Fixing Leaks on Undermount Sinks

Undermount Stainless Steel Kitchen Sink and Tile Counter
Undermount Stainless Steel Kitchen Sink and Tile Counter. Image Source / Getty Images

Other than a relatively few integrated kitchen sinks where the countertop and sink are all a single molded unit, most kitchen sinks are separate units that fall into one of two categories. Kitchen sinks are typically either drop-in sinks (also called self-rimming), or they are undermount sinks that are secured beneath the cutout opening in the countertop, essentially hanging from the bottom of the countertop.

Both styles have their advantages and disadvantages. Drop-in sinks are easy to install since they are simply set into the countertop cutout and are supported by a rim resting on the countertop itself. But because the rim is raised up above the surrounding countertop, water can spill out onto the countertop. Undermount sinks, on the other hand, have no problems with water spillage onto the countertop, since they hang down below the top of the countertop. But undermount sinks are considerably harder to install since their heavy weight is suspended from the bottom of the countertop with clips and adhesives. 

This makes an ​undermount kitchen sink particularly susceptible to leaking around the recessed edge of the sink—the seam where the top lip of the sink meets the underside of the countertop. Unless the sink has been installed perfectly, you may well find that water leaks around this seam, spilling water onto the floor inside the sink base cabinet.​

Whether you installed the undermount sink yourself or had it installed by a pro, it's not uncommon for leaks to develop in a matter of days or weeks after the installation. When this happens, the problem likely has one three causes:

  • The bottom of the countertop (where it contacts the sink) was not cleaned thoroughly. Denatured alcohol should be used to clean off all surfaces prior to caulking. Dust on granite or solid-surface (such Corian or SileStone) can often compromise the caulking and should be cleaned up before installation. 
  • Installation was not secure. The clamps and/or the epoxy used to secure the sink to the bottom of the countertop were loose enough to cause the caulk to quickly pull away. 
  • The installer may have failed to use a true kitchen sealant. Moen, maker of sinks and fixtures, recommends pure, 100% silicone sealant for undermount sink installation. Silicone sealants are designed for resilient flexibility and have good adhesive properties. If an ordinary caulk was used to seal the sink, it likely will fail relatively quickly. 

How to Fix It

Though the process is cumbersome, you can likely remedy leaks on an undermount sink yourself. Start by crawling under the sink and first checking to make sure the clamps are securely holding the sink to the bottom of the countertop. Then, from inside the cabinet, apply new 100% silicone sealant to the seam around the sink. You really cannot do anything about epoxy that incorrectly applied, but if you make sure the sink is tightly clamped then recaulk, you likely will eliminate any leaking. 

Next, from the top side, scrape away as much existing caulk from the joint between the sink and countertop and clean the surfaces thoroughly with denatured alcohol. When the joint is clean and dry, apply the recommended silicone sealant or caulk around the top edge of the sink, sealing the gap with the countertop.