Ask any stone fabricator how they anchor or attach undermount sinks and you will get as many answers as there are stone types. From mechanical anchor systems to adhesives to rail installation systems, your choice is varied.
There are two basic ways to secure undermount fixtures to a stone top. The first way is by using a mounting bolt. The second way is by using a frame, or brace, that anchors to the cabinet. The sink is then placed on this frame. Many of the stone tool suppliers call these sink setters. Most shops attach mounting bolts by affixing them to the bottom of the stone. This is done by drilling, dovetailing or epoxying a threaded bolt to the bottom. Theses are known as chemical anchors. A wood frame can be built to cradle the undermount sink and hold it against the stone. Typically the frame is attached to the cabinet below the countertop. The sink then sits on this frame. The top of the sink would be caulked in advance of the installation; then the excess would be allowed to ooze out and it would be cleaned up after the installation of the top. The sink or fixture can be shimmed as necessary between the wood and the lip.
The most common methods are dovetailing, the Keil Anchoring System, using wood blocks, using laminated stone, using a wood frame and using a supporting subtop. Dovetailing can be done using a common drill and a carbide-tip masonry or diamond bit. The Keil Anchoring System is a fast efficient method, but it requires a special drill, bit and anchor. Wood blocks are typically used to avoid the risk of drilling through the face of the stone. The laminated stone method can be used when the top is not structurally sound enough to drill into for anchoring. A wood frame is typically recommended when excessive loads are expected. A supporting subtop does not seem to be as popular as the other methods mentioned, but it can eliminate a step for the fabricator because the cabinetmaker or carpenter typically does the preparation for the fixture.
1. Place the top (with cut out) face down on a clean, smooth, flat surface. Shim the stone as necessary to provide proper support.
2. Check the product's MSDS (material safety data sheet) to determine the PPE (personal protective equipment) necessary for the task. Be sure to put on your PPE before beginning any cutting, grinding or drilling.
3. Lay out and mark locations for mounting bolts according to the fixture manufacturer's specifications. Be sure to use the correct number for the anticipated weight load.
4. Mark the drill bit for the correct depth. You can use a gauge provided as an accessory for the drill or mark the bit with tape or other easily seen device. On 3⁄4-in. (2cm) material, I typically recommend drilling 3⁄8 in. deep. On 11⁄4 in. (3cm) you can drill a 1⁄2 in. or more.
5. Use a wet sponge or other water supply to keep the bit and the stone cool.
6. Because you are not trying to drill through to top, you should drill slowly and without using the hammer motion equipped on some drills. Drill to the desired depth at a 90-degree angle to the bottom of the top.
7. Once you have reached the desired depth, then slowly lower (while the drill is still running) the bit to a 20-degree angle to the bottom of the stone. When the drill has reached that angle, use a rocking motion to create a dovetail large enough to accommodate the head of your mounting bolt.