Determining Sink Hole Size

Question:
When fabricating a countertop, how do I determine how big to make a sink hole? Not all sinks are the same.

Answer:
Dealing with the sink’s geometry is a routine but critical part of making a high-quality concrete countertop. Complicating matters are the many different styles of kitchen sinks available. Here are the styles you are most likely to encounter and pointers for sizing the countertop properly to accommodate them.

Drop-in sink. This traditional sink style (also called a self-rimming or top-mount sink) has a finished rim that fits over the countertop. Faucets and other plumbing hardware are mounted directly to the sink and do not need holes in the countertop for them.

Drop-in sink

Undermount sink. Unlike a drop-in sink, this style mounts underneath the countertop so that the inner edge of the sink hole in the countertop is visible. Faucets and other plumbing fixtures are mounted separately to the countertop.














Undermount sink

Undermount sinks can be mounted in one of 3 ways:

Step mounting is when the hole in the countertop is a bit larger than the sink bowl so that some of the flange shows (sometimes called “positive reveal”). Clients tend not to like this mounting method because it leaves a shelf inside the sink where food or liquid can collect, making cleaning more difficult.


Flush mounting is when the inner edges of the countertop slab line up with the sides of the bowl where they curve into the top flange (sometimes called “zero reveal”). Alignment and shaping are very critical because a flush-mounted sink will look bad if the hole is not perfectly shaped or precisely installed.

True undermounting is when the hole in the countertop is slightly smaller than the sink hole, with the inner edges of the concrete overhanging the bowl edges by 1/8 to 1/4 inch (sometimes called “negative reveal”). This overhang allows for slight misalignment and extra maneuvering room while providing a clean, professional look.

Apron-front sink. This style has a large apron in the front and sits on a short cabinet. Usually the countertop wraps around the sink on only three sides, leaving the front of the sink exposed with no countertop in front of it. Apron-front sinks can be drop-in, undermount or freestanding, depending on whether the back and sides sit on top of the countertop, mount under the countertop or are flush with the countertop, respectively.















Most commercially available sinks come with a template showing the size and shape of the sink’s rim or flange and the size and shape of the countertop opening for the sink. For drop-in sinks, the hole in the countertop is a rough opening because the sink’s finished rim covers the hole. The hole needs only to be the right size, shape and location to accommodate the sink.

For undermount sinks, complications can arise because most templates, especially those for stainless steel sinks, call for a finished countertop opening larger than the sink’s bowl opening. This creates a step-mount reveal, where part of the sink’s flange is exposed. But as previously mentioned, most clients don’t care for this and will choose an undermount sink, partially because they like the extra depth and the look of the exposed countertop edge around the sink, but mostly because of the ease of cleaning. With a simple wipe of a sponge, crumbs, liquids and other debris can be swept off the countertop and into the sink, with no ledges, rims or other impediments that could trap dirt. Step mounting partially defeats this functionality. This means you may need to modify or even forego using the template altogether and make a new template directly from the sink to create the desired mounting geometry.

It is strongly recommended that you ask your clients which mounting method they want and specify it in the contract. Educating your clients will help them make the best choice and eliminate potential problems. When in doubt, use the true undermounting method, and write in your contract that unless otherwise specified, this is the method you will use.