Correct Way to Plumb a Double Sink With Garbage Disposal

Struggling with a messy under-sink cabinet or recurring drainage issues? You’re not alone. Installing a garbage disposal on a double sink can be tricky—but when done right, it saves time, reduces odors, and keeps your kitchen running smoothly. In this guide, we’ll walk you through the correct way to plumb a double sink with garbage disposal, ensuring everything works efficiently and meets local plumbing codes.

Why Proper Plumbing Matters for a Double Sink With Garbage Disposal

Before jumping into the wrench work, it’s essential to understand why correct plumbing is non-negotiable. According to the American Society of Home Inspectors (ASHI), nearly 30% of kitchen plumbing issues stem from improper sink waste-line configuration—especially in dual-basin setups with disposals.

A poorly plumbed system can lead to:

• Slow drainage or backups
• Foul sewer odors
• Water leakage under the sink
• Violation of local plumbing codes (which may affect home resale value)

The good news? With a clear plan and the right materials, you can avoid these pitfalls—whether you’re a DIYer or supervising a plumber.

What Tools and Materials Do You Need?

Having the right gear saves time and prevents mid-job trips to the hardware store. Here’s your checklist:

✅ Tools:

• Adjustable wrench
• Pipe wrench
• Plumber’s putty or silicone sealant
• Bucket & towels (for spills!)
• Teflon tape
• Hacksaw or PVC cutter

✅ Materials:

• Garbage disposal unit
• Dishwasher drain hose (if applicable)
• Continuous waste kit (includes tailpieces, P-trap, and connecting pipes)
• Drain baskets for both sinks
• Mounting hardware (usually included with disposal)

Pro Tip: Always check local codes. Some municipalities require an air gap for dishwasher connections or specific trap configurations. When in doubt, consult your city’s building department or refer to the International Plumbing Code (IPC) guidelines on Wikipedia.

Step-by-Step: How to Plumb a Double Sink With Garbage Disposal

Follow these steps carefully. Precision here prevents future headaches.

Step 1: Remove Old Plumbing & Prep the Area

Turn off water supply and disconnect existing pipes. Clear the cabinet completely. Place a bucket underneath to catch residual water.

Step 2: Install Drain Assemblies

• Apply plumber’s putty under the flange of both sink drain baskets.
• Insert and tighten from below with a locknut. Wipe away excess putty.

Note: Only the disposal side (usually the smaller basin) will connect directly to the garbage disposal. The other basin drains into the same waste line via a “continuous waste” setup.

Step 3: Mount the Garbage Disposal

• Attach the disposal to the sink flange using the included mounting ring and snap ring.
• Ensure it’s secure—wiggle it gently to test stability.

Step 4: Connect the Non-Disposal Basin

• Install a baffle tee (also called a continuous waste fitting) below the non-disposal sink.
• Connect its tailpiece to the side inlet of the baffle tee.
• The center outlet of the baffle tee will lead to the P-trap.

Step 5: Link Disposal to Drain System

• Attach the disposal’s discharge tube to the baffle tee’s other side inlet (opposite the non-disposal tailpiece).
• Use slip nuts and washers—hand-tighten plus a quarter-turn with a wrench to avoid cracking.

Step 6: Install the P-Trap

• Connect the baffle tee’s center outlet to the vertical inlet of the P-trap.
• Then connect the P-trap’s horizontal outlet to your main drainpipe in the wall.
• Ensure the trap is level and properly sealed—no gaps!

Step 7: Final Checks

• Run water in both basins simultaneously.
• Check for leaks at every joint.
• Grind a few ice cubes in the disposal to test operation and flush debris.

Critical Detail: The P-trap must be installed within 24 inches of the sink’s vertical tailpiece (per IRC P3201.6). This prevents siphoning and maintains the water seal that blocks sewer gases.

Common Mistakes to Avoid

Even experienced DIYers slip up here. Steer clear of these errors:

Expert Insight: “The key to success is maintaining a consistent downward slope—¼ inch per foot—from both sinks to the trap,” says Maria Lopez, a licensed master plumber with 18 years of experience in residential installations.

Garbage Disposal + Dishwasher? Here’s How to Connect Both

Many double-sink setups include a dishwasher. Most disposals have a knockout plug on the side for dishwasher hose connection.

Steps:

1. Remove the knockout plug with a screwdriver and hammer.
2. Attach the dishwasher drain hose to the disposal inlet.
3. Use a high-loop or air gap to prevent backflow (required by code in most areas).

Safety Note: Never skip the high-loop—it’s a legal and hygiene requirement. Water from a backed-up sink could otherwise flow back into your dishwasher.

Maintenance Tips for Long-Term Performance

Your plumbing is only as good as its upkeep. Follow these habits:

• Monthly: Run cold water + ½ cup baking soda + 1 cup vinegar through the disposal. Let sit 10 mins, then flush with hot water.
• Quarterly: Inspect connections for moisture or corrosion.
• Never pour grease, coffee grounds, or fibrous foods (celery, onion skins) down the disposal.

FAQ Section

Q1: Can both sinks in a double sink setup have a garbage disposal?

A: Technically yes, but it’s rare and usually unnecessary. Most homes install one disposal on the smaller basin (often the prep sink). Two disposals require extra space, wiring, and plumbing—and can overwhelm your drain line.

Q2: Which side should the garbage disposal go on?

A: Traditionally, it goes on the smaller, right-hand basin (for right-handed users). But it ultimately depends on your cabinet layout, dishwasher location, and personal preference.

Q3: Do I need a separate P-trap for each sink?

A: No. A double sink with one disposal uses a single P-trap connected via a baffle tee. Installing two P-traps can cause airlock and drainage issues (a violation of most plumbing codes).

Q4: How do I know if my plumbing is up to code?

A: Key signs:

• No sewer smells
• Both sinks drain quickly
• No leaks under the cabinet
• P-trap is present and properly installed
  When in doubt, hire a licensed plumber for a quick inspection.

Q5: Can I install a garbage disposal myself?

A: Yes—if you’re comfortable with basic plumbing and electrical work (many disposals are plug-in; hardwired units require an electrician). Always turn off power at the breaker before starting.

Q6: What if water backs up into the other sink when I run the disposal?

A: This usually means a clog downstream of the baffle tee or an improperly sloped pipe. Check the P-trap and main drain line for blockages.

Final Thoughts

Mastering the correct way to plumb a double sink with garbage disposal isn’t just about tightening pipes—it’s about creating a system that’s efficient, odor-free, and built to last. With the right materials, attention to slope and seal, and respect for plumbing codes, you’ll enjoy a fully functional kitchen for years to come.

Found this guide helpful? Share it with a friend or on social media—your DIY-savvy neighbor will thank you! And if you’ve tackled this project yourself, drop your tips in the comments below. 🛠️💧