If you are not sure if your bathroom sink is an undermount sink, the answer is simple. Undermount sinks sit underneath a countertop rather than on top. They attach into the counters and are usually deeper than other kinds of sinks. Sometimes, they break and have to be replaced.
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Preparing the Area
Start this project by taking out everything in the cabinet and turning off the water supply valves located under the counter. Then, disconnect the sink's P-trap and drain. Unscrew one end of the trap from the drain tailpiece with a screwdriver and loosen the slip nut with a pair of tongue-and-groove pliers. Carefully unscrew the waste trap arm's other end and empty the excess water into a bucket.
Then, you can take apart the P-trap assembly and place a rag snugly into the drainpipe opening. Now, you can use the pliers to unscrew the drain tailpiece. Unscrew the assembly's mounting nut, tap the strainer and pull the drain assembly up and out of the sink.
Cutting Out the Undermount Sink
Undermount sinks are most often caulked into the counters with sealant, but larger ones may also have clips that hold them in place. To remove the clips, you can use a putty knife under them and apply pressure until the clips break away. Look on the underside to see where the caulk, silicone or other adhesive connects the sink's rim to the counter.
For the next part, you will need to hold the sink in place, and you may need to ask a friend to help you with this. With the bottom of the sink supported, start cutting through the sealant and continue completely around all its edges. If the sealant is a thicker material like epoxy, you may need to use a hammer and chisel to help the rim separate it from the counter.
Now take a pry bar and wedge it in to get the sink loose. A heat gun or hair dryer can also help soften the material if needed. Once the sink is out, clean up any residual sealant and remove the support anchors.
Installing the New Undermount Sink
Before buying a new undermount sink, take careful measurements to ensure that it will fit into the hole in the countertop. Take a few wooden boards and place them under the counter as a temporary sink stand. You also have the option of cutting a hole to fit a specific undermount sink, but all of these decisions should be made before you go to the store.
Using a caulk gun, apply a thick layer around the edges of the hole in the counter. Raise up the sink until it presses up against the opening to form a watertight seal. If sink clips are needed, install them as directed. Once this is done, clean up any extra caulk and let everything dry for at least 24 hours.
Now, you can reconnect your new sink's drain assembly by following the directions that came with it. You'll also have to reconnect the sink to your plumbing, and once this is finished, your new sink should be ready to use.