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Undermount SinksContinued

Supporting an Undermount Sink

The installation instructions provided by undermount sink manufacturers can be vague, leaving the installer to decide how to support the sink's weight. Adequate support is especially important for large sinks or those made of cast iron. "People want bigger and deeper sinks all the time," says Rick Brown, product developer for Kindred Industries, a sink manufacturer. "If you hang it from the counter, you have to think about how much it weighs when it's full of water." Some manufacturers, including Corian, recommend that their sinks be supported by a site-built wooden cradle installed in the base cabinet. Eljer's instructions explain, "The structure built to support the sink must be able to support a minimum of 350 pounds. There are many ways to install an undermount sink, and the details of the installation will depend on the quality of your cabinets and countertop. It may be necessary for you to reinforce the sides of the cabinet for it to support the 350-pound minimum." Some sink manufacturers understand that an easier installation system needs to be devised. "We're working on developing a new system for attaching undermount sinks," says Tom Dewane, associate product manager at Kohler. "But for the time being, it's still necessary for the installer to build a box." Vance Industries has developed an installation kit for undermount sinks (Figure 6). The kit includes two weight-supporting aluminum rails equipped with bolts to clamp the sink against the countertop. The Sink Undermounter kit is available from Vance in three sizes, with list prices from $37 to $68.

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Figure 6.

Many undermount sinks, especially large sinks and cast-iron sinks, need to be supported on rails. Vance sells the Sink Undermounter kit, which includes two aluminum rails. The rails have bolts that hold the sink flange against the countertop.
    Most site-built cradles are made of plywood, 2x4s, or angle iron, and support the sink under the sink flange. "Cast-iron sinks are never totally flat," says Socinski. "They sometimes crest in the middle. The contractor needs to check the height of the sink with a long straightedge from one side of the cabinet to the other." The rails should be located so the highest point of the undermount sink ends up at exactly the same height as the top of the base cabinets. If the sink flange varies slightly in height, any discrepancies will be easily sealed with silicone caulk. While some installers insist that an undermount sink must always be supported on site-built rails, others depend on clips attached to countertop anchors, or even just silicone caulk. "People don't like to see 2x4s propping up their sink," says Tony Pelcher. Regardless of the method used to support the sink, it is prudent to use a reversible installation method. When an undermount sink is hung on angle-iron rails, or when there isn't much clearance in the base cabinet, there may be no way to pull the sink out from underneath. Since lifting a stone countertop after the silicone has cured can damage the stone, it is wise to consider how you would go about removing a damaged sink.

Installing an Undermount Sink

The job of attaching an undermount sink to the countertop is sometimes performed by the builder, sometimes by the countertop sub, and sometimes by the plumber. With a solid-surface countertop, the solid-surface contractor usually attaches the sink to the countertop in the fabrication shop. With an unwieldy stone countertop, the sink is almost always installed on site. "Generally, I don't want to attach the sink," says Socinski. "But sometimes the plumber doesn't want to touch it, and so I'll do it." Solid-surface countertops When a solid-surface countertop receives a solid-surface sink, the countertop is usually fused to the sink to create a single integral unit, using the same techniques as for joining countertop seams. The sink opening in a solid-surface countertop is usually cut with a router, and then the rim is sanded with 120- and 180-grit sandpaper. A variety of methods are used to mount sinks that are made out of material other than solid-surfacing. Pelcher has had no problems attaching stainless steel sinks to solid-surface countertops with silicone alone, so that the sinks are hanging by a bead of caulk. "I would be leery of using that method, because of the weight of the sink when it's filled with water," commented Tom Dewane from Kohler. "Our company wouldn't recommend it." For heavier sinks, including vitreous china and enameled cast-iron sinks, Pelcher makes custom clips from solid-surface blocks. First, he prepares small blocks, the same thickness as the sink flange. These blocks are glued to the underside of the countertop, around the perimeter of the sink flange. Then larger solid-surface blocks are glued to the small blocks, forming a lip that extends over the sink flange. Other fabricators make one-piece rabbetted mounting blocks, with the depth of the rabbet corresponding to the thickness of the sink flange (Figure 7).

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Figure 7. Some solid-surface fabricators attach undermount sinks with custom sink clips made from blocks of solid-surface material. The blocks are attached with solid-surface adhesive. Some sink manufacturers recommend that their undermount sinks be mounted to solid-surfacing with stainless steel sink clips bolted to brass anchors. These expanding anchors, which have a female machine thread to receive the bolts, are inserted into 1/4- or 3/8-inch-diameter holes drilled 1/2-inch-deep in the underside of the countertop. Each stainless steel bolt gets a wing nut, which is tightened down against the sink clip. Before the sink is attached, the sink flange and the contact area of the underside of the countertop are cleaned with alcohol. Then a continuous bead of silicone sealant is applied, and the sink is attached. Stone countertops. Stone fabricators cut out the sink opening with a wormdrive saw equipped with a diamond blade. The countertop is delivered to the job site before the sink is attached. Usually the holes for mounting the faucet have already been drilled, although these holes are sometimes drilled on site with a diamond-core bit. A stainless steel or composite undermount sink is attached to a stone countertop with sink clips, which are generally provided by the sink manufacturer. The clips are held by bolts screwed into metal anchors that are epoxied into the stone (Figure 8). Kohler is the only major sink manufacturer to provide anchors with its undermount sinks.

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Figure 8. Undermount sinks are attached to stone countertops with clips that bolt into anchors. The anchors, which have female machine threads, are epoxied into shallow holes drilled in the stone. The number of anchors required is usually specified by the sink manufacturer, varying from only four anchors for some lavs up to 15 or more for a large kitchen sink. "With cast-iron sinks, we don't even drill anchors," says Anita Socinski. "We just make the contractor provide a wood frame to support the sink." Most stone anchors are epoxied into 3/8-inch-diameter, 1/2-inch-deep holes drilled with a conventional diamond bit. However, a German company has developed a tool that is able to drill a flared hole in stone, with the bottom of the hole wider than the top. This undercut hole receives a proprietary anchor. This tool, called the Keil Anchoring System, is available for about $3,800 from Southwest Equipment of Naples, Fla. (800/476-3737). Some installers attach the sink to a stone countertop while the countertop is upside down on the floor. Others wait until after the countertop is installed. "The piece of granite is a big monster," says David Delp, of Prime Construction. "When the granite is installed, you want to be able to slide it, without a bump." Regardless of how an undermount sink is supported, it is always attached to a stone countertop with silicone caulk (Figure 9).

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Figure 9. Before the stone countertop is lowered for the last time, a generous bead of silicone caulk is applied around the sink flange. This stainless steel sink is supported by custom-made angle-iron rails, which also help support the granite.  

Because cold temperatures can interfere with the curing of the silicone, a stone countertop should be allowed to acclimate to room temperature before the sink is attached. The silicone should cure for several hours before the plumber attaches the sink basket and tailpiece.

Martin Holladay

is an associate editor at The Journal of Light Construction. Special thanks to A&M Stoneworks of Colchester, Vt., and Vermont Solid Surface of St. Johnsbury, Vt., for their assistance with this article.