Why Rough In Plumbing for Back-to-Back Sinks Matters (And Why Most People Get It Wrong)
You’re installing two sinks — maybe in a dual-purpose bathroom, a large kitchen island, or a commercial prep station — and they’re positioned back-to-back. Sounds simple, right?
But here’s the truth: rough in plumbing for back-to-back sinks is one of the most commonly misjudged tasks in residential and light commercial construction. Get it wrong, and you’ll face slow drainage, foul odors, code violations, or even water damage behind the walls.
If you’re a contractor, DIYer, or homeowner planning a renovation, this guide gives you the exact steps, measurements, and code insights you need to get it right — the first time. No guesswork. No callbacks. Just clear, proven plumbing logic.
What Exactly Is “Rough In Plumbing” for Back-to-Back Sinks?
“Rough in” refers to the initial installation of water supply lines and drainpipes before walls are closed up, fixtures are mounted, or finishes are applied. For back-to-back sinks, this means coordinating two separate sink drain and supply systems through a shared wall — often with limited space.
According to the International Residential Code (IRC), every sink must have:
A dedicated trap (P-trap)
A vent within 6 feet of the trap
A minimum 1.5-inch drain pipe (2 inches for kitchen sinks)
Proper slope (¼ inch per foot)
The challenge? You’re not just installing two independent sinks — you’re creating a shared system that must function as one cohesive unit.
“A poorly vented back-to-back sink setup is one of the top 3 causes of drain backups in new builds.” — Mike Holmes, Master Plumber & TV Host
Can You Share a Drain Between Two Back-to-Back Sinks? (Spoiler: Yes — But Only If You Do It Right)
Many assume you can simply tie both sinks into one drain line. You can — but only under strict conditions.
✅ Allowed: Dual Sinks on a Single Drain Line (With Proper Venting)
Both sinks must be on the same side of the wall (e.g., both in bathrooms facing each other).
Use a double fixture fitting (also called a “wye and 1/8 bend” or “combination fitting”) to join the two drains.
The combined drain must be at least 2 inches in diameter if one sink is a kitchen sink.
Venting is mandatory — and must be within 6 feet of both traps.
❌ Not Allowed:
Tying two sinks into a 1.25-inch pipe (too small — code violation).
Using a single P-trap for both sinks (illegal — creates siphon risk).
No vent at all (leads to gurgling and sewer gas entry).
⚠️ Pro Tip: Always use ABS or PVC for drain lines in residential applications. Avoid galvanized steel — it corrodes fast.
Step-by-Step: How to Rough In Plumbing for Back-to-Back Sinks (With Measurements)
Follow this sequence. Do it in order. Skip a step, and you’ll regret it later.
Step 1: Plan Your Wall Layout (Before You Cut Anything)
Measure the center-to-center distance between the two sink basins. Standard spacing: 30–36 inches.
Mark the exact location of each sink’s drain outlet on the floor plan.
Confirm the wall thickness — standard 2×4 wall = 3.5” space. You need at least 4” clearance for pipes and fittings.
Step 2: Install the Main Drain Line (Minimum 2” Diameter)
Run a 2-inch PVC or ABS drain pipe vertically through the wall from the floor to the ceiling (to connect to the main stack).
Slope it ¼ inch per foot toward the main stack. → Example: If the pipe runs 6 feet, it must drop 1.5 inches total.
Step 3: Add the Double Fixture Fitting
At the point where the two sinks will connect, install a double sanitary tee or double wye fitting.
This fitting has two inlet ports (one for each sink) and one outlet to the main drain.
Angle the inlets at 45 degrees to reduce turbulence and clogs.
🔧 Pro Tool Tip: Use a pipe cutter and primer/glue rated for DWV (Drain-Waste-Vent) systems. Don’t use “all-purpose” PVC cement — it’s not code-compliant.
Step 4: Install Individual P-Traps and Tailpieces
Each sink gets its own P-trap (minimum 1.5” for bathroom sinks, 2” for kitchen).
Connect each trap to its sink’s tailpiece with flexible tailpiece adapters (easier to align).
Keep the vertical drop from sink to trap under 24 inches to prevent airlock.
Step 5: Venting — The Most Overlooked Part
You cannot skip this.
Install a wet vent or individual vent within 6 feet of each trap.
Best practice: Run a 1.5-inch vent pipe vertically from the double fitting up to the main vent stack.
If the roof is too far, use an air admittance valve (AAV) — but only if your local code allows it (most U.S. states do under IRC 909).
Slope too little (flat) or too steep (1/2”+) — causes separation of solids and water
FAQ: Your Top 6 Questions About Rough In Plumbing for Back-to-Back Sinks
Q1: Can I use a single P-trap for two back-to-back sinks?
No. A single P-trap serving two sinks is a code violation (IRC 2708.1). Each sink must have its own trap to prevent cross-contamination and siphoning. If you tie them into one trap, wastewater from one sink can be sucked out when the other is used — leaving the other trap dry and allowing sewer gas into your home.
Q2: How far apart should the sinks be for rough-in?
Standard spacing is 30 to 36 inches center-to-center, depending on sink size. For a kitchen island with double sinks, 36” gives enough room for prep and cleanup. Always check your sink manufacturer’s cutout dimensions — some require up to 38” for proper installation.
Q3: Do I need a separate vent for each sink?
Not necessarily. You can use a common vent (one pipe serving both traps) — but it must be installed within 6 feet of both P-traps and be at least 1.5 inches in diameter. If the distance is too far, install an air admittance valve (AAV) near each sink — a legal and space-saving alternative in most U.S. jurisdictions.
Q4: Can I run the drain horizontally under the floor instead of through the wall?
Yes — but only if the floor is framed with adequate clearance. You’ll need at least 6 inches of vertical space below the subfloor for the 2” drain pipe and slope. Running under the floor can be easier for basements or slab-on-grade homes — but requires careful planning to avoid joist interference.
Q5: What’s the best material for rough-in pipes?
For drains: PVC or ABS (easy to cut, glue, and inspect). For water supply: PEX (flexible, freeze-resistant, affordable) or copper (longer lifespan, more expensive). Avoid galvanized steel — it rusts internally and reduces water flow over time.
Q6: Do I need a permit for rough-in plumbing?
Yes. Every U.S. jurisdiction requires a plumbing permit for rough-in work. Skipping it can void your home insurance and cause major issues during resale. Always schedule a pre-drywall inspection before closing walls.
Peace of mind knowing your system will last 20+ years
Don’t cut corners. Don’t guess. Use the steps above — and if you’re unsure, consult a licensed plumber before closing the walls.
💡 Pro Move: Take a photo of your rough-in before drywall goes up. Save it on your phone. Future homeowners or plumbers will thank you.
Found This Helpful? Share It!
If you’re a contractor, DIYer, or homeowner tackling a double-sink project, share this guide with your team or friends. Plumbing mistakes are expensive — but knowledge is free.
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