Installing a sink under a granite or stone counter

An undermount sink not only looks sleek and stylish but it is also practical: the counter is easier to clean, and the sink has no exposed rim under which residue can build up, as often happens with traditional drop-in sinks.
This type of undermount installation requires both a different installation technique and a sink designed especially for this purpose, which is often available by special order only. It is therefore wise to seek expert advice before you buy your sink.

The sink is held in place with simple mounting clips, which are glued to the underside of the counter with a special adhesive. Though it may not seem especially strong, this type of installation is the norm and is strong enough to support the weight of a sink filled with water.

The following steps are for fitting an undermount sink under a granite or stone counter. Since this type of material is extremely heavy, the sink must be fitted under the counter once the counter is in place, which means working inside the cabinets from below, making the work a little more complicated.

The procedure for installing an undermount sink under a solid surface (synthetic material) counter is simpler, and is presented in another guide.
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Caution
Before you start
Preparing the material

Caution

While these are our recommended steps for installing an undermount sink yourself, we suggest you call on professional help from installation experts. Installation techniques vary depending on the counter and sink material, and it is essential that the right techniques are used in order to guarantee solidity and water-tightness. Another reason for hiring professional help is that the pieces involved can be heavy and difficult to handle. Finally, bear in mind that manufacturers' products and accessories may call for the use of techniques that are different from those presented in this guide. For further information, refer to the supplier installation manuals.

Before you start

First, ask a professional to cut the hole in the counter; a template is provided with the sink. Since this step is carried out by a third party, the procedure is not included here.

Items such as the strainer and the cutting template, as well as the hardware required for undermount installation, are usually provided with the sink. You will nevertheless need all the following items in order to install the sink professionally:
  • A set of bar clamps of suitable length
  • A piece of 2" x 4", longer than the width of the sink
  • Solvent, such as denatured alcohol, to clean the surface
  • Transparent silicone caulk
  • Epoxy resin adhesive for granite or natural stone
  • 80-grit sandpaper
  • Utility knife (with new blade)
  • Scriber

Preparing the material

  1. Remove the mounting clips from the base plate and rethread the cap screws to stop glue from filling the threads.
  1. Sand the underside of the counter around the rim of the hole so that the adhesive will bond well. Clean well to remove all traces of dust and dirt. This step is essential to the success of your project.
  2. Working from the underside of the counter, line the sink up with the hole and clamp it in position to prevent it from moving.
  3. Mark out the position of the sink rim with a scriber.
  4. Identify and mark out the locations of the base plates used to fasten the sink. These must be installed ¼" in from the rim of the sink and positioned lengthwise (long side running along the edge of the sink). You will need 2 clips on each side, for a total of 8 clips.
  5. Remove the clamps and the sink.
  6. Clean the underside of the counter around the hole with alcohol so that the glue will bond properly to the surface.
  7. Apply epoxy adhesive to the base plates, then position them in the locations marked in step 5. Hold them in place for 10 seconds until bonded.
  8. Put more adhesive on the plates and leave the resin to set for 30 minutes.
  1. Mount the clips back on the base plates.
  2. Clean the rim of the sink with alcohol.
  3. Run a bead of silicone around the rim of the sink, about 3/8" from the edge.
  4. Lay the piece of 2" x 4" across the top of the hole in the counter.
  5. Put the sink in position.
  6. Attach one end of a bar clamp onto the 2" x 4", pass the clamp through the main drain hole, and attach the other end under the sink. Partially tighten the clamp, ensure the sink is positioned properly, then tighten the clamp until the sink is held firmly under the counter. You will need two clamps if you have a double-bowl sink.
  1. Position the clips around the rim of the sink and screw firmly in place (do not overtighten).
  2. Dab a bead of silicone on the head of the cap screw as well as on the thread of the wing nut to prevent them from loosening due to vibrations.
  3. Cut away any excess silicone around the edge of the sink using the utility knife.
  4. Leave to set for 24 hours before removing the clamps and the length of 2" x 4".
All you need to do now is to hook up the drain and the faucets, and you’re all set to enjoy your new sink!
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