What you need to know before you start
A double sink with garbage disposal uses the same basic drain setup as a single sink, but the plumbing splits into two paths under the counter. One sink drains directly to the trap; the other drains into the garbage disposal, which then feeds into the same trap. The disposal sits between the second sink and the main drain line, so water from both sinks eventually reaches the same place—but the disposal grinds food waste before it gets there.
This is a job you can do yourself if you are comfortable working with PVC pipe, slip nuts, and basic hand tools. You will need to measure twice and understand how the trap and tailpieces connect. If you hit cast iron drain lines, hit a main vent stack, or find that your drain opening is in an awkward spot, call a plumber—those situations require experience to solve without creating leaks or venting problems.
Key Takeaways
- The sink without the disposal drains directly into a P-trap; the disposal drains into the same trap from a different angle, using a tee fitting or a double-outlet trap.
- You must install a dishwasher drain line into the disposal's side inlet before you connect anything else, or you will have to disassemble the disposal later.
- Slip nuts hand-tight plus one-quarter turn is the standard; over-tightening cracks plastic fittings and under-tightening causes leaks.
- Test the entire system with both sinks running before you call the job done, because leaks often show up only when water is actually flowing.
- If your existing drain line is cast iron or if the trap location does not line up with both sink openings, you may need a plumber to reroute or adapt the drain.
Measure the distance from each sink to the trap location
Before you buy any fittings, you need to know how far each sink's drain opening sits from the wall and how far down the trap will be. Measure from the center of the left sink drain to the wall, then the right sink drain to the wall. Then measure from the bottom of the sink cabinet to where the main drain line enters the wall or floor. Write these numbers down.
The reason this matters: if the two sink drains are far apart, you may need a longer horizontal tailpiece on one side, or you may need to use a double-outlet trap instead of a standard P-trap with a tee. If the trap location is too high or too low, you will have to buy longer or shorter tailpieces, or the whole assembly will sit at an angle that does not drain properly. A trap should slope downward toward the main drain at a pitch of about one-quarter inch per foot—not level, not steep.
Choose your trap configuration: standard P-trap with tee, or double-outlet trap
A standard P-trap with a tee is the most common setup. One sink's tailpiece connects directly to the trap inlet. The other sink's tailpiece connects to a tee fitting that sits on top of the trap, so both drains feed into the same trap from slightly different angles. This works well when both sink drains are roughly the same distance from the wall.
A double-outlet trap has two inlets built into the top of the trap itself, so you can connect both tailpieces directly without a separate tee. This is neater and takes up less space, but it costs more and is harder to find in some areas. If your sink drains are very far apart or at very different heights, a double-outlet trap may be your only option.
For most kitchens, a standard P-trap with a slip-nut tee works fine. Buy the trap and tee in the same material—usually PVC plastic for new installations. If your existing drain line is metal, you will need a rubber adapter to connect plastic fittings to it.
Install the garbage disposal and connect its drain outlet
Mount the disposal under the sink according to the manufacturer's instructions—usually a ring clamp that tightens under the sink flange. Before you connect the disposal to anything, install the dishwasher drain inlet on the side of the disposal if you have a dishwasher. This inlet is a small port with a rubber cap; you push or screw it in depending on the model. If you skip this step and connect the drain lines first, you will have to take the whole disposal apart to add it later.
The disposal's outlet is a threaded port on the bottom or back. Screw a disposal drain adapter (a short tailpiece with threads on one end) into this port hand-tight, then add one-quarter turn with a wrench. Do not over-tighten; plastic threads strip easily. This adapter will connect to your trap assembly.
Connect both sink tailpieces to the trap and tee
A tailpiece is the short pipe that runs from the sink strainer basket down to the trap. If your sink does not have one, you will need to buy one in the right diameter (usually 1.5 inches) and length. Slide the tailpiece up through the sink strainer basket and hand-tighten the slip nut that holds it in place—the nut that sits under the sink, between the strainer and the cabinet.
Once both tailpieces are in place, connect them to the trap. If you are using a tee, one tailpiece connects directly to the trap inlet with a slip nut. The other tailpiece connects to the tee with a slip nut, and the tee connects to the trap inlet with another slip nut. The disposal's drain adapter connects to the tee as well, or directly to the trap if you are using a double-outlet trap.
Hand-tighten every slip nut, then use a wrench to turn each one one-quarter turn more. This is enough to seal the joint without crushing the rubber washer inside the nut. If water leaks from a joint after you test it, tighten that nut another one-eighth turn—do not go further without taking the joint apart and replacing the washer.
Seal the trap to the main drain line
The outlet of the trap connects to the main drain line that runs into the wall or floor. If the main line is PVC plastic, use a slip nut and rubber washer, the same way you connected the tailpieces. If the main line is metal (cast iron or galvanized steel), you will need a rubber adapter—a short piece of rubber hose with stainless steel bands on each end—to bridge the gap between plastic and metal. Slide the adapter over the trap outlet, then over the main drain line, and tighten the bands with a screwdriver.
Make sure the trap sits level and slopes downward into the main drain. If it tilts or sits too high, water will pool inside the trap and smell. If it sits too low, the vent line may not work properly. Adjust the height by loosening the slip nuts, moving the trap up or down, and re-tightening.
Test both sinks and the disposal before you finish
Fill the left sink with water and let it drain. Watch under the cabinet for leaks at every slip nut and connection. Then fill the right sink and watch again. Then run the disposal with water flowing into it. Finally, run both sinks at the same time and watch the trap area for drips or sprays.
Leaks often take a minute or two to show up, so do not rush. If you see a drip, let it run for another 30 seconds to see if it stops or gets worse. A single drop every few seconds usually means the slip nut is just slightly loose; tighten it one-eighth turn and test again. A steady stream means the washer is damaged or seated wrong; take the joint apart, replace the washer, and reassemble.
Once both sinks drain without leaking and the disposal runs smoothly, the job is done. If you see water pooling in the trap or hear gurgling sounds when the disposal runs, the vent line may be blocked or the trap may be installed backward—call a plumber to check the vent.
When to call a plumber instead
Do not attempt this job if your main drain line is cast iron and you are not comfortable cutting and adapting it. Do not do it if the drain opening in the wall is more than a few inches away from where your trap will sit, because you will need to reroute the line. Do not do it if you are not sure whether a vent line is present or working, because a trap without proper venting will drain slowly and smell bad.
If you hit any of these situations, a plumber can solve them in an hour or two. The cost is worth it, because a poorly vented or misaligned drain will cause problems for years.
Frequently Asked Questions
Can I use the same trap for both sinks if they are far apart?
Yes, but you will need longer tailpieces or a horizontal run of pipe between the two sink drains before they meet at the trap. The key is that water must flow downward into the trap, not sit level in a long horizontal section. If the horizontal run is more than a few feet, ask a plumber whether you need a second trap or a vent line adjustment.
What size slip nuts and tailpieces do I need?
Most kitchen sink drains are 1.5 inches in diameter. Measure the opening under your sink or check the old tailpiece if you are replacing it. Slip nuts come in matching sizes; buy them at the same time as your tailpieces and trap so they fit together.
Do I need a separate trap for the disposal?
No. The disposal drains into the same trap as the other sink. Both drains feed into one trap, which then connects to the main drain line. A second trap would actually cause problems by trapping air and slowing drainage.
What if water backs up into the sink without the disposal?
This usually means the disposal is clogged or the trap is blocked. Run the disposal with hot water for 30 seconds. If that does not work, the trap itself may be full of food waste or grease; you will need to disassemble it and clean it out by hand.
Can I install this under a granite or quartz countertop?
Yes, but you have less room to work. Measure carefully before you buy fittings, and consider asking a plumber to do the final connections if the space is very tight. Granite and quartz are expensive to repair if you crack them while working underneath.