Undermount Kitchen Sinks

By Tim Carter

Summary: Undermount kitchen sinks are sleek and gorgeous. Kitchen sinks that undermount below the countertop are perfect for granite, marble, solid surface or engineered stone countertops. A granite undermount kitchen sink is what I have in my own home.

DEAR TIM: Are undermount kitchen sinks a good idea? I have never had one, and am worried about leakage where the undermount sink contacts the underside of the countertop. I want a granite undermount kitchen sink, but am worried the sink will pull away from the granite countertop. This would be a nightmare, especially if the sink was filled with water. What undermount sink installation tips can you share that will allow me to buy one with complete confidence? Christy F., Flemington, NJ

DEAR CHRISTY: The first thing I can tell you is an undermount kitchen sink will give you a lifetime of leak-free performance if it is installed correctly. I have several undermount sinks in my own home, and they simply do not leak where the sink touches up against my granite countertops.

Years ago I felt the same way you did about undermount kitchen sinks. It was the plumber in me that made me a Doubting Thomas. After all, a standard double-bowl sink that has a disposer on one side and is completely filled with water weighs well over 100 pounds. I think it is reasonable for a person to wonder how in the world the sink will stay permanently attached to the countertop with this kind of weight regularly trying to pull the sink down into the base cabinet.

High-strength epoxies
and silicone caulk are the simple answer. The undermount sinks are held in place with metal anchors that are glued to the underside of the stone with a two-part epoxy that cures rapidly and has incredible tensile strength once cured. Furthermore, a bead of silicone caulk helps bond the sink to the stone.

One installation tip is to make sure the underside of the stone countertop is dust-free. Dust will cause the epoxy bond to fail. Any installer worth his salt will take a brush to remove stone dust from underneath the countertop. Dust in this location is a strong possibility because of the cut made to create the hole for your sink.

The granite countertop fabricator had to cut a special-shaped hole in the stone top to create the hole for your undermount sink. Once the cut is made, the fabricator has to polish the rough stone edge. The water that is used during the polishing process gets very small pieces of stone in it, and this slurry can roll to the underside of the countertop. Once the water component of the slurry evaporates, the stone grit or dust is left behind.

Most undermount sinks come with very clear written installation instructions. Every set of instruction I have seen calls for a bead of clear silicone caulk to be placed around the flat lip of the undermount sink. This continuous bead of caulk acts as the waterproof seal between the sink and the countertop once the undermount sink is pressed permanently against the stone countertop.

Most professional installers can install an undermount kitchen sink in 30 minutes or less. The tools required are a 2x4 longer than the sink cutout is wide, one or two bar clamps, the special two-part epoxy and the caulk and caulk gun.

Once the installer knows the underside of the stone top is dust-free, he applies the silicone caulk to the sink lip. The sink is then pushed up against the underside of the stone top and centered under the polished cutout. The 2x4 is then placed on top of the countertop spanning across the sink. One end of the bar clamp goes through the large drain hole in the sink and the other end goes on top of the 2x4. A double-bowled sink needs two clamps, one for each drain hole.
The clamps are partially tightened and the reveal of the sink is checked to make sure it is even around the entire cutout of the granite. The clamps are then fully tightened. It is now time to mix the two-part epoxy and apply it to the small bolt posts that are glued next to the lip of the undermount sink. The installers are on their backs in the sink base looking up when they do this.
The epoxy sets rapidly, usually in ten minutes or so. After this wait period, the rest of the hardware that clamps the sink to the bolt posts is installed. It is good to wait at least one day before the plumber arrives to connect any drain piping or a heavy disposer to the undermount sink.

Before the drain piping is attached to the sink, it is wise to tighten the nuts that hold the clamps in place. Do not over tighten. Simply make sure they are snug. The adhesive properties of the silicone caulk in conjunction with the epoxy hold the sink permanently in place.

The biggest mistake you or an installer can make is trying to rush the job. All too often an aggressive plumber, remodler or homeowner will try to finish the entire job the same day. They will wait six or seven hours thinking the epoxy is plenty hard, only to discover that the weight of a heavy disposer pulls the sink away from the countertop.