How a double sink drains

Under most double kitchen sinks, each basin has its own tailpiece. The two join at a tee, often through a horizontal piece called continuous waste, and then share a single P-trap. If one side has a disposal, the disposal outlet feeds into that same connection.

Because both basins share one trap and one trap arm, anything that narrows the pipe past the tee affects both sides. Water from the basin you are draining has nowhere to go, so it takes the easy path and rises up the other side. That is not a second clog; it is one blockage past the point where the basins meet.

Reading the symptom

Match what you see to where the problem most likely is.

  • IfOnly one basin drains slowly and the other is fineThenCheck that basin's strainer and tailpiece; the blockage is above the tee.
  • IfDraining one basin fills the otherThenThe clog is at or past the tee, in the trap or the trap arm.
  • IfThe disposal side fills when the disposal runsThenGround food may be packed into the trap or the line just beyond it.
  • IfBoth basins are slow and a nearby tub gurglesThenSuspect the main line and stop using water.

Safe first steps

  1. Stop adding water

    Turn off the faucet and pause the dishwasher. Each gallon you add raises the level in both basins.

  2. Power off the disposal

    Switch it off before reaching near the sink. Do not run it to try to push water through.

  3. Seal the other basin

    If you try a plunger, plug the second drain firmly with a stopper or a wet cloth held down, or the pressure escapes up that side.

  4. Check the trap

    With a bucket below, the trap can be removed and checked for packed food or grease. Stop if fittings are corroded or seized.

  5. Call if it stays

    If the trap is clear and water still rises, the clog is in the trap arm or branch line and needs a cable.

Parts under a double sink

Tailpiece
The short vertical pipe below each basin's strainer.
Continuous waste
The fitting that joins both basins into one outlet.
P-trap
The U-shaped bend that holds water to block sewer gas.
Trap arm
The pipe from the trap into the wall or floor.
Branch line
The drain carrying the sink to the main drain of the house.

Questions for the plumber

  1. Is the continuous waste fitting sloped correctly, or is it holding water?
  2. Was the disposal installed in a way that sends food into the other basin's path?
  3. Is the trap arm long enough or flat enough to collect grease?

Common questions

Why does water come up the other side when I run the disposal?

The disposal pumps ground food and water into the shared trap. If the trap or the line past it is narrowed, the flow backs up into the open basin.

Can a badly installed double sink cause repeat clogs?

It can. A continuous waste fitting with too little slope, or a long flat trap arm, can hold water and food. A plumber can tell you whether the layout is contributing.

Is it safe to leave standing water overnight?

Usually, if it is only kitchen water and nothing is leaking. Do not keep adding water, and keep the cabinet below dry. If sewage appears anywhere in the house, treat it as a main line problem.

General information for Fullerton, not a quote or professional advice. In an emergency, call 911.