How To Replace Undermount Kitchen Sink Step-by-Step

Can I replace my drop-in sink with an undermount sink? Yes, you can replace a drop-in sink with an undermount sink, but it often requires cutting the countertop to make a larger opening. This guide focuses mainly on replacing an existing undermount sink, but we will touch upon the transition from a drop-in later.

Replacing an undermount kitchen sink is a big job. It requires removing the old sink, preparing the area, and installing the new one correctly. Taking your time ensures a leak-free and beautiful finish. This process is much easier with a helper. Get ready to get your hands dirty!

Pre-Installation Checks and Preparations

Before you start wrenching things apart, you need to plan. Good preparation saves a lot of headaches later. You must confirm the new sink will fit the space.

Selecting the Right Replacement Sink

Your new sink must match the existing cutout size as closely as possible, or be slightly smaller. If the new sink is much smaller, you’ll have a large gap in your countertop that needs filling. If it’s too big, you’ll have to cut your countertop, which is a major step.

Key Measurements to Check:

Sink Basin Size: Measure the length and width of the old sink bowl.
Faucet Placement: Where will the faucet mount? Does the new sink have holes, or will you use the existing countertop holes?
Cabinet Clearance: Ensure the new sink depth fits below the cabinet structure and any plumbing pipes.

Gathering Necessary Tools and Materials

Having everything ready before you turn off the water is smart. Here is what you will likely need for the job.

Tool Category	Specific Items Needed	Purpose
Safety Gear	Safety glasses, gloves, dust mask	Protect eyes, skin, and lungs.
Demolition	Utility knife, putty knife, pry bar	Cutting sealant and separating the sink.
Plumbing	Adjustable wrench, basin wrench, bucket, towels	Disconnecting water lines and drains.
Installation	Drill, clamps, mounting hardware (clips, bolts), plumber’s putty or silicone caulk	Securing the new sink and sealing edges.
Cleanup	Scraper, cleaner, rags	Removing old adhesive and grime.

Step 1: Disconnecting Plumbing and Drains

Safety first! Always shut off the water supply before touching any pipes under the sink.

Turning Off Water Supply

Locate the hot and cold shut-off valves under the sink cabinet.
Turn both valves clockwise until they stop.
Turn on the faucet to drain any remaining water from the lines. Keep the faucet open slightly during the rest of the process to relieve pressure.

Removing Disposal and Drains

If you have a garbage disposal, you must unplug it first. If you are keeping the disposal, you might be able to leave it attached to the sink flange for now. However, for a complete undermount sink installation, it is best to remove it.

Place a bucket directly under the P-trap (the curved pipe).
Use an adjustable wrench to loosen the slip nuts connecting the P-trap to the drain tailpiece and the pipe going into the wall. Let the water drain into the bucket.
If you have a separate strainer basket, you need to remove the locking nut holding it in place. This often requires a special tool or a large pair of channel locks.

Detaching Faucet Supply Lines

Use a basin wrench or adjustable wrench to loosen the nuts connecting the flexible supply lines to the faucet shanks coming down through the counter.
Have towels ready, as some water will drip out.

Step 2: Removing the Old Undermount Sink

This is often the hardest part. The sink is held in place by strong clips and thick adhesive. You must carefully break this bond.

Locating and Loosening Mounting Hardware

Look up inside the cabinet, directly beneath the edge of the sink basin. You will see replacing kitchen sink clips or mounting bolts. These are usually metal tabs tightened against the underside of the counter.

Use a socket wrench or screwdriver to loosen every single mounting clip. Do not remove them completely yet, just loosen them enough so they no longer press hard against the counter.

Cutting the Sealant

The perimeter of the sink is sealed to the bottom of the countertop, usually with silicone caulk or strong adhesive.

Use a sharp utility knife. Carefully run the blade along the entire seam where the sink meets the stone or laminate underneath.
Go slowly. You are trying to cut through the old caulk, not damage the countertop material.
For stubborn spots, use a thin putty knife to gently pry the edge away from the counter surface.

Separating the Sink

Once the clips are loose and the sealant is cut, the sink should be free, but gravity might hold it.

Have your helper ready.
If the sink is very heavy (like cast iron), be prepared for the weight shift.
From above the sink, gently push down and then try to lift one corner. If it doesn’t move, check for missed clips or stubborn adhesive.
Work slowly around the perimeter. Once loose, carefully lift the old sink out of the opening and set it aside. This is where removing old undermount sink takes teamwork.

Cleaning Up the Underside of the Countertop

Now you have access to the mounting area. You must prepare this surface perfectly for the new sink.

Use a stiff scraper or razor blade to remove all traces of old silicone, caulk, or adhesive from the stone or laminate. Be very careful not to scratch the finished surface.
Wipe the area down with denatured alcohol or mineral spirits to ensure the surface is completely free of grease and oils. A clean surface is vital for the best adhesive for undermount sink to stick properly.

Step 3: Preparing the New Sink and Countertop

If you are simply swapping one undermount sink for another of the exact same size, this step is quick. If you are replacing drop-in sink with undermount, you will need to address the counter opening size now.

Addressing Countertop Openings

If you are moving from a drop-in to an undermount, your current hole is likely too small and possibly has a visible lip.

If the Hole is Too Small: You must cut the countertop. This is advanced work, especially with granite or quartz. Unless you have experience with wet saws and diamond blades, hire a professional fabricator for this. Cutting the material incorrectly can ruin the entire counter. This is the main reason why many DIY guides suggest cutting countertop for undermount sink should be left to experts.

Installing Faucets and Drains on the New Sink

It is always easier to install the faucet and drain baskets onto the new sink before mounting it under the counter.

Install Strainer Baskets: Apply plumber’s putty around the rim of the strainer basket. Insert it into the drain hole from above. From below, tighten the locknut firmly. Wipe away excess putty immediately.
Install Faucet: Feed the faucet supply lines and mounting shank(s) through the holes in the sink deck. Hand-tighten the mounting nuts from underneath. Do not fully tighten the faucet until the sink is mounted and you can check alignment from above.

Step 4: Mounting the New Undermount Sink

This stage uses the undermount sink mounting hardware to secure the basin firmly beneath the counter.

Applying Sealant

You need a watertight seal between the sink rim and the countertop underside.

Apply a generous, continuous bead of high-quality 100% silicone caulk around the top edge (the perimeter flange) of the new sink basin. This bead will form your primary seal. This is crucial for how to seal undermount sink correctly.

Positioning the Sink

With your helper, carefully lift the prepared sink up into the opening from below.
Align the sink precisely within the space. Make sure the faucet is pointing straight ahead and the drain alignment is correct.
Press the sink firmly against the bottom of the countertop so the silicone squeezes out slightly.

Installing the Mounting Clips

This step secures the sink against the pressure of the cured silicone and the weight of the sink itself.

Install the undermount sink clips. Place them evenly around the perimeter. Space them according to the sink manufacturer’s instructions. These clips usually have a bracket that hooks onto the sink lip and a screw that tightens against the countertop.
Use your wrench to tighten the clips gradually and evenly. Tighten opposite clips sequentially (like tightening lug nuts on a tire) to distribute the clamping force evenly. You are tightening these until the sink is firmly held against the sealant, but do not overtighten, especially if your countertop is thin or delicate. The goal is firm support, not crushing the stone.

Sealing Sink to Granite Countertop (or other material)

If you have granite or quartz, applying the sealant correctly is key to sealing sink to granite countertop for longevity.

Allow the clips to hold the sink firmly for at least an hour.
Use a putty knife or gloved finger dipped in mineral spirits to smooth the squeezed-out silicone around the entire perimeter from underneath. A smooth bead looks cleaner and ensures no gaps remain.
Note: Some installers prefer to apply a bead of silicone directly to the bottom of the counter edge before lifting the sink in, rather than relying only on the bead applied to the sink rim. Both methods can work, but ensure full coverage.

Step 5: Reconnecting Plumbing and Final Checks

With the sink securely fastened, you can put the plumbing back together.

Reinstalling Drains and Disposal

If you removed the strainer basket, install it now. Ensure the plumber’s putty seal is intact.
Reattach the P-trap assembly and connect it to the new sink’s drain tailpieces. Hand-tighten the slip nuts first, then use the wrench for a quarter-turn snugness. Do not overtighten plastic nuts.
If you removed the garbage disposal, lift it back onto its mounting ring and secure it according to its specific instructions. Plug it back in.

Reconnecting Water Lines

Attach the flexible supply lines from the faucet shanks back to the shut-off valves. Use a basin wrench or adjustable wrench. Tighten them securely, but again, avoid excessive force.

Testing for Leaks

This is the moment of truth.

Slowly turn the hot and cold water shut-off valves back on (counter-clockwise).
Fill the sink basin halfway with water. Watch the supply line connections closely for any immediate drips.
Pull the sink stopper and let the water rush down the drain. Watch the P-trap and drain connections closely, both underneath and on the visible pipes.
Run the faucet for several minutes, checking all connections again. If you find a small drip, gently tighten that specific nut just a little more.

Curing Time

Silicone caulk needs time to cure fully. Do not put heavy items in the sink or use the garbage disposal for at least 24 hours, or as directed by the caulk manufacturer. Respecting this cure time ensures a strong, lasting seal.

Transitioning: Replacing Drop-In Sink with Undermount

If you started with a replacing drop-in sink with undermount, you likely completed the major difficulty already: cutting countertop for undermount sink.

If the existing hole is too big for the new undermount sink, you cannot proceed with standard mounting. You would need a custom piece of solid surface material (like a matching piece of granite or stainless steel) to bridge the gap between the sink rim and the existing counter edge. This effectively turns the setup into a custom drop-in style, which defeats the sleek aesthetic of a true undermount.

For a true undermount transition:

The old drop-in sink must be removed.
The lip of the drop-in sink must be cut off the countertop.
The new opening must be perfectly sized for the new sink flange, allowing the stone edge to overhang the sink bowl by about 1/2 inch to 3/4 inch on all sides.
Proceed with Step 4 (Mounting) as described above.

Advanced Tips for Success

Achieving a professional result requires attention to detail. Pay attention to these areas, which are often covered quickly in basic guides, but are critical for longevity.

Importance of Clamping During Curing

When you are securing the sink, the pressure provided by the undermount sink mounting hardware must be maintained while the silicone cures. If the pressure lessens, air can get into the seal, leading to weak spots or failure. Some professionals use specialized clamps that grip the countertop edge from above and press down onto the sink flange from below, offering better, more consistent pressure than standard clips alone.

Choosing the Right Adhesive for Undermount Sink

While silicone is standard for the final seal, some heavy sinks benefit from an extra layer of structural support.

Silicone Caulk: Excellent for waterproofing and flexibility. Use 100% silicone, mold-resistant kitchen/bath grade.
Epoxy or Construction Adhesive: Some installers apply dabs of a heavy-duty construction adhesive (like Liquid Nails for heavy-duty applications) alongside the silicone for extra bonding strength, especially on granite. This is often seen in heavy cast iron sink installations. Always follow the manufacturer’s guidelines regarding when these products can be removed or covered.

Getting Help with Video Resources

For visual learners, searching for an undermount sink installation video is highly recommended before starting. Seeing the torque applied to the clips and how the sealant squeezes out can be invaluable context for tackling the physical labor involved.

Frequently Asked Questions (FAQ)

Q: Do I need clips if I use construction adhesive?

A: Yes, you still need clips or mounting hardware. Adhesives provide bonding strength, but the clips provide the necessary clamping force to push the sink firmly against the counter while the adhesive cures and to support the sink’s weight permanently against gravity.

Q: How long does the sealant need to dry before I can use the sink?

A: For standard 100% silicone kitchen caulk, you should wait a minimum of 24 hours before subjecting the seal to water pressure or heavy loading. Always check the specific product label for the manufacturer’s cure time recommendation.

Q: What happens if my new sink is slightly smaller than the old one?

A: If the opening is slightly larger than the new sink flange, you must use filler material or specialized brackets to secure the new sink firmly using the available mounting points. If the gap is large, you risk water intrusion or the silicone failing prematurely because the support structure is too far apart. It might require professional countertop work to patch the opening first.

Q: Can I reuse the old mounting clips?

A: It is strongly recommended that you use the new undermount sink mounting hardware that comes with your new sink. Old clips might be rusted, bent, or not perfectly sized for the new sink’s lip profile, leading to uneven pressure or failure.

Q: My countertop is laminate. Can I use an undermount sink?

A: While technically possible, it is very risky. Laminate countertops are made of particleboard layers. If water penetrates the seal, the particleboard swells rapidly, destroying the counter and potentially pulling the sink down. If you must do this, use extra sealant and check the seal religiously. Stone (granite, quartz, solid surface) is the preferred material for undermount sinks.