How to Plumb a Single Sink into a Double Sink: Easy Step-by-Step Guide

Converting a double sink cabinet to accommodate a single, larger basin is a common renovation goal, whether you are aiming for a spacious washing station or seeking to install a statement vessel sink. This process requires careful planning and precise execution to ensure proper drainage and a finished look that integrates seamlessly with your existing plumbing. Success hinges on understanding the core principles of redirection, not just the physical removal of fixtures.

Assessing Your Current Double Sink Configuration

Before any pipes are cut, you must evaluate the existing setup to determine the work required. Most modern double sinks connect to a shared tailpiece that then ties into a single waste line, while older installations might feature completely separate drains leading to a central tee-fitting. You also need to identify the water supply lines; even if the sink is not currently used for dishes or laundry, the valves under the countertop will likely control both basins.

Tools and Materials You Will Need

Gather the necessary tools before starting to avoid mid-job frustration. You will need an adjustable wrench, channel-lock pliers, a basin wrench for the confined space under the sink, a utility knife, and a tape measure. For materials, plan on purchasing a new single-configuration drain assembly, a length of PVC pipe to replace the removed tailpiece, PVC cement, a new supply line if the valves are too far apart, and plumber’s tape to ensure a tight seal.

Removing the Old Drain Assembly

Start by placing a bucket under the pipes to catch residual water. Disconnect the flexible supply lines from the faucets, unscrew the nuts securing the P-trap to the tailpiece, and remove the entire assembly. If your current setup has two distinct drain lines, you will need to cap or remove one; if it is already a shared line, you can proceed to modify the tailpiece length. Keep the old wax ring and supply lines on hand only if you are reinstalling an old faucet; otherwise, they will be discarded.

Modifying the Tailpiece and Waste Line

The critical step in how to plumb a single sink into a double sink is altering the tailpiece to the correct height. The goal is to ensure the new drain sits directly over the waste tee without stressing the connections. Cut the existing tailpiece down to the appropriate length so that the new trap assembly can hang freely without being pulled down by gravity. Use a tubing cutter for a clean edge, and sand off any burrs before applying PVC cement to secure the new pipe firmly into the existing waste tee.

Connecting the New Drain Assembly

Once the waste line is configured, attach the new single-configuration drain. Apply a thin layer of plumber’s tape to the threads of the tailpiece to prevent leaks, then screw on the new tailpiece nut and the trap adapter. Hand-tighten these components initially, then use the basin wrench to secure them tightly. The trap should be positioned to allow for easy disassembly later, so avoid packing it down too tightly against the cabinet.

Adjusting the Water Supply

With the drain successfully installed, turn your attention to the faucets. If the existing valves are spaced for two fixtures, you will need to run a single supply line from the valves to the faucet. Use compression fittings to connect a length of braided stainless steel tubing to the hot and cold valves, routing them down the center of the cabinet to the single mounting holes. If the valves are too close together, you might need to install a simple splitter bridge to combine the lines into one feed.

Testing for Leaks and Final Adjustments

Before securing the sink to the countertop, turn the water supply back on and run the faucet slowly. Check all connections—the supply line valves, the trap joints, and the seam where the sink meets the countertop—for any signs of moisture. Tighten any loose fittings immediately, but avoid over-tightening chrome or plastic components, as this can cause cracks. A successful installation will provide a steady stream of water with zero drips and a waste flow that vanishes without gurgling or backup.