Tired of waiting your turn at the sink? Whether it’s a cramped kitchen or a shared bathroom, upgrading from one sink to two can dramatically improve daily convenience—and even boost your home’s resale value. But how to change plumbing from one sink to two without causing leaks, code violations, or costly mistakes? You’re not alone. Many homeowners hesitate because they assume it’s too complex. The good news? With the right tools, planning, and this step-by-step guide, you can tackle this project confidently—even as a beginner.
Why Would You Add a Second Sink?
Before diving into pipes and fittings, it’s worth understanding why this upgrade matters. According to the National Association of Home Builders (NAHB), 68% of homebuyers consider dual sinks in the master bathroom a “must-have” feature. In kitchens, a second prep sink can streamline cooking and cleanup, especially in open-concept homes.
Common reasons include:
Reducing morning bottlenecks in shared bathrooms
Adding a beverage or prep sink in the kitchen island
This is the #1 question DIYers overlook. Not every plumbing system is ready for an extra fixture. Here’s what to check:
1. Water Supply Lines
Most residential sinks use ½-inch supply lines. To support two sinks, you’ll typically tee off the existing hot and cold lines using a ½” x ½” x ½” brass tee fitting. Ensure your main water pressure is at least 40–60 PSI—standard for U.S. homes. You can test this with a $10 pressure gauge from any hardware store.
2. Drain System Capacity
The drain must handle double the wastewater without clogging. The International Plumbing Code (IPC) requires:
1¼-inch drain for lavatory sinks
1½-inch for kitchen sinks
If your current drain is undersized or shares a line with a toilet or shower, consult a plumber. Overloading drains can cause slow drainage or sewer backups.
💡 Pro Tip: Always check local building codes. Some municipalities require permits for plumbing modifications. Visit your city’s official website or call the building department before starting.
Tools & Materials You’ll Need
Gather these before you begin (most available at Home Depot or Lowe’s):
⚠️ Warning: Always shut off the water supply at the main valve before cutting into any line. Open faucets to relieve pressure.
Step-by-Step: How to Change Plumbing from One Sink to Two
Follow these steps carefully. Rushing leads to leaks!
Step 1: Plan Your Layout
Measure the distance between sink centers. The standard is 30–36 inches apart for comfort. Mark locations on the countertop or wall. Ensure both sinks align with cabinet openings and don’t interfere with drawers.
Step 2: Install New Shut-Off Valves
Turn off main water supply.
Cut the existing hot and cold supply lines near the original shut-off valve.
Solder or push-fit a ½” tee onto each line.
Attach new angle stop valves (¼-turn type recommended) to the tee’s open port.
Reconnect the original sink’s supply lines to the tee’s other outlet.
🔧 Note: Use two wraps of Teflon tape clockwise on all threaded connections to prevent leaks.
Step 3: Run Supply Lines to the Second Sink
Connect new braided stainless steel supply lines (typically 20″ long) from the new shut-off valves to the second sink’s faucet inlets. Hand-tighten, then give a quarter-turn with a wrench—do not overtighten.
Step 4: Modify the Drain System
Remove the existing P-trap.
Install a dual-outlet drain tee (also called a “double sanitary tee”) under the countertop.
Connect both sink tailpieces to the tee using slip nuts and washers.
Reattach the P-trap to the tee’s outlet, leading to the wall drain.
📏 Critical Detail: Maintain a ¼-inch per foot slope toward the main drain to ensure proper flow. Use a level to verify.
Step 5: Test for Leaks
Slowly turn on the main water supply.
Open both faucets and check all connections.
Place paper towels under joints—any dampness = leak.
Run water for 2–3 minutes to test drain flow. No gurgling or standing water?
If all looks good, proceed to caulk and finalize.
Step 6: Secure & Finish
Apply silicone caulk around the base of both sinks to prevent water intrusion.
Reinstall cabinet doors or access panels.
Clean up and celebrate—you just doubled your sink power!
✅ Increases home value (up to 3–5% in master baths)
❌ May need permits in some areas
✅ Minimal cost if DIY ($100–$250 in materials)
❌ Risk of leaks if done incorrectly
✅ Customizable (e.g., add filtered water or soap dispenser)
❌ Countertop modification may be needed
Common Mistakes to Avoid
Ignoring venting: Each sink needs proper venting to prevent siphoning. If your original sink was vented, the second usually shares the same vent—but confirm with local code.
Using cheap plastic fittings: Brass or stainless steel lasts longer and resists corrosion.
Skipping the pressure test: Always test before closing up walls or cabinets.
Misaligning drains: Uneven sinks cause poor drainage and odor issues.
FAQ Section
Q1: Do I need a plumber to add a second sink?
A: Not necessarily. If you’re comfortable with basic plumbing and your system can support it, DIY is feasible. However, if you’re rerouting pipes inside walls or adding vents, hire a licensed plumber.
Q2: Can I add a second sink in a bathroom with only one set of supply lines?
A: Yes—by installing tee fittings on the existing hot and cold lines, as described above. Just ensure your water heater can handle the extra demand (most standard 40–50 gal tanks can).
Q3: How much does it cost to add a second sink?
A: DIY: $100–$250 for materials. Professional installation: $400–$1,200, depending on complexity and location (HomeAdvisor, 2024 data).
Q4: Will two sinks slow down water pressure?
A: Only if your main line is undersized (<¾ inch) or you run both at full blast simultaneously. In practice, most households won’t notice a difference.
Q5: Can I use the same P-trap for two sinks?
A: No. Plumbing code requires each fixture to have its own trap arm, but they can share a common P-trap outlet via a double sanitary tee—not by connecting both sinks to one trap.
Q6: What if my countertop isn’t pre-cut for two sinks?
A: You’ll need to cut a new opening. Use a template from your sink manufacturer and a jigsaw with a carbide blade. Seal edges thoroughly to prevent water damage.
Conclusion
Knowing how to change plumbing from one sink to two empowers you to enhance functionality, comfort, and value in your home—without breaking the bank. With careful planning, the right materials, and attention to code requirements, this project is well within reach for motivated DIYers.
If this guide saved you time or stress, share it with a friend who’s renovating their kitchen or bathroom! And don’t forget to pin it on Pinterest or tweet it—your next home upgrade could inspire someone else’s.
🛠️ Remember: When in doubt, consult a professional. A $100 consultation can prevent a $1,000 mistake.
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