Why Is My Undermount Sink Leaking and How To Fix It

Undermount sinks are the perfect solution for a seamless, integrated look in your kitchen or bathroom. While they are an excellent choice, they must be installed correctly. Unfortunately, sometimes you do everything right in the installation, and the sink still fails after some time.

If your undermount sink is leaking and you’re unsure how to fix it, we can help. So, before you call in a professional, read through this guide to see if the culprit of your leaky undermount sink is an easy, doable fix.

Why Is My Undermount Sink Leaking?

Whether you handled the installation yourself or had a professional install your undermount sink, leaks aren’t a novelty with this type of sink. Regardless of whether you have an undermount kitchen or bathroom sink, the culprit of the leak issue is usually one of three causes.

Dusty Countertop

The first issue could fall to the cleanliness of the bottom of the countertop at the time of installation. It’s important that the underside of the countertop, where it contacts the sink, is completely clean. If it wasn’t, the sink might not have adequately adhered, causing it to succumb to gravity after a few days or even weeks.

You need to use denatured alcohol to clean the surface before caulking thoroughly. If you’re working with granite or solid-surface countertops, you need to be especially careful. Any dust coating the counter can compromise the caulking and lead to a faulty seal.

Installation Problems

Undermount Sink

Another problem that may have occurred might fall to issues with the installation. In some cases, the clamps or epoxy you used to secure the sink to the bottom of the countertop were loose. If they were loose enough, gravity would have done its job, causing the sink to fall away from the countertop.

This would have caused a gap between the undermount sink and the countertop, thus compromising the caulk. Holes in the caulk may develop, allowing water to seep through.

Improper Sealant

When installing an undermount sink, you need to use a proper kitchen sealant. If you use the sealant designed for alternate purposes, you may end up with a leaky sink. Considering the sink is supposed to hold water, not spill it all over the place, this becomes somewhat problematic.

Moen, a well-known brand of sinks and various fixtures, recommends pure silicone sealant for undermount sink installation projects. Ensure you get 100% pure silicone sealant, as it’s designed for durable flexibility yet has excellent adhesive properties.

If you cut corners and used a basic, ordinary caulk to seal the sink, there’s a good chance it will fail within a short time.

How Do You Fix A Leaking Undermount Sink?

If your undermount sink is leaking, thus allowing water to seep into the cabinet below, there are a few steps you’ll have to go through to fix it. While you could just leave towels in the cabinet below to soak up the moisture as it comes down, or even a bucket to catch the water, then you have to remember to switch out the towels or empty the bucket. So, it’s best to fix it and do it right the first time.

This process can be time-consuming, but it’s worth it in the long run (we don’t want mold growth in the cabinet below).

If The Sink Is Still Secure

Empty the cabinet beneath the sink if it isn’t already empty. Then, crawl under the sink and check if the clamps are still firmly fixing the sink to the bottom of the countertop. If this is the case, you may be able to rig up a quick fix with silicone.

Here’s what you’ll need:

  • 100% silicone sealant

Gorilla White 100% Silicone Sealant Caulk, 10 Ounce Cartridge, White, (Pack of 1)

From the inside of the cabinet, apply the silicone sealant to the seam around the edges of the sink. You could also try recaulking the area to see if that eliminates the leak. Allow it to dry, then check if the sink still leaks. If it does, proceed to the next section.

Remove The Sink

Generally, the best solution is to remove the entire sink and reseal the sink to the countertop. Here’s what you’ll need:

  • Scraper tool
  • Denatured alcohol
  • Clean cloths
  • 100% silicone sealant
  • Clamps

Start by shutting off the water to the sink. Drain the water lines to the faucet by turning the tap on. Once the water lines are empty, you can start removing the plumbing.

Remove the plumbing supply and drain pipes from underneath the sink. Once that is out of the way, unclamp the undermount sink from the countertop. Given that the silicone caulk currently fixing the sink in place has failed, it should be relatively easy to pull the sink away.

Set the sink aside, then scrape off as much caulk as you can from the top flange of the sink. Try to get all of the caulk off, as it can interfere with the seal. Clean the surfaces that adhere to each other (both on the sink and the countertop) thoroughly with denatured alcohol and a clean cloth.

Allow the area to dry. After the joint is clean and dry, apply a thin bead of silicone sealant to the top flange of the sink. Once you apply the adhesive, you’ll need to move quickly. Re-install the sink before the silicone sealant cures.

Clamp the sink into place. Using a clean cloth, wipe away any excess sealant that oozed out of either side of the sink during installation. For the most part, silicone sealants cure entirely within 24 hours. Check the packaging of the sealant to clarify the particular time frame.

Once the sealant is dry, crawl under the sink and visually check for any areas that could be an issue. If you don’t find any, reinstall the plumbing supply and drain pipes beneath the sink. Turn the water back on and check for leaks.

If you’re not comfortable handling the repair process, call in a professional to fix the leaky sink for you.

Frequently Asked Questions (FAQs)

How do you seal an undermount sink to granite?

Kraus KWU110-32 Kore inch Undermount 16 Gauge Single Bowl Stainless Steel Kitchen Integrated Ledge and Accessories (Pack of 5), 32 Inch, 32"-Workstation Sink

Let’s say you have a stainless steel undermount kitchen sink that you’re mounting on your granite countertop. The process is the same as resealing an undermount sink, which we outlined above.

Generally, the installer will apply a generous bead of silicone caulk between the sink and counter to fix it into place. After the sink is installed, they may use a second bead of caulk along the rim to prevent moisture seepage.

Silicone sealant won’t hold the sink in place, so you’ll need an additional measure to hold the sink in place, whether it’s brackets or a gorilla glue epoxy.

Do undermount sinks fail?

They can – usually, it’s a combination of things. For example, let’s say you have a quartz or granite countertop, garbage disposal, and an undermount sink. The weight moisture and vibration from the garbage disposal can pull the sink away from the countertop.

This allows the sink to leak, thus creating a mess in your sink cabinet. In general, undermount sinks can be problematic, as they rely on caulking and quality installation to remain sturdy. Over time, gravity will work its magic, potentially causing the sink to pull away from the countertop, creating a gap.

It’s crucial that you use the proper materials and complete the installation correctly for the sink to remain solid.

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