Q&A / 

Undermount Kitchen Sinks

DEAR TIM: Are undermount kitchen sinks a good idea? I have never had one, and am worried about leakage where the undermount sink contacts the underside of the countertop. I want a granite undermount kitchen sink, but am worried the sink will pull away from the granite countertop. This would be a nightmare, especially if the sink was filled with water. What undermount sink installation tips can you share that will allow me to buy one with complete confidence? Christy F., Flemington, NJ

DEAR CHRISTY: The first thing I can tell you is an undermount kitchen sink will give you a lifetime of leak-free performance if it is installed correctly. I have several undermount sinks in my own home, and they simply do not leak where the sink touches up against my granite countertops.

Years ago I felt the same way you did about undermount kitchen sinks. It was the plumber in me that made me a Doubting Thomas. After all, a standard double-bowl sink that has a disposer on one side and is completely filled with water weighs well over 100 pounds. I think it is reasonable for a person to wonder how in the world the sink will stay permanently attached to the countertop with this kind of weight regularly trying to pull the sink down into the base cabinet.

This stainless-steel undermount kitchen sink was installed in minutes with just some epoxy and hardware supplied by the sink manufacturer. PHOTO CREDIT: Tim Carter

This stainless-steel undermount kitchen sink was installed in minutes with just some epoxy and hardware supplied by the sink manufacturer. PHOTO CREDIT: Tim Carter

High-strength epoxies and silicone caulk are the simple answer. The undermount sinks are held in place with metal anchors that are glued to the underside of the stone with a two-part epoxy that cures rapidly and has incredible tensile strength once cured. Furthermore, a bead of silicone caulk helps bond the sink to the stone.

One installation tip is to make sure the underside of the stone countertop is dust-free. Dust will cause the epoxy bond to fail. Any installer worth his salt will take a brush to remove stone dust from underneath the countertop. Dust in this location is a strong possibility because of the cut made to create the hole for your sink.

The granite countertop fabricator had to cut a special-shaped hole in the stone top to create the hole for your undermount sink. Once the cut is made, the fabricator has to polish the rough stone edge. The water that is used during the polishing process gets very small pieces of stone in it, and this slurry can roll to the underside of the countertop. Once the water component of the slurry evaporates, the stone grit or dust is left behind.

Most undermount sinks come with very clear written installation instructions. Every set of instruction I have seen calls for a bead of clear silicone caulk to be placed around the flat lip of the undermount sink. This continuous bead of caulk acts as the waterproof seal between the sink and the countertop once the undermount sink is pressed permanently against the stone countertop.

Most professional installers can install an undermount kitchen sink in 30 minutes or less. The tools required are a 2x4 longer than the sink cutout is wide, one or two bar clamps, the special two-part epoxy and the caulk and caulk gun.

Once the installer knows the underside of the stone top is dust-free, he applies the silicone caulk to the sink lip. The sink is then pushed up against the underside of the stone top and centered under the polished cutout. The 2x4 is then placed on top of the countertop spanning across the sink. One end of the bar clamp goes through the large drain hole in the sink and the other end goes on top of the 2x4. A double-bowled sink needs two clamps, one for each drain hole.

The clamps are partially tightened and the reveal of the sink is checked to make sure it is even around the entire cutout of the granite. The clamps are then fully tightened. It is now time to mix the two-part epoxy and apply it to the small bolt posts that are glued next to the lip of the undermount sink. The installers are on their backs in the sink base looking up when they do this.

The epoxy sets rapidly, usually in ten minutes or so. After this wait period, the rest of the hardware that clamps the sink to the bolt posts is installed. It is good to wait at least one day before the plumber arrives to connect any drain piping or a heavy disposer to the undermount sink.

Before the drain piping is attached to the sink, it is wise to tighten the nuts that hold the clamps in place. Do not over tighten. Simply make sure they are snug. The adhesive properties of the silicone caulk in conjunction with the epoxy hold the sink permanently in place.

The biggest mistake you or an installer can make is trying to rush the job. All too often an aggressive plumber, remodeler or homeowner will try to finish the entire job the same day. They will wait six or seven hours thinking the epoxy is plenty hard, only to discover that the weight of a heavy disposer pulls the sink away from the countertop.

The bolt posts and nuts are not common items found at hardware stores. It's best to get these parts from a granite or marble-top fabricator. You can also Google the term - "undermount sink hardware".

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20 Responses to Undermount Kitchen Sinks

  1. We need to remover our undermount sink and replace it. Our countertop is silestone and we hoping to do this ourselves. How do we accomplish this.

  2. My contractor is going to install a very heavy cast iron double sink in my new house.
    The countertops will be 5/8" plywood with 2cm Silestone over top.
    What kind of reinforcement should we do before the installer comes?
    Our cabinets are maple and well made but I dont' think they are sturdy enough.

  3. Hi,
    My stainless stell under mount sink came unglued from granite counter top on one side. I wanted to reglue the sink with 100% Silicon by scraping off the old glue. My question is, if the job is properly done will I see the same issue infuture? If this method if ok, then how do you suggest I support the sink from below while the silicon is adhering?

    Thanks for your time.

    • Roopa, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  4. I want an undermount sink. I started paying attention to undermount sinks in other peoples homes and business. I've noticed the seal looks messy in every single one of them. By messy, I mean there's not a smooth seamless feel to the sillicon seal. Is there a way to make sure I get a seamless feel, or is that just the nature of the beast?

    • Leata, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  5. Hi,
    We just purchased a 32in undermount sink from Amazon. We are getting new granite counter tops. Anyway, the cabinet is 351/2 inches wide, and I was looking at the specs later that said a 36" cabinet is required. Will the installer be able to modify or do we have to get a different sink? Would appreciate your help. This will be a hassle and the timing is not good.

  6. Hi,
    I have a white porcelain undermount sink, but I want to replace it with a stainless steel undermount. Is it possible to replace it without damaging the granite countertops and cabinets?

  7. I have an under counter
    sink in my bathroom with granite counter held by epoxy. The sink fell out. Should clamps have been used?.

  8. Just replaced our stainless steel,sink that fell off of granite! Silicon dried up and rotted away. Put the board across counter top with the clamp on through the sink hole. Now we have a dent In sink from clamp being too tight. What's the best way to remove this dent. It's right in front of sink hole. It's more like a "bubble" effect then a dent. No crease in sink either.

  9. can you recommend a fabricator in the chicago area. My sink is separating from the granite countertop. There are so many advertisements, but I'm not so sure. I've had a lot of bad experiences in the past.
    Rose Scott

  10. My under mount sink just pulled off the countertop. Of course, it was full of water but miraculously, not a drop spilled. House is 9 years old. Good help was hard to find back then. Not a happy camper!

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