How Does an Undermount Sink Mount to a Granite Countertop?

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One of the benefits of using a solid surface countertop like granite is the ability to undermount the sink. Previous to this method of installation, sinks were hung from their rims on the counter. This was an effective method of keeping them in place, but the rim was a collection site for debris and moisture.


Undermounted sinks use a variety of techniques to keep them in place beneath the granite. All of these, however, are invisible from above the counter once complete.

Silicone Caulk

  • Silicone caulk is the same substance used to install a drop in sink that hangs by its rim. This clear, fast grabbing adhesive is applied to the upper portion of the sink's rim, rather than the lower or under portion. The sink is centered below the cutout and pushed upward into place. The caulk helps to hold the sink in place while the mounting hardware is connected. The caulk also helps form a water-proof seal around the edge of the sink, which helps prevent leaks below the counter.

C-Clamp

  • Once the sink has been pushed into place with the silicone, it needs to be held securely in place while the adhesives are drying. To accomplish this, C-clamps are used. A piece of 2-by-4 that is longer than the sink is placed over the sink, extending onto the counter. A long C-clamp is placed on the top of the 2-by-4 and extended down into the sink, through the drain. For multiple bowl and drain sinks, one C-clamp is required for each drain or section of sink. It is not uncommon for the C-clamp to remain in place after installation for up to 24 hours to ensure a good bond.

Epoxy and Clips

  • The silicone caulk and the C-clamp position and hold the sink in place initially. The final installation, which makes the sink immovable involves epoxy and mounting clips. Mounting clips, which come with the sink, consist of two parts: a nut and bolt and a mounting clip. The bolts are epoxied to the underside of the granite at regular intervals around the sink. After about 10 minutes, during which time the epoxy begins to set up, the clips are epoxied to the sink directly across from each bolt.

    Once the epoxy has had a chance to harden, the clips are fitted over the bolts and the nuts are tightened into place. The epoxy can hold several hundred pounds and keeps the sink securely in place.

Causes of Failure

  • While the combination of epoxy, clips, silicone and the C-clamp works together to hold a sink securely to the underside of the granite, there are times when they can fail. This usually occurs for one of two reasons. First, epoxy must bond to a clean surface. Both the underside of the granite and the sink itself must be free of stone dust or oils before attaching. Failure to clean the sink and counter ahead of time can cause the sink to pull away. Second, the drain, garbage disposal and any waste pipes add additional weight to the sink. Failure to wait at least 24 hours for the epoxy and silicone to completely harden before installing these items can cause the bond to fail.

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