal side with the disposal side's drain plugged to force pressure through the shared line.
- Remove the trap under the sink where the two drain lines join and clear out debris.
- Run a drain snake through the trap arm toward the main line if clearing the trap alone doesn't help.
- Check that the drain line and trap were installed with proper slope, especially if this started right after a DIY plumbing change.
- If both sides are still affected after clearing the local trap, suspect a downstream or venting issue.
Tools and parts
- Plunger
- Bucket
- Adjustable pliers
- Drain snake
When to call a pro
If plunging and snaking the trap don't resolve it, a plumber should check the branch line, tee fitting, and venting.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.