A double bowl kitchen sink has two drains but only needs one trap. That surprises a lot of people, and it is also where most double bowl plumbing goes wrong: two traps stacked in series, a tee set at the wrong height, or a disposal outlet that sends food straight into the other bowl. Get the layout right and a double sink drains just as cleanly as a single one. Here is how the common setups work, what parts you need, and how to put it together.
One trap, two bowls
Each bowl has a strainer and a tailpiece. Those two tailpieces join into a single line, and that line feeds one 1 1/2 inch P-trap, which then runs to the wall drain. Most plumbing codes allow two sink compartments to share one trap as long as the outlets are close together and the trap sits within the allowed distance below each one. In practice that means the two drains should be no more than about 30 inches apart and the trap weir no more than about 24 inches below either outlet.
What you must not do is put a trap on each bowl and then join them. Two traps in series is called double trapping, and it traps a pocket of air that makes both bowls drain slowly and gurgle.
The three common layouts
| Layout | How it works | Best for | Typical parts cost |
|---|---|---|---|
| End outlet | Horizontal waste arm from the far bowl to a tee under the near bowl, then one trap | Sinks without a disposal, deep cabinets | About $15 to $35 |
| Center outlet | Tailpieces meet in a center tee, trap sits in the middle | Drain stub centered behind the sink | About $15 to $35 |
| Disposal with continuous waste | Disposal outlet joins a waste arm into the tee, then one trap | Kitchens with a disposal | About $20 to $45 |
| Two separate traps on one stub | Each bowl trapped, joined downstream | Not recommended | Usually fails inspection |
If you have a disposal, the trap layout matters even more. Our guide to the garbage disposal P-trap covers how to stop the disposal from pushing water into the second bowl.
Parts you will need
- A 1 1/2 inch continuous waste kit (end outlet or center outlet) with a baffle tee.
- A 1 1/2 inch P-trap with a J-bend and trap arm.
- Slip nuts and washers in the right size, usually included.
- A tailpiece extension if the bowls are deep.
- A hacksaw or tubing cutter and a deburring tool.
Plastic kits are the norm. Chrome brass looks better in an open cabinet but costs more, typically $40 to $90, and the thin wall versions corrode over time.
How to install a double bowl trap step by step
- Install both strainers with plumber's putty or the supplied gasket and tighten them from below.
- Measure the height of the wall drain stub. The trap outlet needs to sit level with or above it so the trap arm can fall about 1/4 inch per foot.
- Fit the tailpiece on the bowl that will carry the trap. Use a baffle tee here if the kit includes one, so water from one bowl is steered down rather than across into the other.
- Run the waste arm from the second bowl to the tee, cutting it so it falls slightly toward the tee.
- Hang the P-trap below the tee and swing the trap arm to the wall stub. Trim the tailpiece if the trap sits too low to reach.
- Make sure each beveled washer faces the right way, with the tapered side toward the fitting it seals into, then hand tighten every slip nut and add a quarter turn.
- Fill both bowls, pull both plugs at once, and check every joint with a dry paper towel.
If a joint drips, the fix is usually a washer turned backward or a nut cross threaded. Our guide on a P-trap leaking at the slip nut walks through it.
Troubleshooting common double sink problems
- Water rises in one bowl when the other drains: the tee is too low or has no baffle, or the trap is partly clogged.
- Both bowls slow: a clog in the trap or the wall line. See P-trap clogged.
- Gurgling: often a vent issue or a trap arm that runs too far. Our guide on a gurgling P-trap explains.
- Smell from one bowl: usually a second hidden trap or a dried seal.
Safety and code notes
Most codes allow one trap for a two or three compartment sink when the outlets are within about 30 inches and the trap is within about 24 inches of each outlet. Dishwasher drains need an air gap or a high loop depending on your local code, and they connect to the disposal or a branch tailpiece, never below the trap. Never double trap. Unplug a disposal before working under the sink, and wear gloves, since kitchen trap water is greasy and full of bacteria.
Frequently asked questions
Does a double kitchen sink need two P-traps?
No. One trap serves both bowls when the drains join above it. Two traps in series cause slow drains.
Which bowl should the trap go under?
Usually the bowl closest to the wall drain, or the center if the stub is centered. With a disposal, it is often under the non-disposal bowl.
Why does water come up in the other side of my sink?
The tee may be too low or lack a baffle, or there is a partial clog in the trap or wall line.
What size trap does a kitchen sink use?
Most kitchen sinks use a 1 1/2 inch trap and waste kit.
How much does a plumber charge to install a double sink drain?
Typically about $150 to $350 for the trap and waste kit, more if the wall stub needs moving.