Fitting an undermount kitchen sink in a granite or stone countertop is a project that can be done yourself, although certain precautions must be taken due to the weight of the materials. The sink is supported from below by mounting clips glued under the counter.
This structure is strong enough to support the weight of the sink when filled with water. 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. An undermount sink is more complicated to install than a drop-in sink. Follow the steps 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 that are different from those presented here. 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.
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.
Granite and stone countertops are very heavy. The sink needs to be fitted under the counter once the counter is in place on the cabinets. This involves working inside the cabinets under the countertop, which makes the task a little more complicated.
For the adhesive to bond properly, the granite countertop must be dry and have been at room temperature for at least four hours.
With an enamelled cast-iron sink, which is heavier than other sinks, holes must be drilled in the granite, about ½" deep. The anchors are then screwed into the holes.
Wear safety glasses and protective gloves when working under the counter.
Turn off the water supply.
1.1. Remove the mounting clips from the base plates.
1.2. 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.
2.7. Position plates lengthwise, long side running along the edge of the sink. You will need two clips on each side, i.e. eight clips in all.
2.8. Remove the clamps and the sink.
3.1. Clean the underside of the counter around the hole with denatured alcohol so that the glue will bond properly to the surface.
3.2. Apply epoxy adhesive to the base plates.
3.3. Position the plates in the locations marked.
3.4. Hold in place for 10 seconds until bonded.
3.5. Put more adhesive on the plates and leave the resin to set for 30 minutes.
4.1. Mount the clips back on the base plates.
4.2. Clean the rim of the sink with denatured alcohol.
4.3. Run a bead of silicone around the underside of the hole, about 3/8" from the edge.
4.4. Lay the piece of 2" x 4" across the top of the hole in the counter.
5.1. Attach the top end of a bar clamp to the 2" x 4".
5.2. Pass the clamp through the main drain hole and attach the bottom end under the sink.
5.3. Partially tighten the clamp and ensure the sink is positioned properly. Tighten the clamp until the sink is held firmly under the counter. You will need two clamps if you have a double-bowl sink.
5.4. Position the clips around the rim of the sink and screw firmly in place (do not over-tighten).
5.5. 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.
5.6. Cut away any excess sealant around the edge of the sink with a utility knife.
5.7. Leave to cure for 24 hours before removing the clamps and the 2" x 4".
6.1. If you plan to install a garbage disposal unit, a strainer is not required.
6.2. Apply a bead of plumber's putty around the sink drain hole.
6.3. Place the strainer in the hole and press down firmly.
6.4. Under the sink, slip a rubber gasket over the neck of the strainer.
6.5. 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.
6.6. Wipe away the excess putty.
6.7. Repeat these steps for the other sink bowl, if installing a double sink.
7.1. 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.
7.2. Fit washers onto the ends of the connections.
7.3. Strengthen with slip nuts, tightened with a pipe wrench.