Garbage Disposal on a Double Sink. Install and Fix Guide
Yes, you can install a garbage disposal under one basin of a double sink. The standard setup places the disposal under the larger basin, typically the right side. Both drains connect to a shared P-trap via a T-wye fitting. The disposal outlet feeds into one arm of the T-wye; the second basin drain feeds into the other arm. Water from either basin flows through the same trap before reaching the wall drain.
If you are dealing with disposal leaks and drainage problems, this guide covers the double-sink plumbing specifically. For broader troubleshooting, see our full guide to troubleshoot disposal leaks and drainage.
Tools and parts you need
Gather everything before you start. Mid-install trips to the hardware store are how mistakes happen.
Parts:
- 1.5” PVC P-trap kit ($15-$25)
- T-wye fitting (also called a sanitary tee or double-trap arm)
- Slip-joint nuts and washers (usually included in the P-trap kit)
- Teflon tape
- PVC primer and cement (if making permanent joints)
Tools:
- Adjustable wrench or slip-joint pliers
- Tape measure
- Hacksaw or PVC pipe cutter
- Bucket (to catch water when removing old trap)
One note on pipe sizing: if your existing drain uses 1.25” PVC, replace it with 1.5”. We consistently see the narrower pipe clog more frequently; 1.5” is the current standard for kitchen drains. A full 1.5” P-trap kit runs $15-$25 at any home center.
If this is a full disposal installation rather than a drain reconfiguration, start with our how to install a garbage disposal guide first, then return here for the double-sink drain connection.
Which side does the disposal go on?
There is no plumbing code that specifies which basin. Either side works mechanically. That said, we recommend the larger basin for the disposal. Most households find the larger bowl easier to use for food prep, and a shorter waste pipe run to the wall drain reduces long-term clog risk.
Two situations override that default:
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Dishwasher drain routing: If the dishwasher drain hose needs to connect to the disposal inlet, install the disposal on the side closest to the dishwasher. The inlet must be reachable without a long hose run.
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Drain rough-in position: If one basin sits closer to the wall drain stub-out, install the disposal under that basin. Fewer inches of horizontal pipe means less opportunity for grease to settle.
For most kitchens, the right basin is slightly larger and closer to the dishwasher. That is why “right side” is the conventional answer, even though left side works fine.
How double-sink plumbing works
Understanding the topology helps you install it correctly and diagnose it when something goes wrong.
The standard configuration:
Both basin drains meet at a T-wye fitting before reaching the P-trap. The disposal outlet connects to one arm; the second basin drain connects to the other arm. The shared outlet runs down into the P-trap, then into the wall drain.
Left basin drain arm ──────┐
├── T-wye → P-trap → wall drain
Right basin (disposal) ────┘
outlet armIn a standard double-sink installation, both basin drains share one P-trap. A T-wye fitting joins the disposal outlet and the second basin drain before feeding into the trap. If the P-trap clogs, water backs up in both basins simultaneously.
T-wye vs standard T fitting:
This is the most common installation mistake we see. A sanitary tee (T-wye) has a 45-degree angled branch. That angle directs flow downstream toward the P-trap, not horizontally into the opposite drain arm. A standard 90-degree T fitting creates a direct flow path between the two drain arms. When the disposal runs, wastewater hits the junction and shoots sideways into the second basin instead of flowing down to the trap.
Using a sanitary tee instead of a standard 90-degree T fitting is critical. The angled branch in a sanitary tee prevents wastewater from the disposal from flowing backward into the second basin drain. The T-wye fitting, not the disposal itself, is what makes a double-sink setup work correctly.
Slope requirement:
Each drain arm must slope 1/4 inch per foot toward the P-trap. Per IAPMO plumbing code drain slope requirements, a flatter run allows grease and food particles to settle and build up. Each drain arm in a double-sink configuration must slope 1/4 inch per foot toward the P-trap. Flat or reverse-sloped runs allow grease and food particles to settle, causing chronic slow drains.
P-trap location:
The P-trap sits 5-8 inches below the disposal outlet. That dimension comes from Lowe’s installation guides and standard disposal mounting heights. If your trap is lower than 8 inches, the trap arm may be too long and can sag, creating a horizontal section where solids settle.
Dishwasher connection: If a dishwasher is present, the drain hose routes through an air gap on the countertop, then into the disposal inlet. For full detail on that connection, see our dishwasher drain connection guide.
Step-by-step: connect the double-sink drain
These steps cover replacing or reconfiguring the drain plumbing under a double sink after installing a disposal.
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Turn off the water supply valves under the sink. Place a bucket under the drain area before disconnecting anything.
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Remove the old drain assembly. Loosen the slip-joint nuts at each P-trap connection and pull the old trap and drain arms. Have the bucket ready.
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Measure the distance from the disposal outlet to the wall drain stub-out. Also measure the distance from the second basin drain to where the T-wye will sit.
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Cut and dry-fit the drain arm from the disposal outlet to the T-wye. Make sure the arm slopes 1/4 inch per foot toward the T-wye. Use a torpedo level to check.
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Cut and dry-fit the drain arm from the second basin to the T-wye. This arm also needs 1/4-inch-per-foot slope. Both arms must slope downward to the T-wye junction.
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Connect the T-wye outlet to the P-trap inlet. The T-wye should be oriented so the disposal outlet and second basin arm both angle slightly downward into the tee body.
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Connect the P-trap outlet to the wall drain stub-out. The P-trap should sit 5-8 inches below the disposal outlet.
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Tighten all slip-joint nuts by hand, then add 1/4 turn with pliers. Do not over-tighten PVC fittings, snug is enough. Excess torque cracks the plastic. If the trap is PVC and more than 10 years old, replace it rather than retightening ($15-$25 for a new kit).
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Test both basins. Run water in both basins simultaneously. Check every joint for drips. Then run the disposal with water flowing and confirm that the second basin does not back up.
Per the This Old House double-sink plumbing guide, plan to clean the P-trap every 6-12 months after installation. Disposal use creates fine sludge that settles at the trap’s lowest point over time.
Fixing the most common double-sink problems
Problem 1: water backs up into the second basin when the disposal runs
Cause: The T fitting is either installed backwards or is a standard 90-degree T instead of a sanitary tee. A standard T creates a direct horizontal flow path between the two drain arms. When the disposal runs, water hits the junction and goes sideways.
Fix: Replace the T fitting with a sanitary tee (T-wye) oriented so both drain arms angle downward into the body of the tee. We find that swapping the fitting resolves the backflow problem in almost every case where a standard 90-degree T was used.
Problem 2: both sinks drain slowly
Cause: P-trap clog. Both basins share one trap, so sediment and grease buildup in the trap restricts both drains equally. This is a DIY job: 30 minutes, bucket and adjustable wrench.
Fix:
- Place a bucket under the P-trap
- Loosen the two slip-joint nuts connecting the trap to the drain line
- Pull the trap, drain it into the bucket
- Clear sludge from inside the trap with a pipe brush or straightened coat hanger
- Flush with hot water, reinstall, and test
If the PVC trap is over 10 years old, replace it rather than reinstalling it. A new P-trap kit costs $15-$25. Professional P-trap cleaning runs $75-$150 if you prefer not to do it yourself.
For garbage disposal won’t drain situations where the disposal side specifically is slow, that guide covers the drain clearing process inside the disposal itself.
Problem 3: disposal drains fine but second basin backs up
Cause: The drain arm from the second basin to the T-wye has insufficient slope or a partial clog in that specific arm.
Fix: Check the slope of that arm with a torpedo level. It needs 1/4 inch of drop per foot. If the slope is correct, snake the arm from the T-wye end to clear the partial blockage.
FAQ
Can you have a garbage disposal on both sides of a double sink?
Yes, but it requires two separate P-traps and two separate drain lines before joining at the wall stack. This is uncommon in residential settings and usually requires a plumber to reconfigure the drain rough-in. Most homeowners choose one-side disposal and use the other basin for rinsing.
Do I need a special garbage disposal for a double sink?
No. Any standard disposal works on a double sink. The difference is in the drain plumbing, not the disposal unit. For HP selection, our garbage disposal replacement guide covers the full range: 1/2 HP for 1-2 person households, 3/4 HP for 3-4 people, and 1 HP for families with heavy daily use.
Can I put a disposal under the smaller basin?
Yes, but the larger basin is the better choice. The larger bowl gives more room for food scraping and reduces the chance of sending oversized pieces toward the unit. The installation process is identical regardless of which side you choose.
What is the T-wye fitting and why does it matter?
A T-wye (sanitary tee) connects both drain arms to the shared drain run. The angled branch design directs flow downstream toward the P-trap rather than horizontally toward the second basin. Using a standard 90-degree T instead of a sanitary tee is the most common installation mistake in double-sink setups, and it causes water to back up into the clean basin every time the disposal runs.
For additional community-sourced troubleshooting on this specific problem, the Terry Love Plumbing forum discussion on double-sink disposal installs is a useful reference.
Do both sinks need separate P-traps?
No. For a standard double sink with one disposal, both basins share a single P-trap. The T-wye fitting joins both drain arms upstream of the trap. Only if you install disposals under both basins do you need two separate P-traps before the lines join at the wall stack.