How to Add a Second Bathroom Sink

A contemporary master bathroom with two sinks
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Overview
  • Total Time: 3 hrs
  • Skill Level: Intermediate
  • Estimated Cost: $50 (not including sink and faucet)

For busy households, a vanity with double sink basins can be a lifesaver. When you're starting with a single-sink vanity, adding a second sink typically involves one of two scenarios: Either you are installing an all-new countertop (presumably a top with two basin cutouts) or you are installing a second sink alongside the first sink by cutting an opening in the countertop and adding the water supply and drain lines to serve the new sink. The second scenario is outined in this project. The first scenario calls for similar plumbing drain and water supply hookups but does not involve the sink cutout.

Are Your Sink and Countertop Suitable?

Before you decide to tackle this project on your own, take into consideration your sink type and countertop material. Laminate, solid-surface, or wood counters are fairly DIY-friendly when it comes to cutting an opening for a second sink. But cutting a granite, quartz, or marble vanity top is almost always a job for a professional, as these materials require specialty tools and techniques. Dangerous dust particles and cracking are two potential issues that professionals know how to deal with.

Cutting laminate, solid surface (such as Corian), and wood can be done with a drill and a jigsaw. A jigsaw makes a fairly rough cut, but if you're installing a drop-in sink, the cut edge won't be visible. On the other hand, if you want to install an undermount sink with a solid surface or wood top (undermounts don't work with laminate), the sink opening must be cut perfectly and receive a profiled edge, and it must be polished to match the rest of the countertop. This requires advanced techniques with a router and other woodworking tools.

What You'll Need

Equipment / Tools

  • Tape measure
  • Marker
  • Drill and 3/8- or 1/2-inch bit
  • Jigsaw with fine-tooth blade
  • Tongue-and-groove pliers
  • Adjustable wrench
  • Caulk gun
  • Bucket
  • Hacksaw

Materials

  • New sink with faucet and drain assembly
  • Two flexible water supply tubes
  • Plumber's putty
  • Silicone caulk
  • 2 PVC drain trap assemblies, with elbows and extensions (as needed)
  • Drain tee fitting
  • 2 dual-outlet fixture shutoff valves

Instructions

  1. Mark the Hole for the Sink

    Measure the underside of the sink to determine the size and shape of the hole in the countertop for dropping in the sink. Sink manufacturers often provide a paper or cardboard template that you can use to mark the countertop for cutting. Proper alignment and spacing in relation to the existing sink are critical here—make sure there is the proper amount of space between the basins, and that they are positioned with the same front-to-back spacing on the countertop. Mark a cutting outline for the hole, using a marker.

    If you want to use the same drain trap for both sinks, as shown here, there should be no more than 30 inches between the sinks. To ensure comfortable use for both sinks, there should be at least 24 inches of space between the sinks.

  2. Cut out the Sink Hole

    Drill a hole through the countertop inside the marked outline, using a 3/8- or 1/2-inch drill bit (it must be large enough to fit a jigsaw blade). Set the jigsaw onto the countertop, inserting the blade into the hole, then make the cutout with the saw. Test-fit the new sink to make sure it fits.

  3. Mount the Faucet

    Mount the faucet on the sink. It is generally easiest to install the faucet and drain fitting in the sink before inserting it in the cutout opening. This involves inserting the faucet tailpieces through the holes on the sink, then threading the mounting nuts onto the tailpieces from the underside of the sink. With some faucets, you will apply a thin layer of plumber's putty between the base of the faucet and the sink. Other faucets have a foam or rubber gasket that makes putty unnecessary. Tighten the mounting nuts with tongue-and-groove pliers.

  4. Add the Water Supply Tubes

    Attach flexible water supply tubes to the hot and cold tailpieces on the faucet. Make sure the tubes are long enough to reach from the sink location to the existing shutoff valves below the countertop. Tighten the supply tubes to the tailpieces using an adjustable wrench.

  5. Install the Drain Assembly

    Install the drain fitting from the drain assembly onto the sink, following the manufacturer's instructions. With most drains, a bead of plumber's putty is placed around the drain opening, then the drain assembly is inserted into the drain opening, and a mounting nut is threaded onto the fitting's tailpiece from below the sink. Use tongue-and-groove pliers to tighten the tailpiece down securely. Some tailpieces and mounting nuts are made with plastic, so take care not to break the parts by overtightening.

    Wipe away any plumber's putty that has squeezed out from around the drain fitting flange in the bottom of the sink.

  6. Install the Sink

    Insert the sink into the countertop cutout, following manufacturer's instructions. This usually means applying a bead of silicone caulk around the cutout opening then lowering the sink into place. Some styles of sink may have brackets that are installed below the sink to hold it in place against the countertop. If necessary, run a bead of caulk around the seam between the rim of the sink and the countertop. Wipe away any excess caulk using a damp rag.

  7. Disassemble the Old Sink Drain

    Adapt the old sink's drain trap configuration so it can accept two sinks. Where the sinks are 30 inches or less apart, they can be served by the same P-trap.

    Place a bucket under the P-trap on the old sink, then disconnect the drain pipes and drain trap, using tongue-and-groove pliers. In some cases, you may need to cut the pipes free using a hacksaw. Remove the tailpiece extension that drops down from the existing sink as well as the P-trap and the trap arm that runs into the drain opening in the wall.

    Most bathroom sinks use 1 1/4-inch trap parts, but you may find some with 1 1/2-inch drains. Various transition fittings may be necessary, depending on your situation.

  8. Assemble the New Drains

    Assemble the pieces for linking both sink drains to a single P-trap. Typically, the drain trap configuration includes 90-degree elbows on the bottom of each drain tailpiece; these meet at a tee-fitting between the sinks. From the tee, a short pipe leads to the P-trap. The outlet on the P-trap is joined to the drain opening in the wall with a trap arm pipe.

    Make sure the all horizontal pipe sections pitch downward (in the direction of the water flow) at a slope of about 1/4 inch per foot. This will ensure proper drain flow. You may need to make adjustments at this point, possibly cutting off the drain tailpieces or adding extensions to create the proper downward slope.

    Once the PVC drain pipes are satisfactory, tighten all slip-joint connections using tongue-and-groove pliers.

  9. Remove the Old Shutoff Valves

    Replace the existing fixture shutoff valves with double-outlet shutoff valves that can feed both sink faucets. Start by shutting off the water at the home's main shutoff valve. Place a bucket under the water supply valves below the existing sink.

    Disconnect the supply tubes serving the existing sink from the shutoff valves, using pliers or an adjustable wrench. This may require two wrenches; one to hold the valve, the other to loosen the mounting nut on the supply tube.

    Remove the fixture shutoff valves from the water supply stub-outs. Normally this is done by loosening the compression fitting nuts with a wrench. If the valves were soldered in place, you may need to cut off the valves, using a hacksaw.

  10. Install the New Shutoff Valves

    Attach new dual-outlet shut-off valves to the water supply stub-outs, following manufacturer's instructions. With compression-style valves, this means fitting a compression ring and compression nut onto the pipe, then securing the valve by tightening the nut with a wrench. Valves with push-fit connectors simply slip onto the end of the stub-out pipe.

  11. Connect the Water Supply Tubes

    Attach the free end of each faucet supply tube (from both sinks) to the proper valve, using an adjustable wrench. Make sure to properly match the hot water supply tubes to the hot water shutoff valve, and cold to cold.

  12. Check for Leaks

    Open the main water supply valve. Open the fixture shutoff valves and test the faucets on both sinks. Watch for leaks both in the water supply fittings and the drain fittings. Tighten any connections that have leaks. Usually, only a very small amount of tightening is necessary.