How to Install an Undermount Kitchen Sink

Thoroughly clean the underside of the countertop around the sink cutout. Use a rag and denatured alcohol to remove any dust, grease, or debris from the stone. This ensures a strong, watertight bond with the silicone sealant. A clean surface is critical for preventing future leaks.

Carefully place the new undermount sink upside down on a protected surface, like a piece of cardboard on the floor. First, install the basket strainer for the non-disposal side. Apply a rope of plumber's putty around the underside of the strainer's flange, press it firmly into the drain opening from inside the sink bowl. From underneath, place the rubber gasket, fiber washer, and threaded locknut. Tighten the locknut by hand, then use channel-lock pliers or a strainer locknut wrench to give it a final quarter-turn. Clean up any excess putty that squeezes out. For the disposal side, install the garbage disposal mounting flange following the manufacturer's instructions, which is a similar process but typically uses a different set of gaskets and a snap ring.

Place two 2x4 boards across the top of the sink cutout in the countertop. Position the sink underneath the counter and have a helper lift it up into position. While they hold it, use one or two bar clamps to secure the sink loosely to the 2x4s, passing the clamp shaft up through the drain hole. Before fully tightening, double-check the sink's alignment with the countertop reveal on all sides. Make any necessary adjustments, then tighten the clamps to hold the sink firmly in position for the next steps. This support system will hold the weight while the sealant cures and you install the permanent clips.

Lower the sink slightly by loosening the clamps. Using a caulking gun, apply a continuous, even bead of 100% silicone sealant to the top flange of the sink. The bead should be about 3/8-inch thick and unbroken all the way around the perimeter. This bead of silicone will create the primary waterproof seal between the sink and the countertop.

With the sealant applied, press the sink back up into its final position and re-tighten the bar clamps. Ensure the sink is perfectly aligned with the countertop opening. You should see a small amount of silicone squeeze out evenly around the entire perimeter between the sink and the countertop. This indicates you have a good seal. Do not wipe the excess yet. The clamps and 2x4s will hold the sink in place while you install the permanent mounting hardware.

Follow the sink manufacturer's instructions to install the mounting clips. These clips hook into a channel on the sink's rim and have a wingnut or screw that tightens against the underside of the countertop, pulling the sink up securely. Install clips every 6-8 inches around the perimeter of the sink. Use a screwdriver or nut driver to tighten them, alternating sides as you go to ensure even pressure. Tighten them until they are snug; do not overtighten, as this can damage the countertop or the sink.

Once the clips are secure, use a rag or paper towel lightly dampened with denatured alcohol to carefully wipe away all the excess silicone sealant that squeezed out from the inside seam of the sink. Work quickly and cleanly, as silicone begins to skin over rapidly. A clean, smooth seam looks professional and prevents grime buildup.

Wait for the silicone to cure according to the manufacturer's directions (typically several hours, but 24 hours is best) before removing the 2x4 and clamp supports. Once cured, you can proceed to connect the plumbing. Lift the garbage disposal unit up to the mounting flange and rotate it to lock it into place according to its instructions. Connect the dishwasher drain hose to the inlet port on the disposal, ensuring you first knock out the plug inside the port with a screwdriver and hammer if it's a new disposal.

Now assemble the drain plumbing. Attach the tailpieces to the bottom of the sink strainer and the garbage disposal discharge. Connect the two tailpieces using a tee fitting. From the tee, install the P-trap, which connects to the drain line in the wall. Use slip nuts and washers at all connections. Hand-tighten all connections first, then give them a final quarter-turn with channel-lock pliers. Do not overtighten, as this can crack the plastic fittings.

Connect the hot and cold flexible water supply lines from the faucet tails to the corresponding angle stop shutoff valves under the sink. The hot is typically on the left, and cold is on the right. Tighten the compression nuts by hand, then use an adjustable wrench or a basin wrench for the final half-turn. A basin wrench is specially designed for reaching the faucet connections in the tight space behind the sink.

Place a bucket and some dry paper towels under the sink. Slowly turn on the hot and cold water shutoff valves all the way. Carefully inspect every connection point for drips: the supply lines at the valves and faucet, and all drain pipe slip nuts. Turn on the faucet and let water run into both bowls, checking the drain connections and P-trap. Fill the sinks partway and then release the water to put a large volume through the drains, which is a good test for leaks. Plug in and run the garbage disposal with cold water running. If all connections are dry, the installation is complete.