What you need to know before starting

An undermount sink is fastened to the underside of your countertop, so the rim sits below the counter surface rather than on top of it. Replacing one means removing the old sink from underneath, then installing a new one in the same opening. The job takes two to four hours if the opening is already the right size and your plumbing connections are straightforward. If the opening needs resizing or your supply lines are corroded, add another two to three hours and budget for a plumber.

The hardest part is usually breaking the seal between the old sink and the countertop—it's held down by adhesive or clips that have set for years. You'll need to work from below the counter, which means either removing cabinet doors or lying on your back in a tight space. If your countertop is granite, marble, or another stone, call a professional; the risk of cracking the counter is too high for a first-timer.

Key Takeaways

  • Turn off the water supply at the shutoff valves under the sink before you disconnect anything.
  • The old sink is held by adhesive or metal clips underneath the counter; you'll need to break this seal and support the counter from below as you work.
  • Measure the opening width, depth, and the distance from the front edge of the counter to confirm your new sink will fit.
  • Undermount sinks require a rim or mounting bracket system; check that your new sink comes with the right hardware for your counter material.
  • If the opening is damaged, the counter sags, or you hit plumbing obstacles, stop and call a plumber rather than force the installation.

Gather your tools and materials

You'll need a wrench or adjustable pliers to disconnect the supply lines and drain, a utility knife to cut through old adhesive, a reciprocating saw or oscillating multi-tool to cut through mounting clips if they won't budge, and a caulk gun with silicone caulk rated for your counter material. Have a bucket under the sink to catch water that drains from the lines.

For the new sink, confirm it comes with a mounting kit—most undermount sinks include clips, brackets, or a rim system designed for your counter type. If you're installing into laminate, you'll need a different bracket system than you would for solid surface or stone. Read the manufacturer's instructions before you buy; some sinks require a specific adhesive or sealant that doesn't come in the box.

Have towels, a flashlight, and a helper on hand. You'll be working upside down under the counter, and having someone hold a light or support the counter from above makes the job much safer.

Turn off water and disconnect the old sink

Locate the shutoff valves under the sink—one for hot water, one for cold. Turn both clockwise until they stop. If the valves are stuck or missing, shut off the main water supply to the house. Open the faucet at the sink to release any pressure in the lines, then disconnect the supply lines by turning the compression nuts counterclockwise with a wrench. Have your bucket ready; there will be water in the lines.

Disconnect the drain by loosening the slip nuts on either side of the P-trap (the U-shaped pipe under the sink). If the nuts won't turn by hand, use an adjustable wrench. Once the P-trap is loose, you can pull it away from the sink drain. If the drain line is glued to the wall or floor, you may need to cut it with a reciprocating saw—but only if you're replacing it with new PVC. If you're keeping the existing drain line, work carefully to avoid cracking it.

Remove the old sink from underneath

Lie on your back under the sink with a flashlight and examine how the sink is fastened. You'll see either metal clips bolted to the underside of the counter, or a continuous bead of adhesive around the rim. If there are clips, unbolt them with a wrench or socket set. If there's adhesive, use a utility knife to cut through it, working your way around the entire perimeter. This takes patience—the adhesive may be an inch thick and rock-hard.

As you cut or unbolt, support the sink from above with your helper or with temporary wooden blocks wedged between the sink and the counter. Once the fasteners are loose, the sink will want to drop; you need to control that descent so it doesn't fall on your hands or damage the counter edge. Slowly lower the sink, then slide it out from underneath the counter opening.

Once the old sink is out, scrape away all remaining adhesive from the underside of the counter with a putty knife or scraper. Wipe the surface clean with a damp cloth and let it dry completely. If the counter edge is damaged, chipped, or the opening is the wrong size, this is the moment to stop and call a professional.

Prepare the opening and install the new sink

Measure the new sink's width, depth, and the distance from the front edge of the counter to the back. Compare these to your opening. The sink should fit with no more than a quarter-inch gap on each side. If the opening is too small, you'll need to cut the counter—a job for a professional unless you have experience with a circular saw and the counter is laminate. If the opening is too large, you can shim the sink level with plastic shims or build up the rim with additional mounting brackets.

Check that the new sink's mounting system matches your counter material. Laminate counters use a different bracket system than solid surface or engineered stone. Install the mounting brackets or rim according to the manufacturer's instructions—this usually means bolting them to the underside of the sink or attaching them with adhesive. Some sinks come with a rubber gasket that sits between the sink rim and the counter; make sure this is in place before you lift the sink into the opening.

With your helper supporting from above, slide the new sink into the opening from underneath. Align it so the front edge is parallel to the counter front and the sink is centered left to right. Once it's positioned, tighten the mounting bolts or clips from underneath, working in a cross pattern (tighten one side, then the opposite side) so the sink pulls down evenly. Do not over-tighten; you'll crack the sink or the counter.

Seal the rim and reconnect plumbing

Once the sink is bolted down, run a bead of silicone caulk around the entire rim where the sink meets the counter. Use a caulk gun and smooth the bead with a wet finger or caulk tool. This seals the gap and prevents water from running underneath the counter. Let the caulk cure for 24 hours before you use the sink—check the tube for the exact time.

Reconnect the P-trap by sliding the slip nuts back onto the drain and tightening them by hand first, then with a wrench. Hand-tight plus a quarter turn is usually enough; over-tightening can crack the fitting. Reconnect the supply lines by threading the compression nuts back onto the shutoff valves. Again, hand-tight plus a quarter turn. Turn the shutoff valves counterclockwise to restore water pressure.

Turn on the faucet and let the water run for 30 seconds to flush air from the lines. Check under the sink for leaks at the supply connections, the P-trap, and the drain. If you see drips, tighten the nut a quarter turn more. If a connection still leaks after that, turn off the water, disconnect it, and wrap the threads with plumber's tape before reconnecting.

Troubleshooting common problems

If the sink rocks or feels loose after installation, the mounting brackets may not be tight enough, or the counter may be sagging. Check that all bolts are snug and that the counter is level using a spirit level. If the counter sags in the middle, you may need to add support from below—a temporary wooden post or a permanent bracket system. This is a structural issue that a plumber or carpenter should assess.

If water leaks from the rim where the sink meets the counter, the caulk may not have sealed properly, or the sink may not be sitting flush against the counter. Let the caulk cure fully, then run a second bead if needed. If the sink is tilted, you'll need to unbolt it, shim the opening, and reinstall it level.

If the drain leaks at the P-trap connection, the slip nuts may be cross-threaded or the rubber washers inside may be damaged. Disconnect the P-trap, inspect the washers, and reconnect carefully, making sure the nut threads smoothly before you tighten it.

When to call a professional

Stop and call a plumber if the countertop is stone (granite, marble, quartz), if the opening needs to be resized, if the counter is sagging or damaged, or if you encounter corroded or frozen plumbing connections. You should also call if the new sink doesn't fit the opening or if the existing drain line is glued and you're unsure how to disconnect it without damage.

A plumber can also help if you want to upgrade your supply lines to flexible braided hoses (which are less likely to kink) or if you need to relocate the drain or supply connections. The cost of a professional installation is usually $200 to $400, which is worth it if the alternative is a cracked counter or a leak inside your cabinet.

Frequently Asked Questions

Can I replace an undermount sink with a drop-in sink?

Yes, but you'll need to enlarge the opening so the rim of the drop-in sink sits on top of the counter. This requires cutting the countertop, which is a job for a professional unless you have experience with a circular saw and the counter is laminate. The drop-in sink will also sit higher than the original undermount, which may look odd if your counter has a finished edge.

What if the old sink is glued down and won't budge?

Use a reciprocating saw with a metal-cutting blade to cut through the adhesive in sections, working from one side of the sink to the other. Cut slowly to avoid damaging the counter. If the adhesive is extremely thick, you can also try heating it with a heat gun to soften it, then cutting through it with a utility knife. This takes time but reduces the risk of cracking the counter.

Do I need a special adhesive for the new sink?

Check the manufacturer's instructions for your new sink. Some undermount sinks use silicone caulk only, while others require a polyester or epoxy adhesive rated for your counter material. Laminate, solid surface, and stone all have different requirements. Using the wrong adhesive can cause the sink to pull away from the counter or fail to seal properly.

How do I know if my countertop can support a new undermount sink?

The counter needs to be at least three-quarters of an inch thick and made of a material that can be sealed (laminate, solid surface, engineered stone, or natural stone). If your counter is thin, damaged, or sagging, it may not support the weight of a water-filled sink. A plumber or countertop professional can assess this in person.

What's the difference between a clip-mounted and adhesive-mounted undermount sink?

Clip-mounted sinks use metal brackets bolted to the underside of the counter; they're easier to remove and reinstall. Adhesive-mounted sinks are glued down and require more work to remove, but they create a cleaner look with no visible hardware. Most modern undermount sinks use clips, which are faster to install and more forgiving if you need to adjust the position.