How to Install a Bathroom Sink

Instructions on Installing an Overmount Bathroom Sink

By Mark J. Donovan

Installing a bathroom sink can really spruce up a bathroom and it is something most do it yourself homeowners can do with a few simple household tools.

Detailed below is a set of instructions for installing an overmount bathroom sink. An overmount bathroom sink has a flange on it so that the sink bowl rests on top of the bathroom vanity countertop.

Frequently the sink faucet attaches directly to the bathroom sink so as part of this set of instructions I include the installation of a bathroom faucet as well.

Tools required for Installing a Bathroom Sink

  • Jig Saw
  • Pencil
  • Drill with 3/8” drill bit
  • Crescent Wrench
  • Bucket
  • Putty Knife
  • Channel Lock Pliers
  • Tape
  • Silicone Caulk
  • Plumbers Putty

Installing a Bathroom Sink – Begins with Finding and Buying the Right One

The first thing that you need to do is purchase the right size overmount bathroom sink for your bathroom vanity countertop. Make sure when measuring the depth of your bathroom vanity and countertop you account for any backsplash. A backsplash will reduce the maximum allowable depth of your bathroom sink by approximately 1-2 inches.

The width of your bathroom sink is dependent on the width of the bathroom vanity cabinet you plan on installing the bathroom sink in. Check the inside of the vanity cabinet to account for any drawers or other obstacles that could limit the size of the bathroom sink that can be installed.

If you are replacing a bathroom sink the task of selecting a new one is a little easier. You have a pre-defined hole that already specifies the size of the bathroom sink you can install. You may be able to, however, install a larger bathroom sink if there are no obstructions underneath the cabinet and you are willing to cut a larger opening.

Note: If there are separate holes for the bathroom faucet handles and spout you will need to account for them in your selection of a new bathroom sink.

Remove old Bathroom Sink

If you are replacing an old bathroom sink, first turn off the water supply valves underneath the bathroom sink.

Next, disconnect the supply and drain lines from the bathroom faucet. Note that a bucket comes in handy at this stage to catch the small amount of water that is located in the J-trap.

Remove any sink hold-down fastener clips that maybe holding the bathroom sink to the bathroom vanity countertop.

Using a putty knife, slide the edge of it around the perimeter of the bathroom sink to loosen any caulk that maybe forming a seal around the bathroom sink.

Now simply pull the bathroom sink out of the bathroom vanity countertop.

Cutting an opening for the to be Installed Bathroom Sink

If you are installing a new bathroom sink, you will need to use the template which should have come with the bathroom sink to trace out the rough opening for it.

Center the template on the bathroom countertop in the location of where you want the bathroom sink to reside. The template should be positioned approximately 2 inches back from the front edge of the countertop.

Once you have located the position of the template secure it with tape.

Using a pencil, trace around the template.

Note: If a template did not come with the bathroom sink, simply turn the bathroom sink upside down and position it properly over the bathroom countertop and trace around it. Then remove the bathroom sink and draw another outline that is approximately ½” in from the first outline you drew. You will cut on this second inner line. This inner line will enable the bathroom sink flange to rest on the bathroom countertop.

Remove the template, and drill a couple of 3/8 inch pilot holes near the inside edges of the template area. Make sure the holes are within the boundary of the template outline.

Using your jig saw, insert it into a pilot hole and begin cutting on the inside edge of your outline. Have someone underneath the bathroom countertop holding the cutout piece in place until you have completely cut around the outline. This will prevent the cutout piece from breaking off and possibly damaging the surface of the countertop. Warning: make sure the person underneath the countertop is wearing eye protection and that his or her hands are away from the saw blade at all times.

You may also need to drill holes for your faucet supply lines and spout, depending on what type of bathroom sink you are installing.

Installing Bathroom Sink

To minimize your work underneath the bathroom countertop it is best to attach the bathroom faucet to the bathroom sink before you install the bathroom sink.

Turn the bathroom sink upside down and attach the faucet to the bathroom sink using a crescent wrench. Make sure to first apply a bead of caulk or plumbers putty to the faucet base before attaching it to the bathroom sink.

Now attach the flexible supply lines to the faucet assembly. Again, use your crescent wrench to tighten the supply lines to the bathroom faucet. Do not over-tighten as you could potentially crack the bathroom sink.

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Clean around the cut out area removing any sawdust or loose debris.

Apply a bead of silicone caulk around the perimeter underside flange area of the bathroom sink.

Turn the bathroom sink/faucet assembly over and position it into the cutout area. Make sure the supply lines are properly fed into the supply line holes if required.

Press the bathroom sink firmly into place and immediately remove any excess silicone caulk.

Underneath the bathroom sink, fasten the bathroom sink to the bathroom countertop with hold-down fastener clips, if fastener clips came with the bathroom sink.

Install Bathroom Sink Drain Assembly and Supply Lines

With the bathroom sink installed, run a bead of plumbers putty around the bottom of the sink drain area and install the sink drain and drainpipe assembly. There is a lock nut washer assembly that holds the sink drain to the bathroom sink. Use a pair of channel lock pliers to tighten the lock nut washer.

Connect the J-Trap assembly to the sink drainpipe using your hands and/or channel lock pliers.

Connect the flexible bathroom supply lines to the house water supply lines using your crescent wrench.

Turn the water supply lines back on and test to see if the bathroom faucet operates correctly and that there are no leaks around the sink drain assembly or supply lines. If there are, turn off the supply lines and gently tighten the offending connection.

After testing the bathroom sink and faucet, let the bathroom sink set up overnight before using regularly.

For help on your bathroom remodeling project, see my Bathroom Remodeling Bid Sheet. The Bathroom Remodeling Bid Sheet provides a request for quote checklist section that you can provide to prospective bathroom remodeling contractors. It also includes a comprehensive bathroom remodeling cost breakdown table, in Microsoft Excel format, that allows the contractor to include his projected remodeling costs for every phase of the project.

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