Double Vanity Plumbing: Two Sinks to One Drain Made Simple

Tired of cramped morning routines? A double vanity can transform your bathroom into a shared oasis—but only if the plumbing works right. Many homeowners wonder: Can you really connect two sinks to one drain? The answer is yes, and when done correctly, double vanity plumbing with two sinks to one drain is not only possible but also code-compliant and highly functional. In this guide, you’ll learn exactly how to do it safely, avoid common mistakes, and ensure smooth drainage for years to come.

Can You Connect Two Sinks to One Drain?

Yes—two bathroom sinks can share a single drain line, provided you follow plumbing codes and best practices. The International Plumbing Code (IPC) allows this setup as long as proper venting, trap configuration, and pipe sizing are maintained.

According to the IPC Section 1002.1, each fixture must have its own trap, but multiple traps can connect to a common drain if spaced correctly and vented properly. This prevents sewer gases from entering your home and ensures wastewater flows smoothly without backups.

💡 Pro Tip: Never connect two P-traps directly to one drain pipe without a proper fixture cross or double sanitary tee. Doing so can create an “S-trap” configuration, which is prohibited because it can siphon water out of the trap and allow dangerous gases to escape.

How Double Vanity Plumbing Works: The Basics

A double vanity setup typically uses one of two configurations:

1. Dual Drains with a Common Waste Line
   Each sink has its own P-trap, but both connect to a single horizontal drain pipe that leads to the main stack.
2. Shared Trap Arm (Less Common)
   Both sinks drain into a single trap—but this is not recommended and often violates code due to trap seal loss risks.

The first method is the standard, code-compliant approach. Here’s how it works:

• Each sink has its own 1.25″ or 1.5″ tailpiece.
• Each connects to its own P-trap (usually 1.25″ or 1.5″).
• The two trap arms join via a double sanitary tee or a fixture cross into a single 2″ drain line.
• The system is properly vented within 5 feet of each trap (per IPC).

For visual reference, see this simplified diagram concept:

1

2

3

[Sink 1] → P-Trap → \

→ Double Sanitary Tee → 2″ Drain → Main Stack

[Sink 2] → P-Trap → /

This setup maintains individual trap seals while sharing a common waste path—efficient and safe.

Step-by-Step Guide: Plumbing Two Sinks to One Drain

Follow these steps carefully. Always turn off water supply and check local codes first—some municipalities have stricter rules than the IPC.

Tools & Materials Needed

• Two P-traps (1.25″ or 1.5″)
• Tailpieces (1.25″)
• Double sanitary tee or fixture cross (2″ inlet, 1.5″ outlets)
• PVC or ABS pipe (2″ diameter for main drain)
• Pipe cutter, hacksaw, or PVC saw
• Plumber’s putty or silicone sealant
• Adjustable wrench
• Level

Installation Steps

1. Position the Vanities
   Ensure both sinks are level and aligned. The drain outlets should be at the same height (±¼ inch tolerance).
2. Install Tailpieces
   Attach a tailpiece to each sink drain using plumber’s putty. Tighten the locknut securely but avoid over-tightening (can crack porcelain).
3. Connect P-Traps
   Attach a P-trap to each tailpiece. Leave the outlet end of each trap unconnected for now.
4. Install the Double Sanitary Tee
   Mount the double sanitary tee to the wall or floor drain stub-out. The tee should be level and securely fastened.
   • Use a 2″ pipe for the main drain line running to the stack.
   • Ensure the tee is installed upright—inverted tees can cause clogs.

5. Connect Trap Arms to Tee
   Cut 1.5″ pipes to connect each P-trap outlet to the side inlets of the double sanitary tee.
   • Keep trap arm length under 24 inches (IPC limit).
   • Maintain a ¼-inch-per-foot slope toward the drain.
6. Vent the System
   Each trap must be vented within 5 feet (IPC 906.1). If your bathroom has a shared vent stack, tie into it using a 1.5″ or 2″ vent pipe.
   • No vent? Consider an Air Admittance Valve (AAV)—approved in many U.S. jurisdictions (check local codes).
7. Test for Leaks
   Run water in both sinks simultaneously for 2–3 minutes. Check all joints for drips. Re-tighten if needed.

✅ Real-World Example: A 2022 study by the National Association of Home Builders (NAHB) found that 78% of new master bathrooms now include double vanities—making efficient, code-compliant plumbing more important than ever.

Pros and Cons of Sharing One Drain

Common Mistakes to Avoid

• ❌ Using an S-trap: Creates siphoning risk. Always use P-traps with proper venting.
• ❌ Ignoring slope: Drain pipes must slope ¼” per foot toward the stack. Flat or uphill runs cause standing water.
• ❌ Overlooking venting: Unvented traps lose their water seal, allowing sewer gas into your home.
• ❌ Mixing pipe materials incorrectly: Don’t glue PVC to ABS without a transition fitting.

For more on plumbing venting principles, see Wikipedia’s overview of plumbing vents .

When to Call a Professional

While DIY plumbing is rewarding, double vanity drain setups can be tricky if you’re unfamiliar with code requirements. Call a licensed plumber if:

• Your local code prohibits shared drains
• You lack access to a vent stack
• You’re working with cast iron or older pipes
• You’ve had repeated clogs or slow drainage

The average cost to professionally install double vanity plumbing ranges from $400–$900, according to HomeAdvisor (2023 data)—a worthwhile investment for peace of mind.

FAQ: Double Vanity Plumbing Questions Answered

Q: Is it legal to connect two sinks to one drain?
A: Yes, under the International Plumbing Code (IPC), as long as each sink has its own P-trap, the trap arms are properly sloped, and the system is vented within 5 feet of each trap.

Q: Do I need two P-traps for a double vanity?
A: Absolutely. Each sink must have its own P-trap. Sharing one trap between two sinks violates plumbing codes and risks sewer gas exposure.

Q: Can I use a Y-fitting instead of a double sanitary tee?
A: No. A standard Y-fitting can cause turbulence and clogs. Use a double sanitary tee or fixture cross, which are designed for dual fixture connections.

Q: What size pipe do I need for the main drain?
A: Use a 2-inch diameter pipe for the common drain line. Sink tailpieces and P-traps are typically 1.25″ or 1.5″, but the shared section should be 2″ to handle combined flow.

Q: Can I install this without a vent?
A: Only if you use an Air Admittance Valve (AAV) approved by your local authority. Never install an unvented trap—it’s a health hazard.

Q: Will two sinks drain slower if they share a drain?
A: Not if installed correctly. A properly sized 2″ drain with adequate slope and venting handles simultaneous use without issue.

Conclusion

Plumbing a double vanity with two sinks to one drain is a smart, space-saving solution that’s fully achievable with the right knowledge and materials. By using individual P-traps, a double sanitary tee, proper venting, and correct pipe slope, you’ll create a system that’s efficient, code-compliant, and built to last.

Whether you’re renovating a master bath or upgrading a guest room, this setup enhances functionality without sacrificing safety.

Found this guide helpful? Share it with a friend planning a bathroom remodel! 🛁✨

Your dream bathroom starts with smart plumbing—one drain at a time.