How to Install an Undermount Sink

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Installing an undermount sink requires woodworking skills.
Image Credit: Stephen Paul for Hunker

The Kitchn says that a pro can install an undermount sink in 30 minutes or less, but the rest of us can expect to spend quite a bit longer on the project — if we can do it at all. Because an undermount sink has no rim, it must be mounted underneath the countertop, and all the support must come from clips screwed or glued to the countertop material or rails installed in the cabinet. Given that undermount sinks are often used with stone and concrete countertops, affixing these clips and rails at the proper positions can be challenging.

Pros use a simple trick to make this job easier, but even if you do as they do, there's another potential problem with an undermount sink installation: the difficulty of cutting a sink opening in a countertop that doesn't have one. The edges of the opening will be visible, so they have to be more clean and accurate than those for a drop-in sink, and they have to be polished. That's a tall order, so even if you're confident in your abilities as a plumber and sink installer, you're better off getting a pro to cut the sink opening and faucet holes in stone and some solid-surface countertops than you are doing it yourself.

Are You Up for the Job?

If you've ever installed drop-in sinks, you know what's involved in hooking up the drain and faucet, and the procedure isn't any different for an undermount sink. The main difference is mounting the sink itself, and the main determinant of the difficulty of this task is the weight of the sink.

Lightweight sinks made of copper or stainless steel are fairly easy to handle, and you usually get all the supplies you need to do it as well as the instructions with the sink. Heavy sinks made of cast iron, composite granite or fireclay are another matter and often require installation of a support structure inside the cabinet to hold them. There's no need to be daunted by this requirement, though, because you can purchase adjustable support rails, but you're probably going to need a helper.

The Trick Pros Use

It's difficult to support an undermount sink — even a lightweight one — while you draw its outline on the underside of the cabinet and set the clips. You aren't likely to find anything to place underneath it that is exactly the right height, and even if you do, wedging the tight-fitting support underneath the sink is bound to be difficult. Ask the Builder offers this easy workaround that makes the job much easier.

Place a 2x4 across the sink opening and lay it parallel to the countertop. Get a helper to hold the sink in roughly the right position, pass a bar clamp through the drain hole, rest it on the wood and tighten it to draw the sink against the countertop. Before you tighten it all the way, adjust the position of the sink until it's exactly where you want it.

Choosing Sink Clips and Supports

When you purchase a new sink, the sink clips come with it, but if you're installing a recycled sink or if you lost the clips, you'll have to buy some. The most common clips consist of threaded posts mounted to flat disks with holes and metal brackets that fit over the posts and get secured with wingnuts. You can also use cabinet brackets that you screw to the sides of the cabinet and wedge in place under the sink rim to push it tightly against the underside of the cabinet.

Heavy sinks may require installing a sink rail, too.

If you're installing a particularly heavy sink, such as a double-bowl cast iron sink, you can also use a sink rail. This is a rectangular metal railing that you screw to the front, back and sides of the cabinet. Two adjustable metal rods pass between the sides of the rail underneath the sink, and they have adjustable leveling feet that you tighten to level the sink and push it against the countertop.

Things You'll Need

  • Acetone or alcohol

  • Sink clips

  • Sink posts and brackets

  • Two-part epoxy cement

  • Silicone caulk

  • Wingnuts

  • Headlamp

  • Damp rag

  • Bar clamps

  • Pencil

  • Faucet

  • Sink-drain assembly

  • Garbage disposal (optional)

  • P-trap assembly

How to Install an Undermount Sink

Step 1: Clean Up and Get Ready

Remove everything from the sink cabinet and take out all the shelves. You'll need all the available working room you can get. Get a headlamp so you can see what you're doing without having to hold a flashlight.

Step 2: Clean the Underside of the Countertop

Wipe dust and oils from the underside of the countertop using a damp rag or a rag soaked with acetone or alcohol. Dust and oil can interfere with epoxy and silicone caulk adhesion, so it's important to remove all of it.

Step 3: Clamp the Sink in Place

Get a helper to position the sink roughly under the sink cutout and then secure it in position with a bar clamp. Be sure to adjust its position exactly before you tighten the clamp all the way. If the sink has two drain holes, use two bar clamps, one for each hole.

Step 4: Install the Posts for the Sink Clips

Draw an outline around the sink rim and mark the position of each sink clip with an X using a pencil. Space the clips according to the sink manufacturer's instructions. If you don't have the instructions and aren't sure, space them no more than 10 inches apart for a lightweight sink and no more than 6 inches apart for a heavy one. Remember that too many clips is better than too few.

Sink clips keep your sink tight against the underside of the countertop.
Image Credit: Stephen Paul for Hunker

Mix enough two-part epoxy resin for all the posts. The epoxy recommended by Ask the Builder is PC-7, but you can also use JB Weld or a similar product. Apply a generous amount to each post, set the post in place and wait overnight for the glue to set.

Step 5: Caulk the Sink Rim

Loosen the bar clamps and lower the sink to the bottom of the cabinet or — if it isn't too difficult — take it out of the cabinet altogether. If desired, you can install the faucet while the sink is out of the cabinet; this is usually easier than installing it with the sink in place.

Apply a bead of silicone caulk around the rim, being careful when caulking to keep the bead on the outside edge of the rim if a portion remains visible (positive reveal). Reposition the sink underneath the cutout, adjust it so that the rim is inside the outline you drew, tighten the clamps and clean up any caulk that oozes out.

Step 6: Install the Brackets to Hold the Sink

Slip a bracket over each of the posts, orient it to overlap the sink rim, screw on a wingnut and tighten it all the way. It's fine to make it finger-tight, but you can use locking pliers if you prefer. It's best to allow the silicone caulk to set overnight before removing the clamps.

Step 7: Install the Plumbing

Install the faucet if you haven't already done so. Connect the faucet to the water-supply valves. Screw a sink-drain assembly to the drain hole or install a garbage disposal if you have one and connect the drain to the P-trap assembly and to the wall drain.

references

Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.

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