Installing Undermount Bathroom Sink Like A Pro

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Last Updated on November 15, 2022 by acechapman

Undermount sinks bring out the beauty and elegance of solid-surface countertops or stone. Installing an undermount bathroom sink is not as difficult or complicated as you might think. Although undermounts are popular in kitchen sinks, they can prove difficult to install because a kitchen sink weighs more than a bathroom sink. Installing an undermount bathroom sink is relatively simple DIY, particularly if the countertop has not been installed. You also have the option to remove it.

No matter if the countertop is attached or not, the sink must be installed from the bottom. This is the only way to align the sink. For a negative reveal, the sink cutout can extend slightly beyond the sink. It can also be flush with it or slightly recessed (for a positive reveal).

Any misalignment is going to be immediately noticeable. Once the adhesive sets, it’s impossible to fix. You will need to support the sink during the adhesive set. This can be done from above using a bar clamp, or below using precut 2×4 lumber with shims.

installing undermount bathroom sink

Fastening An Undermount Sink to the Countertop

Copper and stainless steel undermount sinks are lightweight. You can attach them to your countertop using construction adhesive, or high-quality silicone caulk. If you are installing heavy porcelain, cast iron, or composite stone sinks, it is risky to rely solely on the glue. Manufacturers usually provide clips that can be attached to the countertop using screws or epoxy cement. If you don’t want your sink to slide off the countertop or fall, it’s important to follow the manufacturer’s instructions.

Moen and Kohler offer mounting clips that are included with their sinks. You can screw these clips into the countertops using insets anchoring into predrilled holes. Installers have different opinions about the use of sink clips. Ask the Builder to suggest using two-part epoxy to secure the sink clips. Home RenoVision DIY recommends that the sink be secured with a single silicone caulk.

Some porcelain sinks have thick rims, making it difficult to use sink clips. It is also possible to install an undermount bathroom sink without using sink clips. Silicone caulk works better than adhesive to seal the sink. You can also use Liquid Nails construction adhesive to secure the sink. Later, you can apply silicone sealant to keep water out. Follow the manufacturer’s instructions if your sink comes with clips.

Cutting Out The Sink Hole

If you are replacing an existing sink, you will need to match the hole in the countertop. It may be possible to make the hole yourself if you have a vanity top. However, natural stone is difficult to finish and edges are not always perfect. To ensure that the sink will fit the hole, it is better to purchase the vanity top and sink together. You can also take your top to a countertop fabricator to cut the hole for a unique sink or if the sink is to fit in a particular area. A variety of faucet types will require drilling holes for the sink faucet.

How to Install an Undermount Bathroom Sink

Tools and materials you’ll need:

  • Denatured alcohol
  • Construction adhesive
  • 100 percent silicone caulk
  • 2×4 board (long sufficient to span across the sink cutout).
  • Drain assembly
  • Plumbers’ putty
  • Faucet
  • Two supply hoses for faucets
  • Drain tailpiece
  • P-trap assembly
  • Sawhorses
  • Rag
  • Caulking gun
  • Bar clamp
  • Tongue-and-groove pliers
  • Wrench

To avoid flooding and leakage, shut off the water supply to your bathroom before you start installing your undermount sink.

Clean the Countertop’s Underside

Place the vanity top upside-down on two sawhorses. Use a denatured alcohol-soaked rag to clean the countertop and remove any oil or dust that might affect glue adhesion. Flip the countertop upside down so that the sinkhole is located between the sawhorses.

Apply adhesive to the sink rim

Apply 1/4 inch of construction adhesive to the outer edge. Keep it as far away as possible from the inner edge. This will prevent it from leaking into the sink bowl. To ensure a watertight seal, you can run a thin layer of silicone caulk around the construction adhesive if the rim is sufficiently wide. After the sink has been set and the adhesive has dried, you can apply silicone to the space between the countertop and sink.

installing undermount bathroom sink

Place the sink and clamp it

Reattach the adjustable grip to a clamp and insert it through the sink drain hole. Attach the clamp to the bar clamp by placing a straight 2×4 across your sink cutout.

As you tighten the clamp, have a helping hand lift the sink and then bring it to within 1/4 inch of your countertop. Adjust the position of the sink until it touches the countertop. Final adjustments can be made while the sink is still moving. Once you are satisfied with the position, tighten the clamp until the sink will not move. Allow the adhesive to set for at least 24 hours.

Caulk the Sink Edge

Use a thin layer of silicone caulk to seal the area where the sink meets your countertop cutout. For a polished look, smoothen the caulk using your finger. Allow the caulk to cure as directed.

Install the Drain Assembly and Faucet

It’s easier to set the countertop on sawhorses than to transfer it to the vanity. Use the plumber’s putty to pack the drain strainer’s underside and tongue-and-groove pliers for the fastening of the gasket or retaining nut included in the drain assembly kit. Use the fasteners provided to secure the faucet to the countertop. Then, attach the flexible supply hose to each faucet inlet and tighten the connection nut using a wrench.

Connect the drain and water supply

Install the countertop on the bathroom vanity. Then, install the P-trap assembly and screw the drain tailpiece onto your drain strainer. Use a wrench to tighten the faucet supply lines onto the shutoff valves. Next, open the valves and inspect for leaks. Then, fill the sink with water and empty it. Also, check for leaks in the drain assembly. Use pliers to tighten any leaky connections.

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