How to Plumb a Double Kitchen Sink with Double Traps (Step-by-Step Guide)

If you’re renovating your kitchen or replacing an old sink, you might be wondering: Can I use two separate P-traps for a double kitchen sink? The short answer is yes—but only if done correctly. Many homeowners attempt this without understanding local plumbing codes, leading to venting issues, sewer gas leaks, or failed inspections. In this guide, you’ll learn exactly how to plumb a double kitchen sink with double traps the right way, step by step, while staying compliant with U.S. plumbing standards and ensuring long-term functionality.

Why Would You Use Double Traps for a Double Sink?

Most double-bowl kitchen sinks share a single P-trap connected via a T-fitting—a standard, code-compliant setup. However, some homeowners prefer dual P-traps, especially when:

• Each bowl has its own garbage disposal.
• The sink bowls are far apart (e.g., in an oversized or custom island sink).
• You want independent drainage for easier maintenance.

But here’s the catch: dual traps can cause airlock or siphoning if not properly vented. According to the International Plumbing Code (IPC), dual traps on a single fixture are only allowed if each trap is independently vented or connected via a continuous waste system that prevents cross-flow imbalance.

💡 Expert Insight: “Using two unvented traps under one sink often leads to one trap losing its water seal due to pressure imbalance—this lets sewer gases enter your home,” says Maria Lopez, a licensed master plumber with 18 years of experience in California.

Is It Legal to Install Two P-Traps Under One Sink?

This is a common point of confusion. The answer depends on your local plumbing code, but generally:

• IPC (used in most U.S. states): Allows dual traps only if each trap arm is individually vented within 5 feet (per IPC 1002.4).
• Uniform Plumbing Code (UPC): Often prohibits dual traps on a single fixture unless special conditions are met.

Before starting, check with your local building department. In many cases, inspectors will reject dual traps unless you install a dual sanitary tee and proper venting.

For more background on plumbing traps and their function, see Wikipedia’s entry on P-traps.

Tools & Materials You’ll Need

Gather these before you begin:

✅ Pro Tip: Use chrome-plated brass or polypropylene traps—they resist corrosion better than cheap plastic and last longer.

Step-by-Step: How to Plumb a Double Kitchen Sink with Double Traps

Follow these steps carefully. This method assumes each bowl has its own drain outlet and you’re using a dual sanitary tee to merge flows properly.

Step 1: Install Sink Strainers

• Apply plumber’s putty under each strainer flange.
• Insert from top, secure with locknut underneath.
• Wipe excess putty.

Step 2: Attach Tailpieces

• Screw tailpiece onto each strainer (include washer and nut).
• Leave 6–8 inches of vertical pipe below each bowl.

Step 3: Install Dual P-Traps

• Connect each tailpiece to its own P-trap.
• Ensure traps are level and aligned toward the wall.

Step 4: Connect to a Dual Sanitary Tee (Critical!)

• Do NOT use a standard T-fitting—it causes turbulence and siphoning.
• Use a dual sanitary tee (also called a “figure-8” fitting) that merges both trap outlets into one horizontal drain arm.
• This fitting has smooth, directional inlets that prevent backflow and maintain trap seals.

Step 5: Connect to Main Drain Line

• Extend the combined drain arm to your existing wall stub-out.
• Maintain a ¼-inch per foot downward slope toward the main stack.
• Secure all slip-joint connections—hand-tight plus ¼ turn with wrench.

Step 6: Test for Leaks & Functionality

• Fill both bowls with 2 liters of water (approx. 22–28°C / room temp).
• Open drains simultaneously.
• Check all joints for drips.
• Listen for gurgling—if heard, you likely have a venting issue.

⚠️ Warning: If water drains slowly or you smell sewage, stop using the sink. You may need an air admittance valve (AAV) or dedicated vent.

Double Traps vs. Single Trap: Pros and Cons

In most residential kitchens, a single trap with a baffle T is simpler and safer. Reserve dual traps for specialized setups—and always consult a pro if unsure.

Common Mistakes to Avoid

• ❌ Using a standard T-fitting instead of a sanitary tee → causes siphoning.
• ❌ Installing traps too high or too low → violates trap seal depth (must be 2–4 inches).
• ❌ Ignoring venting requirements → leads to dry traps and odors.
• ❌ Over-tightening slip nuts → cracks plastic fittings.

According to a 2023 study by the National Association of Home Builders (NAHB), 32% of DIY plumbing failures in kitchen renovations were due to improper trap or vent installation.

FAQ Section

Q1: Can I use two P-traps under a double sink without a vent?

No. Without proper venting (either through a roof vent or an approved Air Admittance Valve), one trap will siphon the other, breaking the water seal and allowing sewer gas into your home.

Q2: What’s the difference between a sanitary tee and a regular T-fitting?

A sanitary tee has a sweeping 90° inlet that directs flow smoothly downward, preventing turbulence. A regular T creates a sharp 90° angle that disrupts flow and can siphon adjacent traps.

Q3: Do I need two garbage disposals for dual traps?

Not necessarily—but it’s strongly recommended. If only one bowl has a disposal, the other bowl’s drain may not align properly, and food debris can back up into the non-disposal side.

Q4: How far can the trap be from the sink drain?

Per IPC, the maximum horizontal distance from the sink’s flood rim to the trap weir is 24 inches. Keep it as short and direct as possible.

Q5: Will dual traps pass a home inspection?

Only if they meet local code. In many jurisdictions, inspectors require individual vents or an AAV for each trap. When in doubt, opt for a single-trap system—it’s universally accepted.

Q6: Can I install this myself, or should I hire a plumber?

If you’re comfortable with basic plumbing, measuring slopes, and reading local codes, you can DIY. But if your area requires permits or inspections, a licensed plumber ensures compliance and avoids costly rework.

Conclusion

Now you know exactly how to plumb a double kitchen sink with double traps—safely, legally, and effectively. While it’s a viable option for certain setups, remember that proper venting and the right fittings (like a dual sanitary tee) are non-negotiable. When done right, your dual-trap system will drain efficiently, prevent odors, and stand up to inspector scrutiny.

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