Need more sink space but only have one drain and water supply? You’re not alone. Many homeowners—especially in older kitchens or compact bathrooms—wonder how to split plumbing from one sink to two to boost functionality without a full remodel. Whether you’re adding a second basin for convenience or upgrading your vanity, this guide walks you through the process safely, legally, and efficiently. No guesswork—just clear, actionable steps backed by plumbing best practices.
Why Would You Split One Sink into Two?
Before diving into the “how,” it’s worth understanding the “why.” Common reasons include:
- Double vanity setups in master bathrooms for couples
- Kitchen efficiency (e.g., one sink for washing, one for prep)
- Rental property upgrades to increase appeal
- Accessibility needs, such as adding a lower sink for children or mobility aids
According to the National Association of Home Builders (NAHB), over 68% of new homes now include dual sinks in primary bathrooms—a trend driven by both convenience and resale value.
Can You Legally Split Plumbing from One Sink to Two?
Yes—but with caveats. Plumbing codes vary by state and municipality, but most follow guidelines from the International Plumbing Code (IPC). Key rules include:
- Each fixture must have its own P-trap (no double-trapping)
- Drain lines must maintain proper slope (¼ inch per foot)
- Water supply lines must support adequate pressure for both fixtures
⚠️ Important: Always check with your local building department. Some areas require permits for even minor plumbing changes. Ignoring codes can lead to failed inspections or costly repairs later.
For more on plumbing standards, refer to the International Plumbing Code overview on Wikipedia .
Tools & Materials You’ll Need
Before starting, gather these essentials:
| Tools | Adjustable wrench, pipe cutter, Teflon tape, bucket, level, drill |
| Materials | ½-inch PEX or copper supply lines, PVC or ABS drain pipes, double sanitary tee, two P-traps, shut-off valves, sink tailpieces |
| Safety Gear | Gloves, safety goggles, towel (for leaks) |
💡 Pro Tip: Use PEX tubing for supply lines—it’s flexible, freeze-resistant, and easier to install than copper for DIYers.
Step-by-Step: How to Split Plumbing from One Sink to Two
Follow these steps carefully. Rushing can cause leaks or code violations.
Step 1: Turn Off Water & Drain Lines
- Shut off the main water supply.
- Open the existing faucet to drain residual water.
- Place a bucket under the P-trap to catch debris.
Step 2: Remove the Existing Sink
- Disconnect supply lines and drain.
- Carefully detach the sink from the countertop or wall.
Step 3: Plan Your New Layout
- Measure the distance between the two new sink basins (standard is 30–36 inches apart).
- Ensure both will align with the existing drain stack and water supply.
Step 4: Install New Supply Lines
- Use a ½-inch tee fitting to split the hot and cold lines.
- Run separate lines to each sink’s faucet.
- Wrap all threaded connections with Teflon tape (3–5 wraps clockwise).
🔧 Precision Note: Maintain at least 40 PSI water pressure at both faucets. If pressure drops, consider upgrading your main line or using a pressure booster.
Step 5: Reconfigure the Drain System
- Replace the single P-trap with a double sanitary tee connected to a shared vertical drain stack.
- Install individual P-traps for each sink—never share one trap (this violates code and causes sewer gas leaks).
- Slope all horizontal drain pipes at ¼ inch per foot toward the main stack.
Step 6: Test for Leaks
- Turn water back on slowly.
- Check all connections for drips.
- Run water in both sinks simultaneously for 2–3 minutes to test drainage.
🚫 Red Flag: Gurgling sounds or slow drainage mean improper venting or slope—address before sealing everything up.
Common Mistakes to Avoid
Even experienced DIYers slip up. Watch out for:
- Double trapping: Using one P-trap for two sinks → sewer gases enter your home.
- Ignoring venting: Each fixture needs proper venting to prevent siphoning.
- Over-tightening fittings: This cracks plastic or strips threads. Hand-tight + 1/4 turn with a wrench is enough.
- Skipping permits: Fines or forced rework can cost 3x more than a permit.
Pros vs. Cons of Splitting One Sink into Two
| ✅ More functional space | ❌ Requires plumbing knowledge |
| ✅ Increases home value | ❌ May need permits |
| ✅ Cost-effective vs. full remodel | ❌ Risk of leaks if done incorrectly |
| ✅ Customizable layout | ❌ Not all spaces accommodate dual sinks |
💬 Expert Insight: “Splitting plumbing is doable for confident DIYers, but if your home has galvanized pipes or complex venting, hire a licensed plumber,” says Maria Lopez, Master Plumber with 18 years of experience in California.
When to Call a Professional
Consider hiring a pro if:
- Your home was built before 1980 (lead pipes or outdated materials)
- You’re unsure about local venting requirements
- The existing drain is clogged or corroded
- You lack confidence with soldering or PEX crimping
The average cost to install a dual sink ranges from $400–$1,200, including labor (HomeAdvisor, 2024). But a botched DIY job could cost $2,000+ in water damage repairs.
FAQ: How to Split Plumbing from One Sink to Two
Q1: Can I use the same drain for two sinks?
A: Yes—but only if each sink has its own P-trap and they connect via a double sanitary tee to a single vertical drain line. Never connect two sinks to one trap.
Q2: Do I need a separate vent for each sink?
A: Not always. If both sinks are within 5 feet of a common vent stack and properly sloped, one vent may suffice. Check your local code—some states (like Florida) require individual vents.
Q3: What size pipe do I need for the water supply?
A: Standard residential sinks use ½-inch supply lines. Splitting with a tee maintains adequate flow, as long as your main line is ¾-inch or larger.
Q4: Will splitting plumbing reduce water pressure?
A: Slightly—but not noticeably if your system is healthy. If pressure drops below 40 PSI, install a pressure-balancing valve or consult a plumber.
Q5: Can I do this in a rental property?
A: Only with landlord approval. Unauthorized plumbing changes can void your lease or insurance.
Q6: How long does the project take?
A: A skilled DIYer can complete it in 4–6 hours. Beginners should allow a full day, especially if cutting into walls or floors.
Conclusion
Learning how to split plumbing from one sink to two empowers you to upgrade your space affordably and efficiently—without sacrificing safety or code compliance. With the right tools, planning, and respect for local regulations, you can enjoy the convenience of dual sinks in your kitchen or bathroom.
✅ You’ve just saved hundreds on labor
✅ Boosted your home’s functionality
✅ Gained confidence in DIY plumbing
If this guide helped you, share it with a friend tackling a similar project! Got questions? Drop them in the comments below—we’re here to help you succeed.
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