Undermount sinks are a popular choice among folks who are updating their kitchens or bathrooms. They look clean and modern, and since an undermount sink actually fits under the countertop, they are also functional. Simply wipe any crumbs or debris right into the sink - there is no sink lip to catch and hold anything.
Undermount sinks can be used in virtually any household application, with the only proviso being they should only be used with solid countertops. Even though a laminate countertop would be sealed, moisture would get in through the surface cracks and the countertop would eventually delaminate and bubble up.
The specific method of fastening the sink to the countertop varies by manufacturer, but in general, you want to fasten the sink to the underside of the countertop, then flip it over and place it. Some manufacturers actually provide precut, predrilled countertops to make the installation easy. However, if you are working with an uncut countertop, you will have to cut the hole yourself.
Installing the sink
1. Kitchen sinks are available in single bowl, double bowl and even triple bowl configurations, while bathroom sinks are available in all kinds of sizes and shapes. The range of sizes and configurations means you not only need to choose your sink, you need to actually have it in your possession before beginning to cut the hole in the countertop.
2. Most new sinks come with a template you can use to determine where to place the new sink. Use the template or the sink itself to mark the countertop for the hole you will cut.
3. Double-check your positioning and be sure it is right. A mistake here would be expensive.
4. Cutting the hole is straightforward. Use a 3/8" bit to drill a starter hole in one corner and then use a saber saw to cut around the outline. If you don't have a saber saw, a keyhole saw for the corners and a handsaw will work just as well, only slower. Cut from the top to minimize any tear out from the saw.
5. Once the hole is cut, flip the counter over, and mark and drill the holes for the mounting screws on the underside.
6. Using denatured alcohol on a rag, clean the underside surface of the countertop around the hole you have just cut as well as the top edge of the sink.
7. Run a bead of latex caulking or plumber's putty around the underside of the hole, and position the sink on top.
8. Tighten the mounting screws. This may cause some of the putty to ooze out around the edges so wipe it up with a clean cloth.
9. Drill the holes for the faucet and attach it while you have access to the underside of the sink.
10. Turn the countertop with sink attached over, and fasten it to the base counter. Following the countertop manufacturer's directions, this may require screws and fastening clips or simply a bead of adhesive.
11. Once the sink is in place, install the sink drain. Put plumber's putty on the bottom of the drain assembly and push it down into the drain opening in the bottom of the sink. There is a lock nut that tightens and holds the drain in place, so tighten it as well.
12. Allow the plumber's putty or caulk to set up overnight.
13. Attach flexible compression fitted water supply lines to the faucet stems and then to the water supply lines.
14. Assemble and attach the S trap and drainpipe to the sink bottom and the household drainpipe.
15. Finally, turn on the water and check for leaks. Hopefully there won't be any, but if there are, tightening the pressure couplings (gently) will usually stop them.
That's it. You've just installed a beautiful and functional undermount sink and countertop.