Installing a sink under a solid surface 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 solid surface (synthetic material) counter. As this type of material is relatively light, the counter can be turned over to install the sink, which gives you much more room for manoeuvre, and then flipped back over again in order to position it over the cabinets. This is not possible with granite or natural stone counters, which are extremely heavy; the procedure for these types of counters is presented in another guide.
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Caution
Before you start
Preparing the material
Installation steps:

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. You should also check with a professional that the weight of the sink is suitable for the type of counter you have chosen. 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.
If you choose a granite sink, which is very heavy, you must first strengthen the solid surface with ½" plywood installed under the counter. This step is not necessary if you have a stainless steel sink or a sink made of a lighter material than granite.
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 clamps of suitable length
  • Solvent, such as denatured alcohol, to clean the surface
  • Transparent silicone caulk
  • Seam adhesive for solid and synthetic surfaces
  • 80-grit sandpaper
  • Utility knife (with new blade)
  • Hot-melt glue (hot glue “gun”)
  • Scriber
  • 4 guide blocks
  • 8 small stop blocks of 2" x 2" made from ½" plywood
  • Cellophane

Preparing the material


  1. Make 4 guide blocks of 3" x 3" from a sheet of ½" plywood. These blocks will be used to indicate the position of the sink edges.
  2. Cut 8 small stop blocks of 2" x 2" from ½" plywood. These blocks are simply temporary “dams” placed between the sink mount hardware and the hole in the counter, to stop the glue from running towards the hole and setting which would prevent you from being able to position the sink properly.
  3. Remove the mounting clips from the base plate and rethread the cap screws to stop glue from filling the threads.

Installation steps:


  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. Glue the guide blocks in the centre of each side using the hot-melt glue (see illustration). These blocks will show you where to place the edges of the sink.
  6. Remove the clamps and the sink.
  1. Clean the underside of the counter around the hole with alcohol so that the glue will bond properly to the surface.
  2. Using cellophane, cover the side of the stop blocks that will be in contact with the glue, so as to be able to remove the blocks easily once the glue has set.
  3. Place a stop block in front of each mark indicating the location of a base plate. The blocks should be on the inside (hole side) and their cellophane-covered edge should be facing the location of the plate. Clamp the blocks in place.
  4. Apply seam adhesive at the locations marked out in step 7 for the base plates.
  5. Put each base plate in position, pushing it down into the adhesive.
  6. Put more glue on the base plates (avoiding the screw) and leave to set for approximately 30 minutes.
  7. Remove the clamps and the stop blocks.
  8. Mount the clips back on the base plates.
  9. Clean the rim of the sink with alcohol.
  10. Run a bead of silicone around the edge of the hole, about 3/8" from the edge.
  1. Turn the sink over and place it between the guide blocks. Clamp it firmly in place.
  2. Position the clips around the rim of the sink and screw firmly in place (do not overtighten).
  3. 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.
  4. Cut away any excess silicone around the edge of the sink using the utility knife.
  5. Leave to set for 24 hours before removing the clamps.
  6. Turn the counter back over and position it on top of the cabinets.
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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