How Double Sink Plumbing Is Different—and How to Install It

Installing a double sink provides convenience and flexibility in a kitchen or bathroom. With two sinks, two people can use the space at once without crowding each other. But installing plumbing for a double sink requires some differences from a single sink. Understanding how double sink plumbing works and the right way to install it will ensure your project goes smoothly.

How Double Sink Plumbing Is Different

While a double sink may simply seem like two single sinks side-by-side, the plumbing involved makes it more complex. Here are some key ways a double sink plumbing system differs from a single sink setup:

Two Drain Lines

With two sinks, you’ll need two drain lines running from the sinks to the main drain pipe. This allows both sinks to drain properly. The drain lines may join together into one pipe before connecting to the main drain.

More Supply Lines

Each sink will need its own hot and cold water supply lines. So a double sink needs four supply lines compared to two for a single sink. This provides water independently to each basin.

Two P-Traps

The distinctive U-shaped pipe under a sink is called a P-trap. A double sink requires one for each basin to prevent sewer gases from coming up through the drains.

Proper Venting

Vents allow air into the plumbing system so water can drain smoothly. A double sink may require adding a vent or upgrading to a larger one to handle the additional drainage capacity.

Wider Drain Pipe Capacity

With two sinks draining into one pipe, the main drain line needs to be wider than for a single sink. This prevents water from backing up when both sinks are used at once.

Shared Supply Lines

Some double sink plumbing setups have hot and cold supply lines that tee off to each sink basin from a shared main line for that water temperature. Others run separate lines from the water source to each sink.

How to Install Double Sink Plumbing

Installing double sink plumbing takes planning and careful execution. Follow these key steps for proper installation:

Step 1: Turn Off Water Supply

Start by turning off the main hot and cold water supply lines and turning on the faucets to depressurize the lines. This prevents water from spraying out during work.

Step 2: Disconnect Existing Sink

Remove supply lines, drain pipes, and P-trap from the existing sink if replacing an old one. Cap the ends of remaining pipes.

Step 3: Install Drain Lines

Attach drain pipes to each sink basin, using a slip nut or compression fitting. Run pipes into a single shared P-trap. Connect horizontal pipes to the main drain line.

Step 4: Install Vents

Add a vent pipe from the shared drain line to equalize air pressure if needed. Connect it to an existing vent stack if possible.

Step 5: Run Supply Lines

Install hot and cold water supply pipes from the main lines up to each sink basin. Make connections with compression fittings.

Step 6: Anchor Sink

Secure the sink base into place according to manufacturer instructions. Seal edges with silicone caulk.

Step 7: Connect Fixtures

Attach water supply lines to the faucets, sprayers, soap dispensers or any other plumbing fixtures.

Step 8: Test Plumbing

Turn water back on and check for leaks. Make sure each sink drains smoothly without gurgles. Adjust as needed.

Step 9: Finish Installation

Complete the job by attaching countertop material, backsplash, sink accessories like strainers, and any other finishing touches.

Common Problems and Solutions for Double Sink Plumbing

Even with proper installation, issues can come up with double sink plumbing. Here are some potential problems and how to fix them:

Slow draining: Clogged pipes, venting issues or insufficient drain slope can impede drainage. Try snaking drain pipes or using a drain cleaner first before replacing pipes.

Leaky pipes: Loose fittings, cracked pipes or deteriorated washers are common causes. Tighten fittings or replace cracked sections. Replace worn out washers.

Low water pressure: Shared supply lines, old galvanized pipes or high sediment buildup can reduce flow. Replace galvanized pipes. Run dedicated lines. Flush sediment from lines.

Dripping faucets: Worn washers, seats or O-rings allow water to leak past. Disassemble faucet and replace worn parts. Replace entire faucet if corrosion is too severe.

Noisy pipes: Water hammer causes banging noises when faucets turn off. Install water hammer arrestors near problem areas.

Backflow into other sink: Drained water from one sink entering the other points to improper venting. Add or upgrade vent pipes according to plumbing code.

Foul odors: Dry P-traps allow sewer gases in. Add water to fill traps. Replace broken or cracked traps allowing gas leakage.

Clogged drains: Hair, grease and other debris gets stuck in drains. Try a drain snake or powder cleaner first. Take apart pipes to fully remove severe clogs.

Tips for a Successful Double Sink Plumbing Installation

Installing double sink plumbing takes some extra planning and work compared to a single sink. Keep these tips in mind for the best results:

  • Carefully measure sink spacing to ensure drain and supply lines will reach properly.
  • Check that existing water supply and drains have adequate capacity for a double sink.
  • Draw out plans for the entire plumbing system including drains, vents and supply lines.
  • Follow all applicable building codes and plumbing standards for your area.
  • Hire a professional plumber if you lack the expertise and tools to install double sink plumbing properly.
  • Use high quality pipes, fittings, washers and other parts less prone to leaks and failure.
  • Thoroughly clean out supply lines before connecting sinks to prevent sediment buildup.
  • Test all new plumbing thoroughly and fix any leaks or drainage issues immediately.

Frequently Asked Questions About Double Sink Plumbing

How much space do I need between sinks in a double vanity?

Most plumbing codes require at least 30-35 inches between the center lines of each sink basin. This leaves room for one person to stand comfortably at each sink.

Should I use shared or dedicated supply lines?

Dedicated lines provide the best water pressure and temperature control. But shared hot and cold mains that split off to each sink work for many residential homes.

What size drain pipe do I need for a double sink?

Typically 1-1/2 to 2 inches is sufficient, but check your plumbing code. Pipe size depends on total drainage fixture units for your bathroom or kitchen.

Can I use the existing drain for a double sink?

You can if it’s at least 1-1/2 inches in diameter and has the capacity for two sink drains. Otherwise, you’ll need to replace it with a wider pipe.

How difficult is it to install double sink plumbing?

For an experienced DIYer, it’s an intermediate task requiring planning and attention to detail. Hiring a pro is best for inexperienced plumbers or complex installs.


While it requires careful planning and installation, a double sink provides great convenience in kitchens, bathrooms and other spaces. Understanding the nuances of double sink plumbing allows you to tackle installation with proper drainage, water supply and functionality. Follow the steps outlined here coupled with adherence to plumbing codes and best practices. With some patience and attention to detail, you can enjoy a stylish and highly useful double vanity sink setup.