Symptoms
- Running water or the disposal on one side makes water rise in the other basin
- Both basins seem fine individually when tested alone
- Gurgling from the unused basin while the other drains
- Problem is worse when both basins are used together
Likely causes
- CommonClog in the shared drain line past where the two basins connectA blockage downstream of the tee fitting forces water back into the path of least resistance.
- CommonClog right at the connecting tee fittingGrease and food debris often collect right where the two lines join.
- PossibleClogged garbage disposal drain lineIf one side has a disposal, its outlet can be a common clog point affecting both sides.
- Less commonVent blockage causing improper flow balanceLess common but can contribute to this symptom.
What to do
- Turn off the garbage disposal switch if either side has one and avoid running it during troubleshooting.
- Plunge one basin at a time with the other basin's drain covered by a wet rag for suction.
- If plunging doesn't help, place a bucket under the sink and disconnect the trap and connecting tee to check for a clog right at the joint.
- Clear any debris found and reassemble the trap and tee connections snugly.
- If the tee area is clear, feed a hand-crank drain snake into the trap arm heading toward the wall to clear a deeper clog.
- Run water in both basins together after clearing to confirm the backup no longer happens.
- If a garbage disposal is involved, check that its discharge tube isn't clogged with food debris.
Tools and parts
- Plunger
- Adjustable pliers
- Bucket
- Hand-crank drain snake
When to call a pro
Call a plumber if clearing the trap and tee connection doesn't stop the cross-basin backup.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.