How to Install an Undermount Kitchen Sink

Begin by preparing the work area for a safe and clean project. Clear everything from the under-sink cabinet. Turn off the hot and cold water supply by twisting the shutoff valves clockwise until they stop. Turn the faucet on to release any remaining pressure and water in the lines. Place a bucket and some old towels under the drain pipes (P-trap) to catch any residual water during disassembly. If there is a garbage disposal, disconnect it from the power source. If it's hardwired, turn off the corresponding circuit breaker at the main electrical panel.

With the bucket in place, start disconnecting the plumbing. Use a pair of channel-lock pliers to loosen the slip nuts on the P-trap assembly that connects the sink drain and garbage disposal to the wall pipe. Water will drain out, so be ready. Next, disconnect the dishwasher drain hose from the garbage disposal, if applicable. Using a basin wrench or adjustable wrench, loosen the nuts connecting the hot and cold flexible water supply lines to the faucet and the shutoff valves. Finally, address the garbage disposal. Insert a disposal wrench into the bottom and rotate to unlock it from its mounting ring, then carefully lower the unit. It can be heavy.

The old sink is held in place by a combination of silicone adhesive and metal mounting clips. First, use a utility knife to carefully slice through the silicone bead between the sink rim and the underside of the countertop. Be thorough and cut all the way around. Next, locate the mounting clips screwed into the countertop's underside. Use a screwdriver or socket wrench to loosen and remove all of these clips. Have your helper support the sink from below, or create a support brace using a 2x4 and a bar clamp. Once all clips are removed and the silicone is cut, carefully lower the old sink and remove it from the cabinet.

A clean surface is essential for a waterproof seal. Use a plastic putty knife or a dedicated caulk remover tool to scrape away all the old silicone residue from the underside of the granite countertop. Be patient and thorough. For stubborn residue, apply denatured alcohol or a specialized adhesive remover with a rag to soften it before scraping again. Once all the gunk is removed, wipe the entire surface clean with a rag and denatured alcohol to ensure it is completely free of dust, oils, and debris. The surface must be perfectly clean and dry for the new silicone to adhere properly.

It is much easier to install components onto the sink before it's mounted under the counter. Place the new stainless steel sink upside down on a soft surface, like cardboard or towels, to prevent scratches. First, install the new faucet, following its manufacturer's instructions. This usually involves feeding the lines through the holes and tightening a retaining nut from below. Next, install the drain assembly (strainer) and the garbage disposal mounting flange. Apply a rope of plumber's putty or a bead of kitchen/bath silicone under the lip of the drain flange before inserting it into the sink's drain hole. From underneath, assemble the gasket, flange body, and retaining ring, and tighten the screws evenly.

Now it's time to mount the sink. First, apply a continuous, generous bead (about 3/8-inch thick) of 100% silicone sealant to the top rim of the new sink. With a helper, carefully lift the sink into the cabinet and press it firmly up against the underside of the countertop. Ensure it is centered correctly in the opening. To hold it in place while the silicone cures and you attach the clips, use a support system. The most common method is to place a 2x4 board across the top of the sink opening on the countertop, and use one or two bar clamps running through the drain hole(s) up to the board to pull the sink up snugly against the counter. Do not overtighten, as this could damage the sink or counter.

With the sink held in place by the support brace, install the permanent mounting clips. The sink kit should include at least 8-10 clips. Position a clip against the sink rim and mark the spot for the anchor on the countertop underside. Drill a shallow hole for the anchor, being careful not to drill through the countertop. Use a drill bit with a stop collar for safety. Tap the anchor into the hole, then position the clip and tighten the screw until it's snug. Repeat this process for all clips, spacing them evenly around the sink's perimeter. Tighten them in an alternating pattern, like tightening lug nuts on a tire, to ensure even pressure. Once all clips are tight, you can remove the 2x4 support brace.

Now, reconnect all the plumbing components. Start by connecting the new faucet's hot and cold supply lines to the shutoff valves. Hand-tighten the nuts and then give them a quarter-turn with a basin wrench or adjustable wrench. Do not overtighten. Lift the new garbage disposal into place, align the tabs with the mounting ring, and use the disposal wrench to turn the ring and lock the unit in place. Reconnect the dishwasher drain line to the disposal's inlet port. Finally, reassemble the P-trap, connecting the sink strainer and disposal outlet to the drain pipe in the wall. You may need to trim pipes with a hacksaw for a perfect fit. Hand-tighten all the plastic slip nuts, then give them a slight snug with channel-lock pliers.

The final step is to check your work. Remove all tools and materials from the cabinet. Slowly turn the hot and cold water shutoff valves back on (counter-clockwise). Immediately inspect all connections for drips: where the supply lines meet the valves and where they connect to the faucet. If you see any drips, tighten the connection slightly. Next, place a bucket under the P-trap and fill both sink basins with water. Let them sit for a few minutes, checking the silicone seal around the sink rim for any leaks. Then, release the water and check the entire drain assembly and P-trap for leaks. Finally, turn on the power to the garbage disposal. Run water and turn on the disposal to ensure it operates correctly and there are no leaks from the vibration. Once everything is confirmed to be dry and working, clean up your tools and the work area.