Installing Two Sinks With Only One Drain

A contemporary master bathroom with two sinks
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No matter how close a family is, more often than not, homeowners prefer their own sinks. Fortunately, if you have space, you can install a second sink without connecting a new drain or hammering through your wall.

Where there’s a will, there’s a way. With the right materials and determination, you can easily install a second sink and connect it to a current drain line.

Determining Your Sink’s Needs

New sinks generally involve one of two scenarios. The first is installing a brand-new bathroom vanity with two new sinks. The second involves adding a second sink to an existing vanity and connecting it to that drain. For the sake of this guide, we’re going to address the latter, which according to ImproveNet, should cost roughly $400.

Take into consideration your countertop material. Do you have laminate or stone? Laminate countertops are DIY-friendly. Before you consider trying to cut a granite, quartz or marble vanity top, call a professional. Dangerous dust particles and cracks are two potential run-ins professionals will prevent for you.

Materials Needed

  • New sink
  • Teflon tape
  • PVC pipe, primer, and cement
  • Assorted fittings
  • Connectors
  • Copper pipe
  • Lead-free solder
  • Copper bell hangers
  • Plumber's putty
  • Caulk

Tools Needed

  • Drill
  • Jigsaw or hacksaw
  • Hole saw
  • Propane torch
  • Tubing cutter
  • Wrench
  • Pliers
  • Bucket

Installing the Second Undermount Sink

Once you have your tools and supplies, you're ready to begin the installation. First, be sure to shut off the water supply in your home.

Take Your Measurements

  1. Measure the shape you need to cut on the existing countertop.
  2. Many sink manufacturers will provide an outline of the circle to cut. This is dependent on the style of your new sink, whether it’s drop-in, vessel or something else.
  3. For all under-mount sinks, the outline of the new sink needs to sit on top of the counter. Therefore, your new sink hole should have a diameter of 1-3 inches less than your new sink. Once measured, drill a pilot hole in the center and then use your jigsaw or hacksaw to cut the outlined hole.
  4. Place the new sink in the hole to ensure it fits. Then, place a bucket below the pipes to prevent spillage of any standing water.

Attach the Pipes

  1. Cut the existing PVC pipes that lead from your current sink to the drain. This makes way for a different PVC pipe that has two entries, one for each sink. Use the hacksaw to cut the existing pipe.
  2. Apply a connector for the new PVC pipes to the copper pipe in the wall. Place your new pipe on the connector, but do not fully attach yet. If your vanity has no cabinet walls, skip to step 6.
  3. Measure the height of the PVC pipe that leads to your new sink. If a cabinet vanity wall is present, drill holes all the way to the new sink. Attach all drain lines/tubes going from original sink to new sink.
  4. Attach all pipes from the drain to the sinks using your PVC cement. Apply cement on the inside and outside of pipes.
  5. Most plumbers like to add shut-off valves to each faucet. Other homeowners forgo adding additional pipes and have two shut-off valves (hot and cold) per sink. If you want separate shut-off valves, continue, if not, skip to step 4 of the next section. You can remove existing hot and cold-water shut-off valves with a wrench.
  1. Install copper tee fittings on both water lines. Clean the tee fittings before connecting.
  2. Solder between the copper pipes and the copper tee fittings and blowtorch them together. They should stick. If you do not feel comfortable using a blow torch, contact a local plumber. They should be able to complete this section of the project within 15 minutes.
  3. Connect copper pipes to the second sink.
  4. Measure the height and drill holes to the new sink. Then, attach copper pipes and tee fittings on the ends. Some homeowners install copper pipe hangers to the back of the cabinet. Essentially, these help hold the copper pipe in place. If you wish, drill the hangers in place.

Install the Sink

  1. Remove the sink from the hole.
  2. Before installing, attach the spouts and faucets. Take some plumber’s putty and wrap it under the faucets and spout. Put it in place and tighten nuts or bolts below.
  3. Apply caulk along the outer, bottom edge of the sink, where it will touch the countertop. Place the sink in position slowly and hold it in place for about 10 seconds. Then, apply some caulk along the outside edge and smooth with your finger. Use a damp rag to quickly remove any excess caulk on the vanity.
  4. Attach pipes, copper lines and faucets to the new sink. Turn the water supply back on and test the water.
  5. If at any point you could use a professional hand, reach out to a local plumber, who according to ImproveNet data, should not charge more than $300.