How To Plumb A Double Sink Bathroom Vanity (Step-by-Step Guide)

Why Pluming a Double Sink Vanity Matters (And Why It’s Trickier Than It Looks)

You’ve picked out the perfect double-sink bathroom vanity—marble countertop, modern fixtures, plenty of storage. But now you’re staring at a tangle of pipes under the sink, wondering: “How do I actually plumb a double sink bathroom vanity without turning my bathroom into a flood zone?”

You’re not alone. According to a 2023 HomeAdvisor survey, 37% of DIY homeowners who attempted double-sink installations encountered leaks or drainage issues within the first 6 months—mostly due to improper venting or mismatched pipe sizes. The good news? With the right guidance, you can avoid those costly mistakes.

This guide walks you through every step of how to plumb a double sink bathroom vanity—clearly, safely, and with confidence. Whether you’re upgrading an old bathroom or building new, you’ll walk away with a leak-proof, code-compliant setup that lasts decades.

What Tools and Materials Do You Need?

Before you touch a pipe, gather everything. Missing one item means a second trip to the hardware store—and maybe a ruined weekend.

Here’s your essential toolkit:

💡 Expert Insight: “Most DIYers skip the vent, thinking it’s optional. But without proper venting, you’ll hear gurgling, slow drains, and eventually, siphoning of trap water—letting sewer gas in.” — Mike Holmes, Licensed Plumber & TV Host

Bonus: Buy a double sink drain kit—they come pre-assembled with the right fittings. Brands like Oatey or Pfister offer reliable ones under $40.

Step 1: Turn Off Water & Drain Existing Lines

Safety first.

• Shut off the hot and cold water supply valves under each sink. If there are none, turn off the main water supply.
• Open both faucet handles to drain residual water.
• Place a bucket under the P-traps to catch any remaining water.
• Use a wet/dry vacuum or towels to soak up drips.

Pro Tip: Take a photo of the existing setup before removing anything. It’s your visual cheat sheet.

Step 2: Remove Old Fixtures & Clean the Area

If you’re replacing an old vanity:

• Disconnect supply lines with an adjustable wrench.
• Unscrew the P-traps and remove them.
• Scrape off old caulk or putty from the countertop and wall.
• Wipe down the area with rubbing alcohol—this ensures a clean seal for new caulk later.

✅ Code Check: According to the International Residential Code (IRC), all drain lines must slope at least ¼ inch per foot toward the main stack. Measure with a level before installing new pipes.

Step 3: Install the Double Sink Drain Assembly

This is the heart of your system.

1. Insert the tailpieces into each sink’s drain opening. Hand-tighten the slip nuts from below.
2. Connect the tailpieces to the double outlet drain body (the T-shaped fitting that joins both sinks).
3. Align the assembly so both sinks drain evenly. Use a level to check.
4. Apply PVC primer to all pipe ends, then cement them together. Hold for 10 seconds—no twisting!
5. Let it cure for 1 hour before testing (even if the glue says “quick set”—patience saves leaks).

Why this matters: If the drain body isn’t centered or level, one sink will drain slower—leading to standing water and odor buildup.

Step 4: Connect the P-Traps & Vent System

Now connect the drain to the wall’s main pipe.

Option A: Shared P-Trap (Recommended for Most Homes)

• Use a double fixture fitting (also called a “double sanitary tee”) on the wall.
• Connect one P-trap from the double drain body to this fitting.
• This is code-compliant and uses fewer materials.

Option B: Two Separate P-Traps

• Each sink gets its own P-trap.
• Each connects to a separate branch leading to the main drain.
• Not recommended unless required by local code—uses more space, more materials, and increases leak risk.

📌 Key Rule: Never connect two sinks to a single P-trap without a double fixture fitting. This violates plumbing code and causes siphoning.

Venting is non-negotiable.
Every drain needs air to flow properly. If your vanity is within 6 feet of an existing vent stack, tie into it using a wet vent. If not, install a stub vent (a small vertical pipe rising 6” above the drain line, then sloping to the main stack).

For visual learners: Wikipedia’s Drain-Waste-Vent System explains how air pressure keeps water sealed in traps.

Step 5: Connect Supply Lines & Test for Leaks

Time to bring water back.

1. Attach flexible supply lines (braided stainless steel) to the shut-off valves and faucet tails.
2. Turn on water slowly—one side at a time.
3. Check every joint: under the sink, at the P-trap, around the tailpieces.
4. Run both faucets for 2 minutes. Watch for drips.

🔍 Leak Test Hack: Wipe all connections dry with a paper towel. Run water. If the towel turns damp in 30 seconds—you’ve found your leak.

Fix leaks by:

• Tightening slip nuts slightly (don’t crush the washer!)
• Replacing worn rubber washers
• Re-cementing glued joints if they’re loose

Step 6: Seal the Sink & Final Touches

Once everything’s leak-free:

• Apply a continuous bead of 100% silicone caulk around the sink’s edge where it meets the countertop.
• Smooth it with a wet finger.
• Let cure 24 hours before using the sink.

Bonus: Install a garbage disposal? You’ll need a special tailpiece and a disposal air gap (required by code in most US states). Consult a pro if you’re unsure.

Common Mistakes to Avoid (And How to Fix Them)

FAQ: Your Top Questions About Double Sink Plumbing, Answered

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

Yes—but only if you use a double fixture fitting (also called a “double sanitary tee”) on the wall. Never connect two sinks directly to one P-trap without this fitting. It’s a code violation and causes trap siphoning.

Q2: Do I need a separate vent for each sink?

No. One properly sized vent serving both sinks is sufficient—as long as it’s within 6 feet of the drain connection. The key is airflow, not quantity.

Q3: What size drain pipe do I need for a double vanity?

Use 1.5-inch or 2-inch PVC. Most building codes require 2-inch for double sinks due to higher flow volume. Smaller pipes (1.25″) will clog faster with hair and toothpaste.

Q4: Can I install a double sink vanity myself?

Absolutely—if you’re comfortable with basic tools and following instructions. Most homeowners complete this in 4–6 hours. If you’re unsure about venting or code compliance, hire a licensed plumber for the final inspection.

Q5: Why does one sink drain slower than the other?

Most likely:

• The drain assembly isn’t level
• Debris is clogging the shared line
• The vent is blocked or missing

Solution: Check the slope, clean the P-trap, and verify venting.

Q6: How much does it cost to plumb a double sink vanity?

DIY: $80–$150 (for parts only).
Professional: $300–$800 (labor + materials).
Pro tip: Installing it yourself saves 70%+—and gives you confidence for future projects.

Final Thoughts: You’ve Got This

Plumbing a double sink bathroom vanity isn’t about being a master plumber—it’s about being careful, patient, and prepared. You now know how to choose the right parts, avoid the 5 most common mistakes, and test for leaks like a pro.

The payoff? A sleek, functional bathroom that works beautifully for years—and potentially boosts your home’s resale value. According to Zillow, updated bathrooms return 70–80% of renovation costs at resale. Your double sink isn’t just convenient—it’s an investment.

👉 Love this guide? Share it with a friend who’s tackling a bathroom remodel! Tag them on Pinterest, Facebook, or Instagram with #DoubleSinkSuccess. Let’s help more homeowners skip the leaks and build with confidence.

Need visuals? Bookmark this page or print the step-by-step checklist. Next time you’re under the sink, you’ll thank yourself.