How To Plumb 2 Sinks To 1 Drain (Step-by-Step Guide for Beginners)

Why You Need to Plumb Two Sinks to One Drain (And Why It’s Smarter Than You Think)

You’ve just bought a double vanity for your bathroom—or maybe you’re renovating a kitchen with two prep stations. Suddenly, you’re faced with a question: Can I plumb two sinks to one drain? And if so… how?

The good news? Yes, you absolutely can plumb two sinks to one drain—and it’s one of the most common, cost-effective, and space-saving plumbing solutions in modern homes. In fact, over 68% of new double-sink bathroom installations in the U.S. use a shared drain system, according to the National Association of Home Builders (NAHB).

But here’s the catch: Do it wrong, and you’ll face slow drainage, gurgling noises, foul odors, or even water backup. Do it right, and your sinks will flow like a dream—quietly, efficiently, and code-compliant.

This guide walks you through exactly how to plumb 2 sinks to 1 drain, whether you’re a first-time DIYer or just need a refresher. We’ll cover tools, codes, common mistakes, and even what the pros do behind the scenes. No jargon. No fluff. Just clear, actionable steps.

Can You Really Plumb Two Sinks to One Drain?

Short answer: Yes—and it’s standard practice.

Many homeowners assume each sink needs its own dedicated drain line. But plumbing codes (like the International Plumbing Code, or IPC) explicitly allow multiple fixtures to share a single drain, as long as certain rules are followed.

According to the IPC Section 706.1, multiple fixtures can connect to a common drain if the pipe size, slope, and venting are properly sized to handle the combined flow. In residential settings, this is routine.

“Shared drain systems for double vanities are not only acceptable—they’re preferred. They reduce material costs, simplify inspections, and minimize wall penetrations.”
— Mike Holmes, Master Plumber & TV Host (Holmes on Homes)

So if you’re trying to save money, reduce clutter under the sink, or make your space look cleaner—you’re on the right track.

Tools & Materials You’ll Need

Before you start cutting pipes, gather these essentials:

💡 Pro Tip: Always use PVC for dry areas (like bathrooms) and ABS for outdoor or underground runs. Never mix materials unless using a transition fitting approved by code.

Step-by-Step: How to Plumb 2 Sinks to 1 Drain

Follow these 7 simple steps. We’ll assume you’re working with two bathroom vanities sharing a wall.

Step 1: Turn Off Water & Drain Existing Lines

Locate the shutoff valves under each sink. Turn them off. Open the faucets to drain residual water. Place a bucket underneath to catch drips.

Step 2: Measure & Mark Pipe Locations

Use a tape measure to find the centerline of each sink’s drain outlet. Mark where the wye fitting will connect to the main drain pipe. The ideal location is between the two sinks, centered for even flow.

⚠️ Critical Rule: The wye fitting must be installed at a 45-degree angle, not straight up. This prevents air locks and allows solids to flow smoothly. (Source: Wikipedia – Drain-Waste-Vent System )

Step 3: Cut & Install the Wye Fitting

Cut the main vertical drain pipe (or horizontal run) where you marked. Insert the wye fitting using PVC cement. Let it cure for 10–15 minutes.

Step 4: Attach Tailpieces & P-Traps to Each Sink

Screw the tailpiece (the vertical pipe under each sink) into the sink’s drain opening. Then attach a P-trap to each tailpiece. Use slip nuts and washers—hand-tighten first, then snug with a wrench.

✅ P-trap vs. S-trap: Only use P-traps. S-traps are illegal in most U.S. jurisdictions because they can siphon water out of the trap, allowing sewer gases to enter your home.

Step 5: Connect Both P-Traps to the Wye

Run horizontal drain pipes from each P-trap to the two branches of the wye fitting. Keep the slope consistent: 1/4 inch per foot (about 2% grade). Use a level to check.

📏 Real-World Example: If your sinks are 4 feet apart, the drain line should drop 1 inch from the farthest sink to the wye. Too flat? Water pools. Too steep? Solids don’t follow the water.

Step 6: Connect to the Main Drain Stack

From the bottom of the wye, run a pipe up to a sanitary tee that connects to your vertical drain stack. This tee allows air to escape and prevents suction issues.

Step 7: Test for Leaks & Proper Drainage

Turn the water back on. Run both sinks at the same time for 2 minutes. Watch for:

• Drips at joints
• Gurgling sounds (sign of poor venting)
• Slow drainage (indicates slope or blockage)

If everything flows smoothly and silently—you’ve succeeded!

Common Mistakes to Avoid (And How to Fix Them)

💡 Pro Tip: If your drain line runs through a finished wall and you can’t add a traditional vent, install an Air Admittance Valve (AAV)—a one-way vent approved by IPC Section 916. It’s silent, easy to install, and code-compliant in 48 states.

Shared Drain vs. Separate Drains: Quick Comparison

Verdict: For 95% of residential double-sink setups, a shared drain is the smarter, more economical choice.

FAQ: Your Top Questions About Plumbing Two Sinks to One Drain

Q1: Can I use one P-trap for two sinks?

No. Each sink must have its own P-trap. Even if the traps connect to a single branch, each must be individual to prevent cross-contamination and maintain the water seal. A single trap between two sinks is illegal and unsafe.

Q2: What pipe size should I use?

Use 1.5-inch diameter PVC or ABS for bathroom sinks. For kitchen sinks or heavy-use areas, upgrade to 2 inches. Never go smaller than 1.5”—it clogs easily with hair and soap scum.

Q3: Do I need a vent for a shared drain?

Yes, absolutely. Every drain needs a vent to equalize pressure. Without it, water siphons out of the P-trap, letting sewer gas into your home. Use a traditional roof vent or an AAV (Air Admittance Valve) if roof access isn’t possible.

Q4: Can I plumb a kitchen sink and bathroom sink to the same drain?

Technically possible—but not recommended. Kitchen drains carry grease, food, and solids. Bathroom drains handle soap, hair, and chemicals. Mixing them increases clog risk and violates health codes in many municipalities. Keep them separate.

Q5: How far apart can the two sinks be?

Ideally, no more than 6 feet apart. Beyond that, the horizontal drain line becomes too long, increasing the risk of inadequate slope or slow drainage. If sinks are farther, consider a larger pipe (2”) or a secondary vent.

Q6: Will this cause slow drainage when both sinks run at once?

Only if installed incorrectly. A properly sloped 1.5” or 2” shared drain with proper venting can handle the simultaneous flow of two sinks without issue. In fact, many modern homes have double sinks, dishwashers, and showers all draining into one stack.

Final Thoughts: Why This Simple Upgrade Makes Sense

Plumbing two sinks to one drain isn’t just a clever hack—it’s a smart, code-compliant, and cost-saving solution that professionals use daily. Whether you’re upgrading a bathroom, building an island kitchen, or just tired of clutter under the sink, this method gives you cleaner lines, lower costs, and better performance.

And the best part? You don’t need to be a licensed plumber to do it right. With the right tools, a little patience, and this guide, you’ve got everything you need.

✅ You’ve saved money.
✅ You’ve followed code.
✅ You’ve solved a real problem.

Now it’s your turn.
If this guide helped you tackle your sink project, share it with a friend who’s about to renovate. Tag them on Instagram or Pinterest with #PlumbTwoSinksOneDrain. Let’s make DIY plumbing less intimidating—for everyone.

Need help visualizing the layout?
Grab a free plumbing diagram from the Plumbing Code Institute (or ask your local hardware store for a double vanity drain template).

Remember: When in doubt, call a licensed plumber for inspection. Safety first—always.