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How to Replace an Overmount Sink With an Undermount

Mary Lougee

Replacing an overmount sink with an undermount sink will hide the sink lip underneath a countertop and provide clean, modern lines. Undercounter sinks form a waterproof seal with the use of silicone caulk, clamps and the appropriate curing time. Replacement steps involve disconnecting plumbing, removal of the old overmount sink, installing the new sink and reconnecting the plumbing.

Step 1

Turn off both the hot- and cold-water supply lines by turning them counterclockwise. Unscrew the nut with an adjustable wrench at the top of the hot-water supply line that leads to the faucet. Turn the nut counterclockwise until it is loose enough to turn by hand and disconnect. Remove the cold-water supply line to the faucet in the same manner.

Step 2

Unscrew the drain line by turning the large plastic nut with adjustable pliers counterclockwise and pull it loose. Unscrew the nut on the other end of the P trap in the same manner. Pull loose and set aside all plumbing parts.

Step 3

Unscrew the mounting clips with a screwdriver from underneath the top-mount sink that hold it to the countertop. Lift the top-mount sink out of the countertop and set aside. Clean the top of the countertop thoroughly with a cleanser and wipe dry to remove any plumber's putty, silicone glue or caulking.

Step 4

Turn the undercounter sink upside down on the countertop. Apply clear silicone caulk to the lip of the sink. Place the sink inside the cabinet under the countertop and press upward into the sink opening while centering. Place a 2-by-4 across the top of the sink with it resting on the countertop on each side of the sink opening.

Step 5

Place a bar clamp on the bottom of the cabinet floor and push the other end through the sink drain hole with the other end resting on top of the 2-by-4. Tighten the clamp to hold the sink in place and tightly to the underside of the countertop.

Step 6

Mix two-part epoxy according to package instructions and apply to the bolt posts on the sink lip. Wait the appropriate time according to the epoxy package for the product to cure completely. Screw wing nuts onto the bolt posts by hand and tighten with pliers. Let the sink sit for 24 hours so that it forms a tight seal.

Step 7

Remove the bar clamps from the drain holes and 2-by-4. Reattach all of the plumbing in the reverse order of its removal. Test the plumbing for any leaks by turning the hot- and cold-water faucets on and observing for any water leaks.