How to Install an Undermount Sink

Cutting and Cleaning the Countertop

Measure the hole in the countertop if it has already been cut. If you are replacing a sink or working with a pre-cut countertop, figure out what sink size you need. Measure the length and width of the hole that will hold the sink. If your sink is over a cabinet, measure the depth of the cabinet as well. If you have an old sink available, measure it. Replace it with a new sink the same size. Keep in mind that hard counters, like marble, quartz, and granite, often work better for this project than laminate or tile countertops.

Choose a spot for the sink if you’re working with a new countertop. Figure out where you want to install the sink. The location depends on a few factors, such as the location of the water supply lines in your home. Most sinks sit in the middle of countertops, but some installers choose a different aesthetic. Stone and concrete countertops work best with undermount sinks. Wood, laminate, and other types of countertops tend to crack during the installation.

Select a reveal style for your sink. The reveal style determines how much of the sink’s rim stays visible after the installation. This is important for appearance, but also has some effects on cleaning. Most people go with a “zero reveal,” where the countertop’s edges are flush with the sink’s opening. Sinks with a zero reveal look professional and don’t leave any extra space for germs and food particles to slip into. Another option is a positive reveal, where about half of the sink’s rim stays visible. The rim will get dirty, similar to a regular drop-in sink. Alternatively, create a negative reveal by hiding the sink’s rim under the countertop. The overhanging countertop edges tend to get dirty and are hard to clean.

Trace the sink’s outline over the countertop. Flip the sink over and trace around it with a pencil. Then, remove the sink and measure the width of the rim. On average, it will be about ⁄8 in (0.95 cm) wide. Make a second outline ⁄8 in (0.95 cm) inward from the original lines. Erase the original outline so you don’t cut it by mistake. A lot of sinks come with cardboard templates. Simply put the template over the countertop, then trace it to size the opening correctly. The outline needs to be smaller than the sink’s opening. Remember that the rim will rest against the countertop after the installation. For the most part, only the bowl of the sink will be visible.

Cut the countertop with a circular saw or jigsaw. Put on safety gear, such as a dust mask and goggles. Then, choose the right saw for the material you wish to cut. If you’re cutting stone, you will need a circular saw with a diamond-coated blade. For a material such as laminate, choose a jigsaw with a special laminate-cutting blade. If you’re working with stone or concrete, consider hiring a countertop fabricator to make the cuts. Not everyone has the tools needed. These countertops are also expensive, so damaged surfaces are not fun to contend with. Be sure to give your countertop fabricator the exact measurements of your new sink so they can cut it with precision. To make holes for the accessories like the faucet, use a drill. Choose a drill suitable for the countertop, such as a masonry bit for stone and concrete.

Clean up the countertop and sink with denatured alcohol. Remove old caulk, sawdust or other debris on the countertop. Use a scraper to eliminate any dried-out adhesive around the opening. Scrub it with a rag dampened in denatured alcohol to remove leftover debris. If you’re replacing an old sink, remove it by cutting through any caulk. Unscrew the brackets under the sink by turning the wingnuts counterclockwise. Remember to shut off the water lines and detach the plumbing before attempting to lift the sink out.

Securing the Sink by Suspension

Stack boards underneath the countertop to hold up the sink. Many undermount sinks need to be lifted into position underneath a countertop. This is a little tricky, but it isn’t too difficult if you use boards to support the sink’s weight. Stack piles of 2 in × 4 in (5.1 cm × 10.2 cm) wood boards until the sink is right beneath the countertop. If you don’t have scrap wood, use other sturdy objects or have someone hold the sink in place until you finish attaching it to the countertop. This method is the best option when your countertop has already been installed. If you’re replacing an old sink, you will install the new undermount this way With this approach, you have the advantage of being able to look down into the sink. You can easily check the sink’s positioning before gluing it in place.

Center the sink in the middle of the hole in the countertop. Adjust the sink and the wood supports as needed. Align the rim of the sink with the cut edges of the countertop if you're going for a zero reveal aesthetic. Look down from above to make sure the sink is exactly where you want it. For other reveal styles, take some extra time positioning the sink. Measure the distance from the countertop edge to the sink’s rim. Make sure it is even on all sides.

Clamp the sink to supports to hold it in place while you install it. You will need an adjustable bar clamp and a couple of 2 in × 6 in (5.1 cm × 15.2 cm) pieces of wood. Set a piece of wood underneath the sink, then set another piece over the hole in the countertop. Pull the drain stopper out of the sink and lower the adjustable clamp through the drain. Clamp the boards so the sink stays suspended about 1 in (2.5 cm) below the countertop. If you’re installing a sink with 2 drains, get a second clamp. Run a clamp through each drain. Another way to do this is with an undermount sink harness. The harness clips to the sides of the countertop. You then tighten the harness to lift the sink into place. It’s a great tool to use if you’re working alone.

Glue the studs to the underside of the counter with an epoxy paste. Get some undermount sink clips along with a bottle each of epoxy resin and hardener. Mix equal amounts of the resin and hardener together with a wooden stirring stick. To install the studs, hold them to the countertop and spread glue over them with the stirring stick. Position the studs in the sink’s corners and around its rim. Glue a stud near each of the sink’s corners first, then spread the other studs out. Keep them no more than 10 in (25 cm) apart.

Spread silicone caulk around the countertop. Cut the tip off a canister of caulk and set it in a caulk gun. Keep steady pressure on the gun’s trigger to spread a thick bead of caulk around the cutout in the countertop. The bead of caulk needs to be above the rim of the sink. Choose a canister of a waterproof silicone caulk from a hardware store. This kind of adhesive holds undermount sinks in place long-term.

Tighten the clamps to bring the sink up to the countertop. Tightening the clamps raises the sink so it presses against the caulk. Check the sink’s positioning again. Adjust it before the caulk solidifies. When you’re satisfied with the sink’s placement, leave it clamped in place so the caulk has a chance to cure. Make any needed adjustments now. Once the caulk solidifies, moving the sink becomes much more difficult.

Install the sink clips and wingnuts over the glued studs. Fit a clip over a stud, then tighten it in place with a wingnut. Position each clip so the slotted end is over the stud and the solid end is underneath the sink’s rim. Twist the wingnuts clockwise to tighten them. When the sink clips are installed correctly, they secure the sink tightly to the countertop. Make sure there aren’t any gaps between the sink and the countertop. If you notice a gap, the clips may not be on tight enough. Always tighten the wingnuts by hand. Overtightening them can damage the countertop.

Wipe away excess caulk with a rag and denatured alcohol. Pushing the sink against the countertop may cause some of the caulk to squeeze out. Wipe it up right away to prevent it from hardening. If it begins to harden, dampen a rag with some of the denatured alcohol, then use it to scrub away the caulk. Once the caulk cures, it becomes difficult to remove. You will need a scraper or sharp blade to cut off the caulk. Scrub the remainder off with denatured alcohol. If you do leave behind some caulk, at least no one will notice it underneath the sink!

Mount the faucet and other accessories over caulk. Set the accessories on top of caulk. After centering them over the holes in the countertop, install the water lines. Run the lines up through the faucet and control knobs to finish the sink. Let the caulk dry for 24 hours before attempting to use it. Get flexible steel supply lines from the hardware store if you need replacements for your faucet.

Installing over the Countertop

Turn the counter so the underside faces upward. In order for you to be able to install the sink this way, your countertop needs to be free from the cabinets underneath it. This method works best for new countertops. Bottom-up installations are quicker and easier, but you don’t get a good view of the sink’s positioning on the countertop. You may need some help turning the countertop back over after the sink is attached to it.

Fit the sink over the hole in the countertop. Turn the sink upside down. Remember that the countertop is upside down, so everything will turn out right side up when you flip it later. Center the sink as perfectly as possible over the hole. If you are able to, take a peek underneath the countertop to help position the sink. If your sink came with a template, use it to position the sink. Center it over the hole, tape it down with masking tape, then fit the sink over it. Templates are much easier to center than bulky sinks.

Trace around the rim of the sink with a pencil. Hold the sink down firmly as you trace it. Make sure the outline is dark and visible. When you’re done, set the sink aside. Remove the template if you used one to fit the sink. The outline serves as a guide for the clips and adhesives needed to attach the sink to the countertop.

Position the sink clips around the edges of the outline. Place a sink clip in each of the corners. The remaining sink clips fit between the corners. Space them out evenly, positioning them 10 in (25 cm) apart or less. Fit the clips so the solid ends hang underneath the sink's rim. The clips are included with most sinks. If you need extras, they are available at most hardware stores. Look for the undermount variety with wingnuts.

Glue the clip sink clip studs in place with epoxy. The best way to do this is by getting epoxy resin and hardener. Use a wooden mixing stick to combine equal amounts of both ingredients. Quickly spread the epoxy over the studs, pushing down on it with the mixing stick. Doing this pushes the epoxy through the holes in the stud and underneath it. When you’re done, let the epoxy dry for 24 hours. Wear rubber gloves when mixing the epoxy. It is a mild irritant, so cover up your arms and other areas of exposed skin. The sink clip studs look like small plates covered in holes. Make sure the studs rest against the countertop, outside of the outline you drew.

Spread silicone caulk along the countertop. You will need a caulk gun and a bottle of caulk. Cut the tip off the bottle of caulk and load it into the gun. Position the nozzle between the outline you made and the edge of the hole in the countertop. Keep pressure on the trigger to lay a steady amount of caulk around the entire hole. A 100% silicone caulk is the best choice for undermount sinks. Silicone caulk is a strong, waterproof adhesive, but it’s also flexible. The changing temperatures in your kitchen won’t damage it.

Place the sink and clip it to the underside of the counter. Center the sink upside-down again. Take a peek underneath the countertop again to make sure you’re happy with the sink’s positioning. Then, slide the sink clips onto the studs and fit a wingnut through each one. Twist the wingnuts clockwise by hand to secure the sink in place. The sink clips look like flat metal bars. Fit the slotted end over the stud you glued to the countertop. Rest the other end over the sink’s rim. Tighten the wingnuts gradually. Alternate sides so the sink doesn’t have a chance to slip out of place.

Wipe up any remaining caulk with a rag and denatured alcohol. Remove any tape or template you used during the installation. Make sure the sink fits tightly against the countertop. Finish by cleaning up the excess caulk, then scrubbing the countertop with more denatured alcohol. Remove the excess caulk as soon as possible to prevent it from hardening. If it hardens, get a knife or scraper and carefully remove the caulk. Scrub the remaining caulk with denatured alcohol to remove it.

Mount the accessories after the caulk dries for 24 hours. After the caulk dries, flip the countertop over and finish installing it. Install the faucet and other accessories the same way you did with the sink. Fit them over the countertop, then glue them in place with caulk. After another 24 hours, your sink will be ready for use. To install water lines, feed them through the faucet. You will need some braided steel flex lines from the hardware store. Connect the other ends to the supply lines in the walls.