Key takeaways
- A double sink is two basins sharing one trap and one run to the wall.
- The other basin is your diagnostic: it says whether the blockage is local, shared or downstream.
- Block the good side before you plunge, or you push water into the basin that was fine.
- Clean the trap by hand before you reach for a snake, and treat chemicals as a last resort.
- Several slow fixtures in the house means the sink was never the problem.
When one basin of a double kitchen sink fills up while the other drains normally, the first instinct is to plunge the side that is backing up. That is usually the wrong side. A double sink is two basins sharing one drain line, and the behaviour of the *other* basin is the best diagnostic tool you have - it tells you whether the blockage is in this side's branch, in the shared trap underneath, or further down the line where both basins meet the main drain.
Both basins share one pipe
Under the cabinet, each basin has its own tailpipe, and the two join before the water reaches the trap. Some sinks use a continuous waste tee, some use a baffled tee, and either way the two sides share everything from that junction onward: one trap, one run to the wall, one connection to the main drain. That is why a blockage anywhere downstream of the junction affects both basins, why a blockage in one branch affects only that side, and why water from the left basin can appear in the right one when the shared section is full.
Three places it can be blocked
The first is the branch: the tailpipe and the short horizontal run from the blocked basin to the tee. This is the most common spot, because it collects food scraps and grease from the basin above it. The second is the shared trap, the U-bend where water sits and where heavy debris settles. The third is downstream: the horizontal run to the wall, or the main drain beyond it, which is where roots, scale and decades of grease live. The first is a twenty minute job, the second is a bucket and a pair of gloves, and the third is the one that may not be yours to fix.
| What you see | Where the blockage is | First thing to do |
|---|---|---|
| One side backs up, the other runs freely | The branch under the blocked basin | Block the good side and plunge |
| Both sides slow, or one gurgles | The shared trap | Bucket under the trap, clean it by hand |
| Water rises in the other basin when the disposal runs | The shared section is partly blocked | Stop the disposal, clean the trap and branch |
| The sink clears, then backs up within a day | Downstream, toward the wall | Snake the wall stub slowly |
| Several fixtures slow, or a sewage smell | The main line or the vent | Stop and call a plumber |
Reading the symptoms
Run water in the side that drains well and watch the other one. If the good side runs freely and the blocked side stays put, the blockage is in that side's branch. If both sides drain slowly, or the good side makes the blocked side gurgle, the shared trap or the run downstream is involved. If water comes up in the other basin when you run the disposal, the shared section is partly blocked and the disposal is pushing water into it. And if every drain in the house is slow, or you can smell sewage in another room, stop working on the sink - the problem is in the building's main line or the vent stack, and that is a call to a plumber rather than a job for a plunger.
Clearing it, in order
Work from cheapest and least invasive outward. Block the drain on the good side with a wet cloth or a plug, fill the blocked side with two inches of water, and plunge with a cup plunger - sealing the other side is what makes the suction travel down the branch instead of pushing water up into the other basin. If that does not clear it, put a bucket under the trap, unscrew the trap and the branch arm, and clean them by hand, which is messy but almost always successful because the debris is sitting right there. If the trap is clean and the sink still backs up, the blockage is downstream and needs a snake through the wall stub, fed slowly with the cable turning. Chemical cleaner comes last, not first, and never in a sink with a disposal.
When it is not your sink
Three signs that the blockage is beyond your cabinet: more than one fixture in the house is slow, water appears in a floor drain or a shower tray when you run the kitchen tap, or the sink drains fine and then backs up again within a day. Those point at the main line, a blocked vent, or a full septic tank. A vent problem often announces itself with gurgling and a smell rather than with standing water, and it is worth knowing that no amount of plunging will fix a blocked vent.
How to stop it coming back
Most double-sink blockages are food and grease that were never meant to go down. A basket strainer in each basin catches the solids before they reach the tee, and it costs less than one visit from a drain company. Beyond that: pour grease into a container rather than down the drain, run hot water for thirty seconds after washing up, and once a week fill the basin and let it drain, which flushes the branch under pressure. That routine, rather than a bottle of chemicals, is what keeps both basins draining the way they did when the sink was new.
The cheapest prevention is the part above the drain: a strainer in each basin. If the kitchen drain has a disposal in it, the cleaning order is in cleaning a kitchen sink drain with a disposal, and if you are choosing the fitting itself, start with basket strainer sizes.
The bottom line
Read the other basin before you touch anything: it tells you whether the blockage is local, shared, or downstream. Seal the good side before you plunge, clean the trap by hand before you reach for a snake, and treat chemicals as the last resort. If several fixtures are slow, the sink was never the problem.
Frequently asked questions
Why does my double sink back up into the other side?
Because both basins share one trap and one drain line. When the blockage is in the shared section, water from the side you are using has nowhere to go but up into the other basin.
How do I find where a double sink is clogged?
Watch the other basin: if the good side drains freely, the blockage is in the branch under the blocked basin; if both sides are slow or the good side makes the other gurgle, it is the shared trap or the run downstream.
Should I plunge the side that is backing up?
Yes, but block the other side first with a wet cloth or plug. That sends the suction down the branch instead of pushing water up into the basin that was draining fine.
When should I call a plumber for a backed-up sink?
When several fixtures in the house are slow, when water appears in a floor drain or shower tray, or when the sink clears and backs up again within a day. Those point at the main line, the vent or a full septic tank.
Related reading
- How to clean a kitchen sink drain with a garbage disposal
- Does baking soda and vinegar really unclog a sink?
- Kitchen sink basket strainer: how to measure, choose and replace it
Premium Pop-Up Sink Drain Stopper with Hair Catcher
$9.99
Shop the pop-up sink drain stopper with hair catcher (from $9.99)