How to Install an Undermount Sink
Updated February 20, 2026
Mount an undermount sink to granite, quartz, or solid surface -- clip placement, silicone application, drain hookup, and the 24-hour cure that makes it watertight.
Overview
Undermount sinks sit below the countertop for a clean, seamless look -- you can sweep crumbs and water straight into the sink with no rim in the way. The trade-off is a more complex installation: the sink hangs entirely from clips and adhesive underneath, with nothing resting on top to support its weight. That means the countertop has to be a solid material (granite, quartz, marble, solid surface) with a pre-cut opening. Laminate will not work -- the exposed edges absorb water and delaminate. Replacing an existing undermount is straightforward. New installation in an existing countertop needs a professional cutout. This guide covers the mounting process with the cutout already done.
What You'll Need
Safety First
- These sinks are heavy -- stainless steel runs 15-25 pounds, cast iron 50-100 pounds. You need a helper to hold the sink in position while you work the clips from below. Trying to hold a heavy sink overhead with one hand while installing clips with the other is dangerous and nearly impossible.
- Drilling into the underside of granite or quartz for clip anchors? Use only the bit type recommended by the clip manufacturer (masonry or carbide-tipped). Wrong bit can crack the stone. Drill slow with light pressure -- let the bit work.
- 100% silicone adhesive only. Not silicone-latex blend, not construction adhesive. Only 100% silicone gives you the waterproof, flexible bond an undermount needs. Full 24-hour cure before you use the sink.
Step-by-Step Instructions
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Prepare the Countertop and Sink
Replacing? Cut through the old silicone from below with a utility knife, remove the clips, lower the sink out. Scrape all old silicone and adhesive from the underside of the countertop with a razor scraper, then clean with rubbing alcohol. The surface has to be completely clean and dry or the new silicone will not bond. New installation? Verify the cutout matches the sink template -- the cutout should be slightly smaller than the bowl, leaving a 1/4 to 1/2 inch reveal of countertop edge visible inside the sink. Dry-fit the sink from below to check alignment.
Tip: Do a full dry run before any adhesive goes on. Have your helper hold the sink in position while you test-fit every clip. Mark clip positions with a pencil. This rehearsal ensures everything fits before silicone goes on and the clock starts. -
Install the Mounting Clips
Clip styles vary: epoxy-mounted (glued to the underside of the stone with two-part epoxy -- most common for granite and quartz), channel clips (slide into a channel on the sink rim), and bolt-through (for solid surface where bolts go through the countertop). For epoxy clips: mix the two-part epoxy per instructions, apply to the clip base, press firmly against the countertop underside so the clip arm engages the sink rim. Space every 6-8 inches around the perimeter. Let the epoxy cure completely (30-60 minutes) before moving on. The clips must be rock-solid before they take the weight of the sink.
Tip: Some manufacturers include a mounting template with clip positions pre-marked. Tape it to the underside and follow it. Even spacing matters -- clips too far apart let the sink sag between them. -
Apply Silicone Adhesive to the Sink Rim
Run a continuous bead of 100% silicone around the top rim of the sink -- the flat surface that presses against the countertop. Generous bead, about 1/4 inch wide, no gaps. The silicone does double duty: waterproof seal and extra adhesive strength beyond the clips. Some installers also put a bead directly on the underside of the countertop around the cutout for extra security.
Warning: You have 15-20 minutes of working time before the silicone skins over. Once it skins, it will not bond properly. Have your helper ready and all clips accessible before you squeeze the first bead. -
Mount the Sink and Secure the Clips
With your helper, lift the sink into position from below. Press the silicone-coated rim firmly against the countertop. Align so the reveal is even on all sides. While your helper holds it, engage the clips. Tighten evenly -- quarter-turn passes around the perimeter, gradually pulling the sink tight. Check alignment after every pass. Once all clips are snug, wipe excess silicone from inside the sink and the countertop underside with a damp rag.
Tip: Heavy cast iron or fireclay sink? Use temporary support from below during the 24-hour cure. Stack boards or use a car jack with 2x4 lumber to hold the sink tight against the countertop. Clips alone may not hold a 75-pound sink overhead for a full day. -
Connect the Drain and Faucet
Install the basket strainer: plumber's putty around the flange, insert from above, tighten the locknut from below. Connect the tailpiece and P-trap. The faucet goes through the countertop holes, not the sink -- undermount sinks do not have faucet holes. Connect supply lines to the faucet and shut-off valves. Garbage disposal? Mount it to the disposal-side drain flange now.
Tip: Since the faucet mounts through the countertop, make sure the spout reach is long enough to direct water into the sink, not onto the countertop edge. Measure from the faucet holes to the sink edge before you buy. -
Allow Cure Time and Test
Do not use the sink for 24 hours -- the silicone needs a full cure to reach bonded strength. After 24 hours, fill with 2-3 inches of water and release. Check under the sink for drips from the drain, strainer seal, and P-trap. Check the perimeter where the sink meets the countertop -- no water should seep between the sink and stone. Run the faucet for several minutes and check everything again.
Tip: After curing, check the silicone bead from above (inside the sink where the rim meets the countertop). Any gaps or thin spots? Apply a thin finishing bead of clear silicone and smooth with a wet finger. Creates a secondary waterproof barrier on the visible side.
Pro Tips
- The countertop cutout should be done by the fabricator, not you. An improperly sized cutout in stone cannot be easily corrected. Provide the sink template to the fabricator so it is cut at the shop.
- Clean both the sink rim and countertop underside with rubbing alcohol right before applying silicone. Dust, oil, or skin residue weakens the bond. Wear gloves to avoid transferring oils from your hands.
- Large single-bowl sink? Consider adding a center support bracket (a metal strip spanning the cutout from below). Big sinks can sag in the middle over time, especially heavy stainless and cast iron.
- Clear silicone if any adhesive will be visible from above through the reveal. White is stronger but shows. Black blends well with dark granite.
- Solid surface countertops (Corian, etc.) use a completely different technique -- the sink is bonded directly with manufacturer-specific adhesive for an invisible seam. That process is best left to a certified fabricator.
When to Call a Pro
Call a pro if you need the cutout made (specialized tools and expertise), if the sink is over 50 pounds (cast iron or fireclay), if you are working with solid surface that needs specialized bonding, if the old installation failed because clips pulled out of the stone (may need repair), or if you are not comfortable working overhead with adhesive and heavy objects.