A leak under your sink often seems like more of a disaster than it actually is. The most unpleasant task ahead of you is drying off everything you store under the sink and throwing away any ruined paper products. Once you've cleaned up the mess, you can locate and fix the leak. The leak could be in the pipes under the sink, or in the strainer in your sink on top of the drain. The strainer assembly connects the sink to the drain pipes and a leak can occur where the strainer meets the drain opening.
Locate the source of the leak by closing the drain stopper and filling the sink with water. Look and feel under the sink for the leak along the drain pipes and strainer assembly. Bail out the sink or let the water drain once you have located the leak.
Tighten the slip nuts on the drain pipes if the leak is in the drain pipe. Turn the slip nuts clockwise with channel-type pliers. The slip nuts look like collars where two pipes meet. Refill the sink to check that you have fixed the leak.
Remove and replace the strainer gaskets and washers if you locate any leaks around the sink's drain opening. Locate the slip nuts on both ends of the pipe directly below the drain, called the tailpiece. Unscrew these slip nuts with channel-type pliers and disconnect and remove the tailpiece.
Locate the locknut part of the strainer assembly. The locknut is a collar almost directly below the sink with evenly spaced bumps, called lugs. Unscrew the locknut with a spud wrench. If the locknut sticks, tap on the lugs with a hammer to loosen it. Remove the friction ring and rubber gasket that rests between the locknut and the sink.
Pull the strainer out of the drain hole and into the sink. If it sticks, slip a putty knife underneath the edge of the strainer.
Scrape away old putty from the indentation around the drain opening and the bottom of the strainer flange that sits inside that channel.
Place fresh plumber's putty in the lip around the drain opening. Put the strainer back into the drain opening and press the edges into the putty.
Place a replacement gasket around the strainer from under the sink, followed by the friction ring. Tighten the locknut into place. Remove the washer from the top of the tailpiece pipe and install a replacement washer. Move the tailpiece into position and tighten the slip nuts on each end. Let the plumber's putty cure according to the package directions before running any water in the sink.
You can replace the entire strainer if it seems damaged.