Badly corroded, leaky kitchen sink strainer assembly (C) Daniel FriedmanRepair a Leaky Sink Strainer / Drain

  • SINK LEAK DIAGNOSIS & REPAIR - CONTENTS: 10 Steps to replace a leaky sink drain basket assembly, how to diagnose and fix a leaky sink in the kitchen, bath, or laundry. Here we describe finding the source of the leak at a sink strainer or at the drain piping and we give step by step instructions for replacing a corroded, leaky sink strainer assembly.
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How to fix a leaky sink drain or sink strainer assembly:

Using photographs and text we show how to diagnose the source of water found under a leaky kitchen sink. We repair the link by repairing or replacing leaky sink trap and drain parts or by replacing a leaky sink strainer assembly.

This article series describes the different types of plumbing leaks that can occur in building supply piping or drain piping. Knowing just what kind of leak is occurring in a building helps pinpoint the problem and also helps specify the necessary plumbing repair.

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How to Repair Kitchen Sink Leaks at the Strainer Assembly

Sink basin or sink strainer leaks occur at the drain assembly of all types of sinks due to improper installation, loose parts, missing gaskets or sealant, or from corrosion.

Watch out: But unless you look closely under the sink - a pose that requires crouching and crawling about - you may not see the real source of trouble. If you miss a corroded leaky sink strainer assembly you may find yourself replacing traps or drain piping that were not the problem.

Leak stains on plastic kitchen sink drain trap and drain piping (C) Daniel Friedman

[Click to enlarge any image]

Above: yellow leak stains and water droplets on the white plastic sink drain and trap parts looked as if the drain parts were loose or damaged, maybe cracked. They were not. The problem was the sink strainer. To see it one has to bend over or nearly crawl under the sink and then look up - taking care not to get rusty crud in one's eyes.

1. Remove the Sink Trap, Tailpiece, Drain Parts

Put a bucket or deep plastic bin under the sink drain so that you can catch the water that will spill out of the P-trap when you remove these parts.

Remove all of the white plastic parts (or equivalent at your sink) that you see in the photos above. These parts should un-screw by hand if they're plastic or you may need to use a large goose-neck slip-joint pliers.

Watch out: Take care to keep all of the drain parts as you will either re-use them (if they're in very good shape) or you'll take them to your plumbing supplier to be sure that you purchase all of the necessary parts to install new drain components at the end of the strainer replacement procedure we give below.

2. Remove the Old Sink Strainer Assembly

The sink strainer kit manufacturers, if they give you any instructions at all, will just say "remove the old strainer assembly".

Badly corroded, leaky kitchen sink strainer assembly (C) Daniel Friedman

Look let's be honest. You may have a hell of a difficult time getting the old sink strainer off.

Sure if the strainer assembly were pretty new and is un-damaged, you might be able to un-screw the large round strainer nut by tapping it counter-clockwise with a hammer and a screwdriver.

Watch out: don't go banging really hard on a cast-iron sink or you may break it.

OK with that warning out of the way, try tapping the protruding lugs on the large round strainer nut - yellow arrow in the photo above. Tap the nut in the direction of the yellow arrow. When that lug breaks off, try the next one. Try tapping at several spaced points around the nut to see if you can loosen it. Try spraying some WD40 or Liquid Wrench on the corroded nut threads.

Sink Strainer Removal Problem 1: the whole strainer assembly just rotates in the sink when I try to unscrew it

OK so let's get a grip on things. The strainer nut isn't loosening. Instead the wole sink strainer assembly is rotating when you tap on the nut to try to turn it. Here's how your assistant can grab onto the strainer assembly from above you go back to pounding from below.

Using a pair of Vise Grip locking pliers to grab the slipping sink strainer assembly from above (C) Daniel Friedman

I use a pair of Vise Grip pliers. Unless your assistant is incredibly strong you'll need two Vise Grips. The first pinches down on the cross member of the sink strainer drain opening - see the close-up photo just below.

Using a pair of Vise Grip locking pliers to grab the slipping sink strainer assembly from above (C) Daniel Friedman

The second Vise Grip pliers grabs onto the first one at an angle so that your assistant can hold the strainer assembly from above you go back to pounding to try to turn the sink strainer lock nut from below.

Watch out: don't get rusty crud in your eyes while working below the sink. You risk abrasion, cuts, or a nasty eye infection. I'd say wear goggles but you won't, or if you do you won't be able to see what the heck you're doing. Instead keep your head out of the way of falling debris.

Sink Strainer Removal Problem 2. The Strainer Nut Won't Budge

Yeah I know. On a really badly corroded kitchen sink strainer assembly the whole shebang may be rusted together. So we continue with Plan B. You may still need your assistant to hold the strainer from above.

Using a hammer and a small 1/2" cold chisel or even a large crummy old big flat-bladed screwdriver, cut through a thin portion of the sink strainer lock nut.

Sink strainer nut is to be cut off at red arrow (C) Daniel Friedman

In the photo above, I'd cut through the thin area of the sink strainer nut pointed to by the red arrow. I'm hammering mostly at right angles against the nut, I'm no longer trying to turn it. If you click to enlarge the photo you'll see the tips of our Vise Grip pliers nosing down through the strainer basket.

Cutting a frozen sink basket strainer nut (C) Daniel Friedman

Above you can see my screwdriver just cutting through the outer flange the strainer nut. With the outer flange of the sink basket assembly nut cut, I was able to drive my screwdriver between the threaded side of the strainer nut and the basket assembly side - photo just below.

Breaking off a frozen sink strainer nut (C) Daniel Friedman

Don't worry about damaging the frozen sink strainer basket nut or basket itself - you're going to throw all of these parts away and install a whole new sink strainer assembly. Once you've cut both the flange and the threaded portion of the sink basket nut you'll be able to pry the nut off of the sink strainer basket, as I illustrate just below.

Prying off the sink basket nut (C) Daniel Friedman

Alternatives to using the cold chisel or old screwdriver to cut the frozen sink strainer nut

  1. A short hacksaw blade held to cut at an angle - this can work but it's difficult
  2. A Dremel tool with a carbide cutting blade - this is very dangerous as when you break off the cutting blade it can cut your eye to shreds.

Step 3. Remove the Sink Strainer Basket

With the strainer nut removed you will push the strainer basket up from below or just pull it off with your still-attached Vise Grip pleirs. Only if some idiot who installed the old sink strainer assembly used an adhesive instead of plumbers putty will we have trouble with this step.

Below at Step 4 you can see the sink, from above, with the old sink strainer basket out of the way.

Step 4: Remove Plumbers Putty from the Sink Surface

Cleaning the sink after sink strainer baseket removal (C) Daniel Friedman

I used a dull flat bladed screwdriver to scrape off the nasty old plumbers' putty to begin preparing the sink to receive the new sink strainer assembly.

Inspect the under-side of the sink around the drain opening and scrape off any plumbers putty or rust crud that may be under there too.

Step 5. Clean the Sink Surfaces Around the Drain Opening

Cleaning the sink in preparation for installing the new sink strainer basket assembly (C) Daniel Friedman

Above, using a paper towel, I'm wiping both sides (the porcelain side you can see and the cast iron under-side of the sink around the drain opening.

Below I'm using alcohol to finish cleaning both surfaces of the sink drain opening. I admit that few plumbers would bother to do this but I want to be extra sure that my plumbers putty will seal nicely against the porcelain/enamel of the upper sink surface, and I want a good clean mating surface for the rubber gasket that's going to go against the underside of the drain opening.

Cleaning the sink in preparation for installing the new sink strainer basket assembly (C) Daniel Friedman

Below: I tried three methods to get the heavy rust off of the cast iron underside of the sink drain opening. I used a wire brush, then a flat bladed screwdriver to remove the high and very rusty material. I finished with 120 grit sandpaper - shown below.

Sandpaper to remove heavy rust on the sink drain opening underside (C) Daniel Friedman

I admit most plumbers won't bother to be so meticulous but I don't want to have to take this job apart because my new sink strainer baseket assembly is leaking. You can take a break from lying on your back under the sink by sanding the under-side of the sink drain opening by reaching through the drain from above as Illustrate in my photo just below.

Alternative method of cleaning around the sink drain opening to prepare for installing a new strainer (C) Daniel Friedman

After scraping and sanding the under-side of the drain opening I wiped it off again with alcohol so that no dust or debris remained.

Final cleaning of the sink drain opening before installing the new sink basket assembly (C) Daniel Friedman

Step 6: Prepare Plumbers Putty & Install It Around the Sink Drain Opening

Install Plumbers Putty around the sink drain opening (C) Daniel Friedman

Above: If your plumbers' putty is old and dried out, buy new plumbers' putty.

Watch out: DO NOT substitute adhesive caulk or something like that for plumbers' putty. If you do, when your sink strainer assembly needs replacement, the next woman who sets out to repair this sink is going to find you and kill you.

Install Plumbers Putty around the sink drain opening (C) Daniel Friedman

Warm the putty up in your hand by kneading it until it's soft. Then form it into a pencil-sized putty rope that you can press down around the sink drain opening.

Install Plumbers Putty around the sink drain opening (C) Daniel Friedman

If your putty won't adhere at all to the porcelain sink surface you should reconsider my suggestion of cleaning the surfaces with alcohol. Lacquer thinner would work too but is more dangerous and more toxic. Here you see I've got my plumbers' putty sealant in place and I'm ready to press the sink basket upper part into place.

Step 7: Collect the New Sink Strainer Basket Parts

Sink drain basket assembly parts ready for installation (C) Daniel Friedman

Above from left to right are the new sink drain basket strainer assembly parts in the order of installation.

  1. The sink strainer basket itself. This part will be pushed down through the sink drain opening from above.
  2. A large black round flat rubber gasket. This gasket will go around the sink basket and up against the under-side of the sink.
  3. A large white round flat plastic gasket. This gasket will go between the rubber gasket and the strainer nut. This allows the nut to turn up tight against the bottom of the sink, securing the strainer basket in place. Without this plastic gasket the nut may tend to deform or ruin the rubber gasket.
  4. The sink strainer assembly nut. This nut will be turned by hand into place then tapped snug with a hammer and screwdriver. Don't over-drive it or you may deform the gaskets. Don't under-drive it or the strainer will be loose, will rotate, and ultimately it will leak.

Step 8: Insert the Strainer Basket Through the Sink Drain Opening

From above, insert the sink strainer basket through the drain opening. You'll see in the photo that I've placed more plumbers' putty right onto the underside of the sink drain basket flange. This isn't necessary. Some plumbers just put their putty on the flange rather than on the sink surface. I do both. I don't worry about too much putty as the excess is going to squeeze out when I tighten the sink drain basket nut.

Sink drain basket ready to install into the sink drain opening (C) Daniel Friedman

Below you see that I've swabbed both the large diameter sink drain basket threads and the smaller diameter threads with teflon pipe joint paste. This will assure that both the drain basket nut and the sink drain tailpiece nut will screw easily in place. As I've used non-hardening teflon paste this will make the next person who has to replace this sink strainer very happy as it'll probably be easier to unscrew the parts.

Teflon paste on threads of the sink strainer basket makes future repair easier (C) Daniel Friedman

I also have the theory that this teflon paste helps resist corrosion of these parts during their service life.

Below you can see the excess plumbers' putty that's oozed out of the connection as I push the sink drain basket in place.

Sink drain basket in place, excess putty oozing out of the way. (C) Daniel  Friedman

I don't touch that oozed putty yet as more is to come oozing out when I tighten the nut that secures the sink drain basket in place.

Step 9: Install the Sink Strainer Assembly Gaskets & Nut

Sink strainer basket gaskets and nut ready to install (C) Daniel Friedman

Above I've taken an extra step also eschewed by most plumbers: I've smeared plumbers putty on the upper side of the rubber gasket that will mate against the rusted and somewhat rough cast iron surface of the sink underside. You can see I've got the three parts in order: rubber gasket, plastic gasket, basket nut.

To hold the sink basket strainer from turning while I tighten the nut below, my accomplice uses a pair of blunt nosed pliers. I don't need the Vise Grip pliers for this step as less force is required than when removing an old sink strainer basket assembly, and Also I want to try to avoid nicking or scarring the sink drain parts.

Holding the sink strainer assembly against turing while the retaining nut is being installed (C) Daniel Friedman

Below you can see from under the sink that the sink drain basket parts are now in place.

Sink strainer basket assembly from below, nut in place (C) Daniel Friedman

Below: more putty has oozed out from below the sink strainer basket flange as I've tightened the retaining nut. Now this putty can be removed. I just used my screwdriver to gently cut it away.

Putty ready for removal after tightening the retaining nut on the sink strainer assembly (C) Daniel Friedman

I also remove any excess putty that has oozed out of the gasket and nut assembly on the sink underside.

Final cleanup of sink basket strainer installation (C) Daniel Friedman

Below is one of my two newly-installed sink drain strainer baskets.

Sink drain strainer basket newly installed (C) Daniel Friedman

Step 10: Re-Connect the Sink Basket to the Drain

Sink drain parts all reconnected (C) Daniel Friedman

Above I've re-connected the plastic drain parts and trap so that the sink is now connected to the building drain-waste-vent system again.

With the plastic drain parts all snugged hand-tight we run water in both sinks and watch for leaks. You should not see leaks at the drain basket assembly but you may find a drip or two at those plastic drain connections if they were not clean and snugged up nicely. A professional plumber would not re-use old drain parts.


Replace One or Both Strainers at a Double Kitchen Sink?

Leaky kitchen sink strainer assembly repair (C) Daniel Friedman

You can see that the main leak problem is at the main or deeper right hand sink of this double sink assembly. If I had to physically remove the whole sink to repair the strainer assembly I'd certainly have done both of them rather than to have to do that a second time a few years later. In this case I decided to replace both of the strainer assemblies anyway.


Continue reading at LEAK TYPES, WATER SUPPLY or DRAIN PIPES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see LEAK CAUSES in WATER PIPING - what causes leaks in metal piping?




If you are de-winterizing or are thawing frozen piping see Thawing Frozen Pipes which addresses how to find frozen and burst pipe leaks in buildings without causing a catastrophe

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