Concrete Sink Knockouts

A sink knockout is used to create a void in the concrete for an under-mount sink. It can be made from different materials, but they are typically foam, rubber, or fiberglass. Rubber and fiberglass knockouts are re-usable for many many castings, while a foam knockout can only be used a few times before it should be replaced.

What You’ll Need
  • CHENG Glossy Form Tape
  • Kitchen Sink Drain
  • Bathroom Sink Drain Form

Step 1 – Foam Knockouts

  • Foam knockouts are great for a do-it-yourself countertop project because they are affordable and readily available for a wide range of sinks.
  • 2″ or 2-1/2″ Thickness: If your countertop is going to be 2-1/2″ thick, the foam knockout has to be the same thickness.
  • Advantages: Affordable, easy to use, readily available
  • Disadvantages: Reusable only a few times, generic
  • Glossy Form Tape: Wrapping the edge of the sink with a glossy tape will leave a very smooth finish on the finished concrete piece.
  • For tips on creating your own foam sink knockout, check out the Glossy Form Tape Guide.

Step 2 – Fiberglass Knockouts

Sink Knockout 2 | CHENG Concrete Exchange
  • Fiberglass knockouts are used by professionals, but can be used in do-it-yourself projects as well. A number of companies produce fiberglass sink molds that can be reused for multiple castings.
  • Fiberglass knockouts can be used to produce integral sinks that only need drain hardware and don’t use a metal sink at all.
  • Advantages: Off-the-shelf availability, durable and reusable, Mid-range in price
  • Disadvantages: Expensive for one time use, not completely unique

Step 3 – Rubber Sink Kockout

Sink Knockout 3 | CHENG Concrete Exchange
  • Rubber knockouts are used by professionals who are doing custom work that they need to be able to reproduce.
  • Advantages: Completely unique and reusable
  • Disadvantages: Expensive, time consuming and difficult to produce

Video – Building Sinks for Concrete Countetops