One of today's trends in kitchen design is a sleek, modern, efficient look — which has given rise to the popularity of the undermount sink. Besides its streamlined appearance, homeowners tend to enjoy it because it makes cleanup easier: just wipe food and debris directly into the sink where a garbage disposer will take it from there.
Given that the sink will be connected under the countertop, consideration must be given to the capacity of the sink — not only the weight of the sink but what it's holding.
For example, if a typical sink made of stone or cast iron is 22" x 33" is filled with 5" of water, it can weigh approximately 125 pounds before any dishes, pots, or silverware are added. This means extreme care must be taken and expertise is paramount when securing this type of sink. If the seal and sink clips fail, it would mean damage to the cabinet, floor, countertop, and plumbing, and may even cause injury.
Most undermount sinks are made of stone, cast iron, china, or stainless steel. The countertop material must be able to support the hanging weight of the sink and the garbage disposer (if it has one), along with the weight of anything put into the sink, such as water and dishes.
Keep in mind that the disposer has a tendency to cause vibration both in the sink and where it attaches to the countertop. A solid counter such as granite, marble, and other stone-like material is best for this type of sink, as well as a kitchen sink support system.
One of the differences between an undermount sink and a standard drop-in sink is that the hole must be cut precisely to match the sink dimensions. It may seem obvious, but it's worth noting that it's crucial to have the sink on hand before the countertop is cut to ensure the cut is accurate.
You'll need to prepare the countertop for mounting the sink. First, smooth the edges of the hole with sandpaper so it won't damage the sink.
Next, clean the dust and stains from the countertop and the opening with a clean cloth and acetone. Be sure to allow the surface material (e.g., granite, wood, stone, concrete) to dry completely after cleaning. You want the adhesive to be able to work properly.
Secure the sink with wooden braces, which extend from the bottom of the countertop to the deck of the cabinet. Basically, you'll be stacking boards (such as 2x4s) under the sink until it's at the exact height it needs to be. These boards will hold the sink in place while you glue it and while it is drying. As an alternative to stacking the board, you could place a 2X4 across the sink opening with straps through the drain holes to hold the sink in place.
Installing the sink can involve either gluing the sink (via a sealant) to the countertop or attaching the sink to studs (via brackets that are screwed tight after drilling into the underneath of the countertop) or both.
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The preferred method is gluing, most often used for bathroom sinks, which eliminates the need to drill holes into the bottom of the countertop for the support studs. This method could weaken the surface material and cause it to crack or break.
Assuming you are using the glue method, apply a continuous bead (about a 1/2") of sealant — such as silicon or latex or the manufacturers' recommended sealant — around the perimeter of the undermount sink.
Secure the sink to the counter with clips. There are different types of clips. Some use a stud that is drilled and epoxied into the counter with a nut that tightens the clip. Other clips are mounted to the counter with a strong two-part epoxy. In any case, follow the countertop manufacturer's recommendations for precisely how the clips should be used.
At this point, before you tighten the clips or leveling bolts and caps, you should adjust the position of the sink. You want to do this before the sealant sets, so you have the opportunity to make it square with the hole in the counter top.
In a typical installation, you should allow 24 hours for the sealant and the epoxy to properly set, before removing the wood braces, connecting the plumbing, or using the sink. This will help to prevent the sink from moving and breaking the waterproof seal.