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Replace an Undermount Bathroom Sink

Undermount sink - cracked

Replace an Undermount Bathroom Sink | The red arrow points to a nasty crack in the china bathroom sink. If you put enough water in the sink, it leaks into the cabinet below. Copyright 2020 Tim Carter

QUESTION: Tim, can you solve a conundrum for me? My bathroom was remodeled about two months ago, and the vanity has a granite top with a china undermount sink. Yesterday, I noticed a long crack in the sink that extends up from the drain hole. The contractor claims I did it by dropping something. I didn’t. He won’t repair it. What can be done? How might it have happened? I feel I need a new sink, but I can’t figure out how to install one because when I lay on my back and look up I don’t see any clips holding the sink to the underside of the stone. I’m so angry at the contractor. Help! Paula H., Ft. Lauderdale, FL

You may have anger issues like Paula caused by contractors who run away from problems instead of running towards them. Based on the incoming help requests at my website, I see an alarming trend of contractors who are reluctant to honor warranties. It’s hard to comprehend, but I feel it’s caused by the huge demand for building services right now. Contractors can just start a new job and make more money rather than go to a past job to work for free fixing an error or faulty product.

I’ve been a master plumber since age 29 and have installed countless sinks, including china undermount ones like Paula’s. Fortunately, the photos that Paula sent to me showed she’s got plenty of room to work under the sink and putting in a new one shouldn’t be that hard.

What could cause an undermount sink to crack?

Let’s first do an autopsy on why the sink might have cracked. The first thing I’d consider would be the drain assembly. The plumber that installed the drain might have over-tightened the nut on the drain assembly that compresses the drain trim ring to the china surface. The drain stopper fits inside this trim ring that you see from above when you brush your teeth.

In all my years, I’ve never seen a china sink come from the factory with a firing crack defect. It’s possible, but it’s very rare. No matter how the crack happened, there’s no acceptable fix in my opinion other than to remove the existing sink and replace it.

Should caulk be used to hold an undermount sink?

The lack of clips or hardware under the sink to hold it tight against the stone top is very unusual. I’d never install an undermount sink this way. It appears the contractor, or his subs, used silicone caulk to hold the sink in place. This is a recipe for long-term disaster. I can’t begin to share with you the number of failed-sink photos homeowners have sent me where sinks were installed just with caulk and suddenly drop once filled with water or heavy dishes.

undermount sink - bottom view - no clips

No clips holding the undermount sink.

How do you remove an undermount sink?

I’d empty everything out of the vanity cabinet below and disconnect the sink from the drain piping. I’d also remove the drain tailpiece from the sink. Then I’d stack some old towels or empty cardboard boxes in the vanity base to catch the sink when it suddenly releases from the stone top.

Next, I’d get a new stiff 1.5-inch putty knife and using a hammer I’d carefully tap the knife at an angle so one tip of the knife, not the entire flat edge, drives between the underside of the stone and the top rim of the sink. Here's the exact putty knife I'm talking about:

stiff putty knife

It may be hard to see, but these putty knives have a very sharp tip that will cut through caulk and will not BEND when you hit them with a hammer. CLICK or TAP HERE or the photo to have one in your hands in days.


I’d do this from above looking down on the sink, not from below lying on my back in the cabinet. Hold the knife parallel with the underside of the stone top and tap it carefully. Avoid having the hammer touch the polished edge of the stone top cutout. You surely don’t want to chip that!

If the sink is held in place with silicone caulk as I suspect, the knife will cut through the caulk. At some point, you’ll start to see the sink sag although it could drop suddenly with no warning. This is why you have the cushioning in place to prevent damage to the vanity.

Once the sink is out, scrape any excess caulk from the underside of the stone top. You can purchase solvents that will soften silicone caulk. Read the safety warnings about working with these chemicals indoors.

undermount sink - top view

What is the proper way to mount a new sink?

You’ll install the new sink using under mount hardware kits. There are any number of them available and many require you epoxy a threaded post to the underside of the stone top. These are easy to work with, you just have to make sure you install them in the right position so they don’t touch the outer rim of the replacement china sink.

It’s important to realize the epoxy will not adhere well to any stone that has silicone caulk smeared on it. You either have to remove the caulk smear or grind it off using a small rotary tool so you expose rough stone that the epoxy will stick to with no issues.

There are numerous online videos that show how to install undermount sinks and how to hold the sink in place as you tighten the hardware to the epoxied threaded posts. Watch these videos for sure.

When you go to re-install the drainpipe assembly understand the cone-shaped washer that fits inside the hole is what helps create the seal. You don’t need the nut overtightened to get a leak-proof seal. Hand tight plus an additional 1/2 to 3/4 turn is all you need.

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