Two Ways to Install an Undermount Kitchen Sink

Text and illustrations by Michael Chotiner

Besides their sleek, cool look, undermount sinks make it easier to clean up a countertop. Because they're adhered to the underside of a counter, there are no visible joints around the bowl to catch food bits and grease as you wipe the counter surface.

Undermount sinks are best installed beneath stone and solid-surface tops. They're not recommended for installation under plastic laminate, ceramic tile or wood tops.

Before you mount a sink, you've got to have a finished opening cut into the countertop with the right size, shape and position. It's also best to have holes pre-drilled for the faucet and accessory mounts.

All sink manufacturers and countertop fabricators recommend that the necessary holes be cut by a pro. I agree, not only because cutting stone and solid surfacing requires a set of special, heavy-duty tools that not every do-it-yourselfer has, but also because it's so messy—and expensive if you accidentally chip or crack the top in the process.

Since you're not going to cut the sink opening yourself, you'll need to tell your countertop fabricator which sink and faucet you plan to install, and which reveal style (more on this in a moment) you prefer so he or she can cut the opening to the correct size and shape. Choose a sink that fits into the base cabinet below the counter with some room to spare. Your fabricator will work from a sink template, usually available from the manufacturer.

Undermount sinks can be installed in any of three "reveal" treatments

The term "reveal" refers how much of the sink edge is visible when looking directly down into the sink.

There are three reveal styles (graphics are a side-cutaway view of a typical deep basin sink):

Positive reveal - part of the sink rim is exposed.
Zero reveal - edges of the countertop opening are flush with sink walls.
Negative reveal - countertop extends past sink walls.

Your choice of a reveal style should be based mostly on your visual preference. Most fabricators I know prefer the zero reveal style—arguing that it's the cleanest. Some experts say that a positive reveal just gives you another narrow, hard-to-clean surface around your sink (and believe me, it'll get dirty as you become accustomed to sponging messes from the countertop right into your undermount sink). A negative reveal also produces a hard-to-clean surface on the counter underside where dirt, grease and germs are likely to build up unseen.

Setting Up Your Workspace

An undermount sink should be attached to the countertop before it's installed on the base cabinets. It's awkward to do it otherwise—there's typically not enough space inside the sink base cabinet to do what you need to do. If for some reason you need to install the sink with the top in place—or reset an existing undermount sink that has separated from the underside of the counter— you're probably going to have to cut the cabinet sides to gain access to clip fittings that secure the sink against the underside of the counter. Installing the faucet will be a pain, too.

When your countertop arrives—holes and all— you should have a work stand set up to support the counter so that you can work above and below without flexing and cracking it. Depending on the length and weight of the top, you might be able to get away with setting a couple of two-bys across sawhorses. A table frame minus a tabletop might also work.

Gather the tools and supplies you'll need to install the sink, including:

One tube of 100% silicone adhesive caulk
Two-part epoxy
Denatured alcohol
Masking tape
Bar clamps (depending on attachment method)
Sink
Sink-mounting clips with glue-on stud bases
Faucet and accessories

Right-side up or right-side down?? This is NOT a trick question!!

Some sink installers like to do the job with the underside of the counter facing up on the support table; others like to work with the finished surface facing up. Each work style has its advantages.

Bottoms-Up Installation...

Installing the sink with the underside up is generally faster and easier, but it's a little more awkward to determine whether the sink is perfectly centered on the opening to produce the desired reveal treatment. To start, set the counter slab on the work stand with the underside facing upward.

Wipe the inside edge of the cutout and the area around it on the underside of the counter with a rag and denatured alcohol. Be sure the surfaces are dust free.
Apply masking tape to the inside edges of the cutout.
Set the sink over cutout, center it to achieve the desired reveal, and trace around the rim to outline the area where silicone adhesive will be applied. Then set the sink aside.
Lay out the positions of the sink clips just outside the line you've drawn around the perimeter of the cutout. Provide at least one clip at each corner of the sink and space others along each side, no more than 10 in. apart.
On a cardboard scrap, mix enough two-part epoxy to adhere the sink clip studs. Working quickly with a wooden popsicle stick, spread epoxy on the base of each stud and press it in place hard enough to squeeze the epoxy through the holes of the stud bases. Allow the epoxy to cure for at least 24 hours.
Once the epoxied-on sink clip studs have cured, apply a generous bead of silicone around the edges of the sink cutout within the line that's been drawn around the perimeter.
Set the sink in position over the opening and center it. Take a peek under the work stand to see that the sink rim is properly aligned and that the reveal is equal on all sides. Adjust as necessary.
When you're satisfied, slip the slotted sink-clip brackets onto the studs so that one end bears on the sink rim. Thread the wingnuts onto the studs and tighten them down on the brackets, alternating sides to prevent the sink from shifting in one direction or another. The nuts should be only finger-tight to avoid undue strain on the countertop.
Clean silicone squeeze-out from around the cutout edges with your finger. Remove the masking tape and clean away any silicone that may have leached underneath. Wipe the edges with alcohol.
Mount the faucet and, if applicable, the accessory dispensers.
Install the counter and join the faucet supply tubes to the water supply.

Or a View from the Top??

Some installers prefer to mount the sink working with the countertop right-side up. Doing so gives you a better view of the fit and look of the reveals during the final stages as the sink clips are tightened to retain the sink rim on the underside of the counter. It's also what you have to do if you're trying to re-install an existing undermount sink that's delaminated and falling away from the countertop.

The trick for installing an undermount sink from above is to suspend it beneath the cutout using bar clamps. If you're trying to reinstall an existing sink that's come loose, you first need to disconnect it completely from the countertop and prop it up beneath the cutout.

Set up some props—stacked 2 x 4 in. boards or some other temporary stand to support the sink an inch or two beneath the cutout.
If you're doing a re-installation, shut off the water supply to the sink, disconnect the faucet supply tubes, P-trap and disposal (if present), and remove the strainer body from the sink drain hole.
Remove any sink clip brackets that may be keeping a sagging sink from falling completely away. Set the sink on the props.
Using a knife, scraper and solvent, clean away all dried adhesive from the underside of the counter and the sink rim.

Once the mounting surface on the underside of the counter is completely clean, install new sink clip studs wherever they may be needed (see Steps 4 and 5, above).

To suspend the sink beneath the cutout:

• Get two scraps of 2 x 6 in. boards that are about 2 ft. longer than the sink is wide. • Remove the adjustable handles from one or more bar clamps (if you're working with a double sink, you should have two clamps on hand). • Thread the bars of the clamps up from beneath the sink, up through the drain holes. • Restore the adjustable handles to the clamps. • Set one 2 x 6 in. board between the sink bottom and the stationary jaws of the clamps. Lay another 2 x 6 in. board scrap on the countertop, spanning the cutout. Position the adjustable clamp jaws on the 2 x 6 in. board and tighten enough to lift the sink off of the props below. • Adjust the clamps to suspend the sink about an inch below the cutout, aligning the sink rim with the cutout edges.

To finish the installation:

Apply a bead of silicone adhesive on the bottom of the countertop all around the cutout.
Tighten the adjustable clamps to bring the sink rim in firm contact with the adhesive and countertop bottom, adjusting the alignment of the sink to achieve the desired reveal.
Slip the clip brackets onto the mounting studs and over the sink rim. Tighten the wingnuts finger-tight. Over-tightening risks cracking the countertop.
Clean away silicone squeeze-out as necessary.
Make or restore all drain and faucet connections as necessary.

Using either upside-down or right-side up installation, any reasonably handy person should be able to install an undermount sink or to reset a sagging one like a pro!

About the author: From his past experience as a contractor and cabinetmaker, Michael Chotiner has accumulated some great "pro how-to" knowledge that he likes to share for The Home Depot. To see a wide array of under-mount and over-mount kitchen sinks, visit homedepot.com.