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Kitchen: Under Sink Leaks

Kitchen sink cutoffChecking under the sink periodically for leaks is a good idea for the sink cabinet houses a higher concentration of potential leaks than practically any other area of the home. Slow leaks can damage cabinets, flooring, and provide an ideal environment for growing mold and mildew. A leak check takes less than a minute and can prevent future problems and damage.

If you do find water in the sink cabinet, the possible sources are:

  • Leaks in the sink drain
  • Leaks in water supply lines or fittings
  • Leaks from the garbage disposer
  • Leaks where drain baskets connect to the sink
  • Leaks where the disposer connects to the sink
  • Leaks at water filters, instant water heaters, or other sink accessories
  • Leaks at the sink faucet

Under sink water filterLocating the source is usually as simple as removing all objects from the sink cabinet, drying up the water, and then testing drains, disposer, accessories, or faucets. If the leak is not immediately obvious, first run some water through each side of the sink bowl (It may be necessary to fill the bowl to create sufficient pressure to cause the leak). Then carefully run your hand along the drain pipes, feeling for wet spots.

Kitchen sink trapIf the drain is the problem, a repair may be as simple as tightening the slip nuts that connect one pipe to another. Be careful not to tighten too much or you may crack the fitting; hand tightening is usually sufficient. If a fitting continues to leak after tightening, you may need to replace the small nylon washer that is under the slip nut.

Supply lines can also be a source of leaks as they are always under considerable pressure. Here again, tightening can sometimes stop the leak. If you are tightening where the pipe joins the cut-off valve, you need to support the valve to keep it from turning, especially if it is mounted on a plastic pipe. I use an adjustable wrench for this, supporting the valve body as I use another wrench to tighten the connecting nut. However, if tightening fails, try removing the nut and applying pipe joint compound to the threads. Re-tightening should eliminate the leak; if not, the threads on the supply tube nut may be damaged and may need replacement. I prefer the woven wire mesh supply tubes. While they are more expensive, they can easily form to any contour and are less prone to damage.

Garbage disposerIf the leak comes from the disposal connection to the sink or from the basket strainer, you may have to remove the disposer or basket drain and re-install it. For the basket strainer, I secure it by inserting a couple of screwdrivers in the holes on the sink side—they make a tool for this, but I’ve never owned one—and use large groove-joint pliers to remove the nut on the underside. Once removed, both the basket strainer and sink must be cleaned, removing the old putty residue.

To re-install, roll a ½” snake of plumber’s putty long enough to reach around the drain hole, making certain that the sink is perfectly clean, and then applying the putty in a circle around the edge of the drain hole. Press the basket strainer into place, secure with the screwdrivers or tool, apply the rubber and paper washers, tighten the large locknut and you’re ready to connect the drain.

For other leaks, follow the suggestions above and you should be able to locate and repair the problem. If the problem is a leaky sink faucet, see the section: Repairing Leaky Faucets.


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