How to Plumb a Single to Double Bathroom Sink Conversion

Switching from a single to a double bathroom sink sounds complicated but you can do it yourself. After the fun part of choosing your new sinks and vanity, it’s a matter of tying together 2 sinks into a one main drain. You’ll also make sure to fit dual shut-off valves in with your water supply lines. 

Step 1 – Shut off Water 

Shut off the water with the shut-off valves under the sink. 

Step 2 – Remove the Existing Vanity 

Loosen the caulk that is between the wall and the vanity by using a utility knife. Carefully remove the vanity countertop so that you don’t damage the wall. Remove the screws or nails that hold the old vanity to the wall. Dispose of it. 

Step 3 – Turn off the Main Water 

Turn off all water going to the house with the main shut-off valve. For added safety, turn off the shut-off valves on top of your water heater. 

Step 4 – Drain 

Find the lowest faucet in the house and drain the water. Once that is complete, you can remove the old bathroom vanity shut-off valves and dispose of them. Put in new double-outlet valves. You do this by slotting in the supply pipe into the valve, making sure to pass the nut and compression washer over the pipe. Tighten that with a wrench. Do the same thing again for the hot and cold lines. 

Step 5 – Turn Main Supply On 

Turn the main supply back on and make sure not to forget the water heater valves as well. Check your new plumbing for leaks. If you find one, tightening the compression nut a bit more should do the trick. 

Step 6 – Install Faucet and Drain 

Before you install the new vanity top, install the faucet and drain assembly to it first. You attach the flexible supply lines by using your wrench to tighten the compression nuts to the threaded posts that are on the faucet. Put the faucet through the pre-drilled holes in the vanity top; tighten the nut included to keep it in place. Put in the drain assembly that comes with the faucet by applying plumber’s putty under the rim of the drain trim. Push it down through the top of the drain hole. Next, tighten the nut from underneath the vanity until the putty oozes out on all sides. This seals the drain to prevent leaks. Get rid of excess putty by scraping it off. 

Step 7 – Screw the Tailpiece 

Now you can screw in the tailpiece that comes from the drain assembly to the drain pipe. Tighten it. Put in the lift lever and the stopper by lining up the lift lever through the hole in the rear of the tailpiece. Place it into the hole in the stopper.

Step 8 – Install the Vanity 

Put in your new vanity. Secure it to the wall studs. Don’t forget to check to make sure the top is level and that all valves are clear of any drawers or other obstacles in the vanity. 

Step 9 – Silicone 

Apply a bead of silicone along the top of the vanity cabinet and set the sink on top. 

Step 10 – Hot and Cold Shut-off Valves 

Connect the supply lines to the hot and cold shut-off valves. Tighten the compression nuts with your wrench. 

Step 11 – Hook Up 

Connect the new drain assembly to the old drain pipe. Use a t-connector and a new p-trap. Tighten. 

Step 12 – Test 

Turn the shut-off valves back on. Keep your eye out for leaks. Turn on the faucet and check for any leaks from the drain line. Fill the sink up with water and let that all drain so that you can check for leaks under pressure. 

Step 13 – Caulk 

Apply caulk to the sides and back of the vanity top to the wall. Let it dry for 24 hours.