What you need to know before you start
Replacing an undermount sink is a job you can do yourself if you have basic plumbing skills and the right tools, but it requires more care than a drop-in sink because the sink is fastened to the underside of the countertop. The sink sits below the counter surface and is held in place by clips, brackets, or adhesive — not by its own rim. You will need to work from underneath the cabinet, disconnect the plumbing, remove the old sink, and install the new one in the same opening.
The job typically takes three to five hours if the plumbing is straightforward. If your countertop is damaged during removal or if the opening needs to be enlarged, the work becomes more complicated and may require a countertop specialist. Most undermount sinks are stainless steel or composite, and the replacement sink should match the dimensions of the existing opening within a quarter inch.
Key Takeaways
- Turn off the water supply and disconnect the drain and supply lines before you touch the sink itself.
- Undermount sinks are held by clips or brackets underneath the counter, which you must remove from inside the cabinet before the sink will come free.
- The countertop opening must match your new sink's dimensions; if it does not, you will need to cut or seal the edges before installation.
- New undermount sinks require waterproof sealant around the perimeter where the sink meets the counter to prevent water damage.
- If the countertop cracks during removal or the opening is damaged, stop and call a countertop professional rather than trying to patch it yourself.
Turn off water and disconnect the plumbing
Locate the shutoff valve under the sink — it is usually a small knob on the supply line coming from the wall. Turn it clockwise until it stops. If there is no shutoff valve under the sink, turn off the main water supply to the house. Open the faucet to release any remaining pressure in the lines.
Disconnect the hot and cold supply lines by unscrewing the compression fittings where they connect to the faucet or the sink. Place a bucket underneath to catch water that remains in the lines. Disconnect the drain line where it meets the P-trap — this is the U-shaped pipe under the sink. If the drain is glued, you may need to cut it with a hacksaw or a reciprocating saw.
If your sink has a garbage disposal, disconnect the electrical cord from the outlet or the switch. Unbolt the disposal from the sink drain if it is attached. Remove the entire drain assembly — the strainer basket, the rubber gasket, and any mounting hardware — so the sink can come free.
Remove the mounting clips or brackets from underneath
Look under the sink from inside the cabinet. You will see metal clips, brackets, or straps bolted to the underside of the countertop and pressing down on the rim of the sink. These are what hold the undermount sink in place. Using a wrench or socket set, unbolt each clip or bracket. Start with one side and work your way around — do not remove them all at once, or the sink may shift and crack the counter.
As you loosen the bolts, the sink will begin to drop slightly. Continue loosening until all the fasteners are removed and the sink is resting only on the sealant bead around its perimeter. If the sink is still stuck, run a utility knife around the edge where the sink meets the counter to cut through the sealant. Do this slowly and carefully — you are trying to free the sink, not gouge the countertop.
Lift out the old sink and clean the opening
Once all clips are removed and the sealant is cut, carefully lift the sink straight up and out of the opening. You may need a helper for this — undermount sinks are heavy, and you need both hands to lift evenly. Set the sink aside or into a bathtub where it will not damage your floor.
Scrape away all the old sealant, adhesive, and debris from the underside of the countertop and the edges of the opening. Use a putty knife, a scraper, or a heat gun to soften stubborn sealant. Wipe the area clean with a damp cloth and let it dry completely. Inspect the countertop for cracks, chips, or damage. If the counter is damaged, this is the time to have it repaired before you install the new sink.
Measure and prepare the opening for the new sink
Measure the opening in both directions — length and width — and compare it to the dimensions of your new sink. Undermount sinks should fit with a gap of no more than a quarter inch on all sides. If the opening is too large, you will need to seal the extra space with a filler strip or epoxy. If the opening is too small, you will need to cut the countertop, which requires a circular saw or a jigsaw and should be done by someone experienced with countertop work.
If the opening is the right size, explore a bead of waterproof sealant — typically silicone caulk or polyurethane — around the entire perimeter of the opening on the underside of the counter. This sealant prevents water from seeping between the sink and the counter and damaging the cabinet below. Use a caulking gun and explore a continuous bead about a quarter inch thick.
Install the new sink and find the mounting clips
Carefully lower the new sink into the opening, aligning it so the edges are even on all sides. Press down gently to seat the sink in the sealant. From underneath the cabinet, position the mounting clips or brackets so they press down evenly on the rim of the sink. Start bolting them in place, working from one side to the other and alternating sides to keep the sink level.
Tighten the bolts gradually — do not fully tighten one clip before moving to the next. Tighten all clips to a snug fit, then go around again and tighten them firmly. The sink should not move when you press down on it. Wipe away any excess sealant that squeezes out from under the sink with a damp cloth before it dries.
Reconnect the plumbing and test for leaks
Reconnect the drain line to the P-trap, hand-tightening the compression fitting first and then using a wrench to snug it. Reconnect the hot and cold supply lines to the faucet, again hand-tightening first. Turn the shutoff valve back on and open the faucet to run water through the new sink.
Watch underneath the sink for drips or leaks at every connection point. If you see water dripping, tighten the fitting a quarter turn with a wrench. Do not over-tighten — this can crack the fitting or strip the threads. Let the water run for a minute, then check again. If the leak persists, turn off the water, disconnect the fitting, and inspect the compression washer inside. If it is cracked or worn, replace it and reconnect.
Run water for several minutes and check the cabinet floor for any water. If everything is dry, the installation is complete. If you see water pooling under the sink, turn off the water and identify which connection is leaking before you use the sink again.
Frequently Asked Questions
Can I reuse the old mounting clips on the new sink?
Usually not. Mounting clips are designed for a specific sink model and may not align with the rim of your new sink. Check the manufacturer's instructions for your new sink — they will specify which clips to use. Most new undermount sinks come with their own mounting hardware, or you can order the correct clips from the sink manufacturer.
What if the countertop cracks when I remove the old sink?
Stop work when ready. A cracked countertop will only get worse if you continue, and water will seep into the cabinet and cause rot. Contact a countertop specialist to assess the damage. Depending on the material — laminate, granite, quartz, or solid surface — the repair may be a straightforward epoxy fill or may require replacing a section of the counter.
Do I need a plumber to do this job?
Not necessarily, if you are comfortable working with plumbing connections and have the right tools. The hardest part is usually removing the old sink without damaging the counter. If you are unsure about any step — especially disconnecting the drain or reconnecting the supply lines — hire a plumber for those parts. The labor cost is usually less than the cost of repairing water damage from a leak.
What kind of sealant should I use around the new sink?
Use a waterproof, kitchen-grade silicone caulk or polyurethane sealant. Avoid regular latex caulk — it breaks down when exposed to water and heat. explore it in a continuous bead around the entire perimeter of the opening on the underside of the counter. Let it cure for the time specified on the tube — usually 24 hours — before running water through the sink.
How do I know if my new sink will fit the existing opening?
Measure the opening in length and width, and compare those measurements to the sink's specifications. The sink should be slightly smaller than the opening — typically a quarter inch smaller on all sides. If your new sink is larger than the opening, you will need to cut the countertop. If it is smaller, you will need to fill the gap with a trim ring or seal it with caulk.