What Is an Undermount Sink?

Undermount sinks are a relatively new innovation, arriving on the scene in response to new developments in countertop materials. In the days before running water and P-trap drains, a washstand was little more than a basin that sat on the kitchen table or counter. Eventually, homesteaders got the idea to cut a hole in the countertop or a recess in front of it so the washstand could rest inside the cabinet, which lowered the rim to a more convenient level. Thus were born the drop-in and farmhouse sinks, and for many years after the development of modern plumbing, these were the only types of sinks available.

Kitchen interior with focus on undermount sink.
credit: alabn/iStock/GettyImages
A stainless steel undermount sink helps make a bright kitchen even brighter.

The undermount sink is a third option that starting becoming popular in the mid-twentieth century along with new synthetic solid-surface countertop materials, such as Corian. An undermount sink is one that is fastened to the underside of the countertop and held in place by anchor posts secured by strong adhesive or screwed into the underside of the countertop. Because this type of sink is normally used in conjunction with an upscale countertop made of a solid material, it has gained a reputation for being more elegant than its top- and front-mounted counterparts. Its popularity has only increased over the years, and it has advantages that often make it the best sink choice.

Why Choose an Undermount Sink?

One of the disadvantages of a traditional drop-in sink is that the rim rests on the countertop. It forms a border around the sink that makes it more difficult to keep the countertop clean. Besides preventing you from wiping crumbs, spilled liquids and dirt directly into the sink, the rim can actually collect debris. You generally seal the rim with caulk when installing the sink, but the caulk can wear out after a few years and allow water to seep underneath the rim and into the sink cabinet. The design of an undermount sink avoids these problems.

Undermount sink bracket.
credit: The Home Depot
You have to glue the sink support brackets to some countertop materials.

An undermount sink also has a rim, but it's sealed to the underside of the countertop, so there's no border around the sink opening. The edge of the countertop hangs over the sink, which makes it easy to wipe countertop debris into the sink. Because the rim isn't visible, you see more of your granite, marble or synthetic stone countertop, and the sink is easier to keep clean. As a bonus, you get that much more countertop work space.

Tips

Undermount sinks work only with solid countertops, such as those made from composite stone, solid-surface material, natural stone, or solid wood. They don't work with laminate countertops because MDF and plywood edges are unattractive and vulnerable to water damage. Nor do they work with ceramic tile countertops, which are usually built on a core of plywood.

Undermount Sinks Have Drawbacks

Although there are many advantages to undermount sink, this is not the best optoin if you're looking for a budget model. Undermount sinks cost, on average, about 50 percent more than a comparable drop-in sink. And you'll need to a slightly higher labor cost for professional installation, because undermounts are more difficult to install.

The biggest potential problem with an undermount sink is that it can fail. The anchors securing the sink to the underside of the countertop can come loose, causing the sink to sag or even fall. Moreover, if the caulk seal between the rim of the sink and the underside of the cabinet fails, water can seep past the rim and into the cabinet. Because you can't see the rim, this seepage could go on for a long time and cause a lot of damage before you're aware that it's happening.

Three Rim Styles

The rim of an undermount sink doesn't have to be completely hidden. It can be partially visible, or it can be flush with the edge of the countertop. The three available rim options include:

  • Negative reveal: The rim is set back from the edge of the countertop so the edge completely overhangs the sink. This is the best-looking option, but it leaves the edge of the countertop more vulnerable to chipping, and it leaves a small lip on the underside of the countertop that can support mold growth.
Undermount kitchen sink.
credit: PlumbingSupply.com
The countertop completely overhangs a negative reveal sink.
  • Positive reveal: The edge of the countertop bisects the rim, leaving part of the rim visible. This makes it easier detect a leak, but the exposed rim can catch food particles and thick liquids.
  • Flush mount: The edge of the countertop coincides exactly with the inside of the sink bowl. This is the easiest configuration to keep clean, but because it calls for the most precision during installation, it may cost more to install.

Undermount Sink Materials

The best undermount sink materials for DIYers are lightweight ones, such as stainless steel and copper.. These materials are easy to support and clean, and they look great with most countertop materials. Composite granite and porcelain-coated cast iron are heavier materials, but they acan provide a graceful complement to ceramic, stone and wood countertops, providing they are sturdy enough to support the sink. The dark color of composite granite, which is manufactured with actual bits of real stone, provides a pleasing contrast in a decor scheme that is predominately white. Cast iron sinks are usually coated with white porcelain, but a variety of other colors, including black, is also available.

What's Involved With Installation?

Before you can install any type of sink, you have to cut a hole in the countertop, and because undermount sinks work only with solid materials, cutting the hole can be challenging. Many countertops, especially stone ones, come with pre-cut holes, though, so you might not have to worry about that.

Before mounting the sink, the installer has to set the anchor posts to hold it. You can often screw these anchors into wood or synthetic stone, but if the countertop is real stone, the best option is epoxy glue. Once the posts are set, the installer positions the sink and fastens the sink to the posts with special clips. Once that's done, drain and garbage disposal installation complete the job.

Detail of elegant kitchen furniture with decorative sculptures of cats near the window
credit: Edita Meskoniene/iStock/GettyImages
If you have an undermount sink, the faucet gets mounted to the countertop.

Tips

The faucet gets mounted to the countertop, so it can be installed before the sink is fastened down and connected, when there more room in the sink cabinet to work.


Chris Deziel

Chris Deziel

Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience, and he is also an avid craftsman and musician. He began writing on home improvement topics in 2010 and worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. He currently contributes a monthly property maintenance blog on Landlordology.com. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.