An undermount sink is as handsome as it is practical; the countertop is easier to clean and no residue can accumulate under the sink edge.
This type of undermount installation involves a specific technique and a sink designed especially for this purpose, often available by special order only.
The sink is supported from below with special mounting clips glued under the countertop. This structure is strong enough to support the weight of the sink when filled with water.
An undermount sink is more complicated to install than a drop-in sink. Follow these steps and the manufacturer's instructions carefully, and if in doubt, call a professional installer.
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While it is possible to install an undermount kitchen sink yourself, we suggest you call on professional help from installation experts if you are in any doubt. Undermount installation techniques depend on the type of counter and sink. The materials can be heavy and cumbersome. Finally, manufacturers' products and accessories may involve techniques different from those presented in this guide. For more information, refer to the supplier installation manuals.
First, ask a professional to cut the sink hole in the counter; a template is provided with the sink. Instructions for this procedure are not included in this guide.
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, the cutting template and the hardware required for undermount installation are usually provided with the sink. You will nevertheless need all the above-listed tools and materials in order to install the sink professionally.
This guide provides instructions on installing an undermount sink, assuming that the plumbing under the counter is in place and ready to connect up. Solid-surface countertops are not particularly heavy. This means that the countertop can be turned over to install the sink, giving you more room for manoeuvre, and then flipped back over again once the sink is in place, in order to position it on the cabinets.
Wear safety glasses and protective gloves when working under the counter.
Turn off the water supply.
1.1. Make four 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.
1.2. Cut eight small stop blocks of 2" x 2" from ½" plywood. These blocks will act as temporary "dams" between the sink mounting hardware and the hole in the countertop. They stop the adhesive from running towards the sink hole and subsequently setting, which would prevent you from positioning the sink properly.
1.3. Remove the mounting clips from the base plate and rethread the cap screws to stop glue from entering the threads.
2.1. Sand the underside of the counter around the rim of the hole so that the adhesive will bond well.
2.2. Clean well to remove all traces of dust and dirt. This step is essential to the success of your project.
2.3. Line the sink up with the hole under the counter.
2.4. Clamp it in position to prevent it from moving.
2.5. Use a scriber to mark out the position of the sink rim.
2.6. Identify and mark 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 two clips on each side, i.e. eight clips in all.
2.7. 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.
2.8. Remove the clamps and the sink.
These blocks act as temporary "dams" between the sink mounting hardware and the hole in the countertop. They stop the adhesive from running towards the sink hole and subsequently setting, which would prevent you from being able to position the sink properly.
3.1. Clean the underside of the counter around the hole with denatured alcohol. This will enable the glue to bond properly to the surface.
3.2. 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.
3.3. 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.
3.4. Clamp the blocks in place.
4.1. Apply seam adhesive to the locations marked out for the base plates.
4.2. Put each base plate in position, pushing it down into the glue.
4.3. Put more glue on the base plates (avoiding the screw).
4.4. Leave to set for approximately 30 minutes.
5.1. Remove the clamps and the stop blocks.
5.2. Mount the clips back on the base plates.
5.3. Clean the rim of the sink with denatured alcohol.
5.4. Run a bead of silicone around the hole, about 3/8" from the edge.
6.1. Turn the sink over and place it between the guide blocks.
6.2. Clamp the sink firmly in place.
6.3. Position the clips around the rim of the sink and screw firmly in place (but do not over-tighten).
6.4. Dab a bead of silicone sealant 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.
6.5. Cut away any excess sealant around the edge of the sink with a utility knife.
6.6. Leave to set for 24 hours before removing the clamps.
Though a solid-surface countertop is relatively light, it may be necessary to add extra support beneath it, over the sink cabinet or dishwasher opening. If the layout includes a lunch counter or other overhang, attach ¾" plywood to the cabinet tops prior to installing the countertop.
7.1. Check that the cabinets are sturdy and level.
7.2. Fasten additional support cleats made from 2" x 4" around the top of the sink cabinet.
7.3. If the counter is too heavy for you to handle alone, ask one or two other people to help you.
7.4. Turn the counter over and place it on the cabinets. Adjust as needed.
8.1. If you plan to install a garbage disposal unit, a strainer is not required.
8.2. Apply a bead of plumber's putty around the sink drain hole.
8.3. Place the strainer in the hole and press down firmly.
8.4. Under the sink, slip a rubber gasket over the neck of the strainer.
8.5. Attach and tighten the lock nut by hand, followed by an extra half-turn with a pipe wrench, to secure the strainer to the sink.
8.6. Wipe away the excess putty.
8.7. Repeat these steps for the other sink bowl, if installing a double sink.
9.1. Use the waste pipe and a P-trap to fit the strainer body to the wall pipe. If the drains are not exactly aligned with your sink, use angle fittings (or elbows) of different sizes to make the necessary connections.
9.2. Fit washers onto the ends of the connections.
9.3. Strengthen with slip nuts, tightened with a pipe wrench.