How to Plumb a Double Sink the Right Way

A double-bowl kitchen sink looks simple under the cabinet until you see the maze of pipes that connect two drains, a garbage disposal, and a dishwasher to a single trap. Learning how to plumb a double sink comes down to understanding the continuous waste system: two bowls feeding into one shared P-trap through a tee, with the right slope so water flows and nothing siphons the trap dry. Get the layout and the pitch right and the rest is straightforward assembly.

Understand the Parts Before You Start

A standard double-sink drain kit includes tailpieces, slip nuts, washers, a continuous waste tee, and a P-trap. Each bowl gets a strainer basket (or a disposal on one side) that connects to a tailpiece. The two tailpieces meet at a tee, and from that tee a single P-trap leads to the wall drain (the trap arm). The trap holds a plug of water that blocks sewer gas from rising into your kitchen.

• Tailpiece: The vertical pipe dropping from each basket strainer.
• Continuous waste tee: The horizontal fitting that joins both bowls into one line.
• P-trap: The U-shaped trap that retains water and connects to the wall.
• Trap arm: The pipe running from the trap to the drain stub in the wall.

Plan the Layout and Slope

The single most important detail is slope. Horizontal drain runs need a fall of about 1/4 inch per foot toward the wall drain. Too little slope and water sits; too much and it can outrun the solids and pull the trap. Dry-fit everything first to confirm the tee lines up under the bowls and the trap reaches the wall stub without strain.

Decide which side gets the disposal, if you have one. The disposal raises and shifts the connection point on that side, so the continuous waste tee usually sits under the non-disposal bowl, with the disposal discharge feeding into the tee from the side.

Step-by-Step: Plumbing the Double Sink

1. Install the basket strainers. Seat each strainer in the drain opening with plumber’s putty or the supplied gasket, then tighten the locknut underneath.
2. Attach the tailpieces. Connect a tailpiece to each strainer with a slip nut and washer. On the disposal side, attach the disposal’s discharge elbow instead.
3. Position the continuous waste tee. Connect one tailpiece into the tee. The horizontal arm of the tee runs across to receive the other bowl’s drain or the disposal discharge.
4. Connect the second bowl. Run the disposal discharge or the second tailpiece into the side inlet of the tee. Cut pipe to length so the run keeps a slight downhill pitch toward the tee.
5. Install the P-trap. Connect the trap to the bottom of the tee and swing the trap arm into the wall drain. Maintain that 1/4 inch per foot slope on the arm.
6. Hand-tighten then snug the slip nuts. Tighten each slip nut by hand, then give a quarter turn with channel-lock pliers. Plastic threads strip easily, so do not overtighten.

Connecting the Dishwasher Branch

If you have a dishwasher, its drain hose typically connects to a dedicated inlet on the garbage disposal or to a dishwasher branch tailpiece. Always loop the dishwasher hose up high to the underside of the countertop (a high loop) or install an air gap on the sink deck. This prevents dirty sink water from siphoning back into the dishwasher. If you are connecting to a new disposal, remember to knock out the dishwasher inlet plug inside the disposal first, or no water will drain through.

Don’t Forget the Vent

Every trap needs a vent to break the suction that would otherwise siphon the water seal out of your trap. Most kitchens have a vent in the wall behind the sink. If your sink sits on an island with no wall to run a vent, you may need an air admittance valve (AAV), where local code permits. A poorly vented double sink gurgles, drains slowly, and lets sewer odor escape.

Test for Leaks

1. Fill both bowls completely, then pull both stoppers at once to stress the system with full flow.
2. Watch every slip nut and connection for drips with a flashlight and dry paper towel.
3. Run the disposal and the dishwasher cycle to confirm those branches drain cleanly.
4. Tighten any weeping joint a quarter turn at a time, or disassemble and reseat the washer if it persists.

Honest DIY Scope and When to Call a Pro

Replacing a double-sink drain assembly with an existing trap and vent is a reasonable DIY job for a confident homeowner with a couple of hours and basic tools. Where you should call a licensed plumber is when there is no existing vent, when you need to relocate the wall drain stub, when you are tying into cast-iron stack, or when you are moving the sink to an island. Anything that touches the drain-waste-vent system inside the walls usually requires a permit and code knowledge.

Tools and Materials You’ll Need

• A double-bowl drain kit with tailpieces, a continuous waste tee, slip nuts, and washers.
• A P-trap sized to your drain (commonly 1-1/2 inch in kitchens).
• Channel-lock or slip-joint pliers for snugging slip nuts.
• A hacksaw or PVC cutter to trim pipe to length.
• Plumber’s putty or silicone for the basket strainers.
• A bucket and rags to catch residual water when you disassemble the old trap.

Most kits come in PVC, which is forgiving and easy to cut, or chrome-plated brass for a more finished look under an open-base sink. PVC is the practical choice for hidden cabinet plumbing.

Common Mistakes to Avoid

• Too little slope: A flat or back-pitched run lets water and food sit, leading to odor and clogs.
• Overtightening slip nuts: Plastic threads strip and crack easily, causing leaks rather than preventing them.
• Forgetting the disposal knockout plug: Leave it in and the dishwasher will not drain.
• No high loop on the dishwasher hose: This invites dirty water to siphon back into the machine.
• Reusing old washers: Cracked or compressed washers leak; use the new ones in the kit.
• Ignoring the vent: An unvented trap gurgles, drains slowly, and can siphon dry.

The Bottom Line

Knowing how to plumb a double sink is mostly about respecting slope and the shared continuous-waste design: two bowls into a tee, one P-trap to the wall, a properly looped dishwasher branch, and adequate venting. Dry-fit first, keep the pitch at 1/4 inch per foot, hand-tighten plastic slip nuts, and test under full flow. Handle the visible assembly yourself and bring in a pro for anything behind the wall, and you will have a quiet, leak-free sink that drains the way it should.