Water coming up in the other sink? What to do when the shared drain backs up

Short answer: Both bowls of a double kitchen sink drain into one pipe below the point where they join. When that shared pipe or the trap under it is clogged, water from the disposal side or the dishwasher has nowhere to go but up the other bowl. The clog is past the point where the two sides meet, so the fix is the trap and the line in the wall, not the disposal.

If you're reading this with dirty water standing in both bowls, skip down to the four steps. The why can wait.

Why the water comes up the other side

Under a double sink, the disposal drains sideways into a tee, and the drain from the other bowl drops into the same tee. From there everything goes through one P-trap and into the drain pipe in the wall. Your dishwasher joins the same line too, usually through a hose onto the side of the disposal.

So three things share one exit. When the trap or the pipe past it is blocked, water pushed down one side meets a dead end, and the easiest way out is up through the other drain. When the dishwasher pumps out, the same thing happens.

The clog is usually in the trap or the first stretch of pipe beyond it. Ground food is fine and heavy, so it settles in the trap. Grease goes down warm, cools in the pipe and sets, then catches whatever comes next. A line can get slower for months before the day it finally backs up.

Is it the shared line or the disposal?

Switch the disposal on for a second. If it hums but doesn't spin, it's jammed, and that's a different problem with a different fix: our guide to unclog a kitchen sink with a garbage disposal covers the hex key and the reset button. If it spins normally and water still rises in the other bowl, the disposal is doing its job and the clog is downstream. Read on.

Stop running water into it

Turn off the disposal and don't start the dishwasher. Every run pushes more water and more ground food into a line that can't take it.

Skip the chemical drain cleaner. It can damage the disposal, and if it doesn't work it sits in standing water that you, or a plumber, will have to put hands into when the trap comes off.

Before you plunge or open anything, unplug the disposal or switch off its breaker, so nobody can turn it on while you're working under the sink.

Four steps, in order

1. Plunge with the other drain sealed. Use a cup plunger, the flat kind made for sinks, on the bowl with the most water in it. Seal the other drain with its stopper, a wet rag or a second plunger, or the pressure just blows up through that side. If the dishwasher hose connects at the disposal, pinch it with a clamp so water doesn't push back toward the dishwasher. Plunge hard for 20 or 30 seconds, then see if it drains.
2. Pull the P-trap. Set a bucket under it. Loosen the two big slip nuts by hand or with slip-joint pliers, drop the trap, and empty it. Look inside it and up into the tee above it, clean it out, and put it back with the nuts snug but not cranked down. Run water and check for drips.
3. Auger the line in the wall. If the trap was clear, the clog is in the pipe going into the wall. With the trap off, feed a small hand auger into that pipe, turning as you push. When it stops, work it back and forth to break the clog up, pull the cable out, and put the trap back.
4. Test it the hard way. Fill both bowls partway and pull both stoppers at once. If they drain together without one rising, the line is clear. Then run the dishwasher's drain cycle and watch the sink.

It only backs up when the dishwasher runs

If the sink drains fine on its own but fills when the dishwasher empties, the line is partly clogged. A slow drain can keep up with a sink, but the dishwasher pumps a lot of water quickly, and the overflow comes up in the sink. Work through the trap and the line as above.

If there's a small chrome or plastic cap on the counter beside the faucet, that's an air gap, and it can clog too. Pull the cover off, clear out any food, and snap it back on. If the air gap spills water while the dishwasher drains, the blockage is between it and the disposal.

One more case: if the disposal was just put in and the dishwasher hasn't drained since, the knockout plug inside the disposal's dishwasher inlet may still be there. Taking the hose off and punching that plug out fixes it. If the dishwasher drained fine until now, this isn't your problem.

It backs up into both sinks

Both bowls filling together means the clog is past the tee, in the trap or the line beyond it. The plunger and the trap usually sort it out.

If other drains in the house are slow too, or a toilet gurgles when the kitchen sink drains, the problem is further down, possibly in the main sewer line. Plunging the kitchen sink won't touch that, and it's time to call. Our toilet guide lists the same main line signs from the bathroom side.

What builds up in a kitchen line

Grease and cooking oil go down as liquid and set in the pipe. Coffee grounds clump instead of washing away. Eggshells leave a grit that sticks to the grease. Potato peels turn to paste, and rice and pasta swell with water in the trap. None of it causes a backup in one day. It's the layer that builds up week after week until the line can't keep up with a full dishwasher.

Run cold water while the disposal runs and for 15 seconds or so after you switch it off, so the food clears the trap instead of sitting in it.

When to call a plumber

Call if you've pulled the trap and run an auger and it still won't clear, if it keeps coming back, if more than one drain in the house is slow, or if water is leaking under the sink. A clog that keeps returning in the same spot is usually something you can't reach from under the sink, and a sewer camera inspection shows what's in the line instead of guessing. We clear it with drain cleaning.

Genzel Plumbing is a family-owned, licensed plumber (Texas RMP-23766) in McKinney, serving Collin County homes since 2001. Call (972) 238-5585 and you will know the price before we start.

Shared drain questions we get asked

Why does water come up the other side and not the side I'm using?

Water pushed down one side looks for the easiest way out, and when the shared line is blocked, that's up through the other drain. Depending on where the clog is and how much water is moving, both sides can fill.

Can I run the dishwasher while the sink is clogged?

No. It drains into the same line, so its water will come up in the sink and can go over the edge onto the floor. Clear the drain first.

Why does the sink gurgle when the dishwasher drains?

Gurgling means air is being pulled through the drain because water can't flow freely. It's usually a partial clog in the shared line. If the line is clear and it still gurgles, the drain vent may be blocked.

Why do both sinks still drain slowly after I cleaned the trap?

Then the clog is past the trap, in the pipe inside the wall or further along. A hand auger reaches the first few feet. If that doesn't help, or other drains are slow too, call a plumber.

Can I use liquid drain cleaner on a double sink?

Don't. It can damage the disposal, and when it doesn't clear the clog it sits in the standing water, which makes plunging or opening the trap dangerous. A plunger and a hand auger are safer and usually work better.

Does the dishwasher need its own drain?

No. Sharing the kitchen drain is the normal setup, through the disposal or a fitting on the sink tailpiece. It only becomes a problem when that shared line is partly clogged.