bathroom sink leaking underneath

DIY Fix For A Bathroom Sink Leaking Underneath – With Video!

Are you dealing with a bathroom sink leaking underneath?  Whether you’re a seasoned DIYer or new to plumbing, this guide will help you identify and repair the source of the water leak.  

Fixing leaky sink plumbing, as I show in the video and detail in the post below, is something I think every homeowner or renter should know how to do.  The good news is that it’s doable even if you’re new to DIY.

You’ll only need a few basic tools, and here are our suggested tools for beginner DIY projects.

AFFILIATE DISCLOSURE:  Some of the links below are affiliate links. This means that, at no additional cost to you, I will earn a small commission if you click through the link and finalize a purchase.  Read the full disclosure policy here.

Video: How to Fix Your Leaky Sink

In this video, I show you how to figure out where the leak is coming from and how to fix a leaking bathroom sink.

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How to Fix a Bathroom Sink Leaking Underneath 

If you’re new to DIY home repair, this brief intro to the basic components of sink plumbing will be helpful.

Understanding Your Bathroom Sink Drain Assembly

sink drain assembly

The sink plumbing (aka drain assembly) consists of several components that work together to remove water from your sink. These components include the tailpiece, the P-trap, the supply lines, and the shut-off valves. 

Familiarizing yourself with these parts will make it easier to identify and fix leaks. So, let’s discuss identifying exactly where the leak is located in the drain assembly.

How to Find the Location of the Leak

With the water running, use a dry paper towel to wipe along each component, starting at the top, where the drain tailpiece meets the sink, to see if it comes back damp.  Focus on the joints, as this is the most likely leak location.

This will help you determine the exact location of the leak so you know exactly what needs to be repaired.

When getting replacement parts for the leak repair, don’t be afraid to take the old piece(s) to the hardware store.  I promise they’re used to it!

4 Main Places Sink Will Leak Underneath

These are the four main areas of a sink’s plumbing where a leak is most likely to occur:

  1. The sink drain flange may be leaking through the seal. 
  2. The P-trap itself may have loose connections or worn-out seals. 
  3. The supply lines that bring water to the sink can develop leaks at the connections. 
  4. And the shut-off valves, located under the sink, can also be a culprit for leaks.

I have found the most common causes of bathroom sink leaks to be the flange or the trap.

How to Fix a Bathroom Sink Leak

Once you’ve determined where the leak is coming from, you’ll need to get the necessary tools and materials.

Tools & Materials Needed:

  • Channel-lock pliers (aka tongue & groove pliers)
  • Replacement pieces for the leak’s location
  • Plumber’s putty, if removing the flange

If you don’t keep common plumbing pieces on hand (I only do because of my house-building day job!), I suggest finding the exact location of the leak first so you know what materials are needed for that specific fix.

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Turn Off The Water Supply

When you’ve identified the leak, have the tools and replacement materials needed, and are ready to tackle the repair, the first step is to turn off the water supply.

Locate the shut-off valves under the sink and turn them clockwise to shut off the water. This will prevent further leakage and allow you to work on fixing the problem without any water flow.

Fixing a Leak at the Sink Drain Flange

Jenny's Note Graphic

This terminology is a bit confusing because the flange is also known as a tailpiece assembly or pop-up assembly.  And Joe even calls it a stem so take your list of words when you go to the hardware store!

This is the part that connects the sink to the drain pipe.  Over time, the rubber gasket between the sink and drain flange can deteriorate or become misaligned, causing water to seep. 

Remember, the first thing to do when repairing anything in a plumbing system is to shut off the water supply! 

tailpiece drain assembly
This close-up of the flange & tailpiece assembly may help understand the directions better.

Steps to Replace Drain Flange Assembly

Remove the retainer that connects the sink stopper to the metal piece hanging under the sink behind the tailpiece.  

Unscrew the ball lever and remove it.

Pull the pop-up out of the assembly inside the sink.

Loosen the nut completely where the P-trap connects to the tailpiece.  Then loosen the second nut on the P-trap completely.  This will allow you to remove the first section of the trap so you have room to work.

Have a bucket ready because this will be full of water!

Loosen the nut at the bottom of the sink and pull the rubber down as far as it will go.

Using the pliers, loosen the nut at the top of the tailpiece.  You can finish loosening it by hand after it’s broken free using the pliers.

Shove up on the stem with your hand and unscrew to remove the flange, or the ring, from the drain inside the sink bowl.  This will separate the underneath portion of the sink flange assembly from the sink.

Remove the old tailpiece, and now we’re ready to add the new one.

Install the New Tailpiece

Take a small ball of plumber’s putty.  Roll it in your hand to create a ¼-inch thick snake.  

Put it around the flange piece (the ring on the drain in the sink bowl).

Push the tailpiece all the way up from underneath and screw the flange onto the tailpiece.

Pull down the tailpiece and tighten the nut underneath the sink at the top of the tailpiece.

Make sure that you get a good squish of putty around the flange.

Wipe off the extra putty.

Drop in the pop-up.

Reinstall the ball lever.  Move it up and down a couple of times to make sure it’s engaged with the pop-up.

Reconnect the retainer of the sink stopper to the metal piece that hangs under the sink behind the tailpiece.  

Reinstall the trap. If you lack hand strength, the pliers may be needed to tighten the nuts.

Turn the water back on and run it while checking the repair. Use a paper towel starting at the top and working your way down, checking all the joints to ensure the leak is fixed and it has a watertight seal.

When getting a replacement piece, I suggest avoiding chromed steel flanges, which tend to rust over time.  Plastic and brass are both good choices.

Fixing a Leak in the P-Trap

The P-trap is the curved pipe from the tailpiece to the main plumbing line. If the P-trap becomes loose or the seals wear out, it can leak.

P-trap is terminology that means it holds a certain amount of water and keeps sewer smells from coming into the house.

This will be one of two styles, depending on whether the trap goes straight down to the floor or into the wall to meet the main plumbing line.

An S-trap goes straight down.  It is very obviously curvier than the J-trap.  This is the style that is shown in the video.

S-style P-trap - S-trap

A J-trap goes into the wall and is a J-like shape.

J-style P-trap - J-trap

To fix a leaky P-trap, I recommend checking at the joints for loose pipe connections first, as this is the most common problem with faulty P-traps and is an easy fix.  Tighten the connections, using pliers if needed.  Use the paper towel test with water running to see if this fixes the leak.

If it still leaks at one of the joints, then the seal is probably bad, and the trap will need to be replaced.

Remove the old one by loosening the nuts at the top and bottom of the trap.  

Have a bucket ready because this will be full of water!

Get a replacement for whichever style of P-trap is needed.

Install the new trap

Compare the new trap to the one that was removed.  If the last section of pipe that goes into the floor or wall is longer than the old pipe, it may need to be cut so that it goes into the main drain at least 2”.

Slide the nut onto the pipe, then slide the ring on, ensuring the flat back is towards the nut and the taper is toward the next piece of pipe.  Otherwise, it will not seal.

Depending on the size of the tailpiece, the ring may be two pieces to go from 1-¼” to 1-½”, and again the flat back towards the nut and the taper towards the next piece of pipe.

Install the new pipe loosely, ensuring all the connections line up, and then tighten the nuts with the pliers.

Fixing a Leak in Water Supply Lines

These are the small pipes that connect the faucet to the water supply valves. Over time, the supply lines can develop leaks due to wear and tear or loose connections. 

How to Replace Water Supply Lines

Measure the length of the old drain lines so the replacements will be the same length.  It’s ok to be a little longer, but not shorter!

Get a braided steel line with nut fittings. Stay away from compression fittings, as they tend to have more issues and are prone to leaks.

The supply lines shown in our video are braided steel.

The braided steel lines are very easy to install.  No plumber’s tape or sealant is needed.  Just tighten the connections and do the leak check one last time.

Just an FYI, in extreme heat or humidity, cold water lines will have condensation due to the cold water colliding with the hot air.  To distinguish between condensation and a leak, dry off the line and let the water in the line come to room temperature.  Don’t run any water at this sink during this time.  Dry it off again and leave it for another 10 minutes.  If the line is still dry after 10 minutes, it’s probably just condensation. 

Replacing Leaking Shut-Off Valves

The last leaky sink repair we’ll cover is replacing shut-off valves.  These are the valves in the sink plumbing that turn the water supply off and on to that sink.

water supply lines and water shut-off valves

Important:  You must shut off the main water supply if replacing shut-off valves under the sink.  This will stop the pressurized flow from the water source.

Due to the variations of supply lines, you will need to remove the shut-off valve and take it to the hardware store to find the exact replacement.

See the video for one way these may be replaced.  This shows the glued-on PVC, and I show how to clean the pipes to ensure a good seal for the new valves.

Other types include threaded-on steel, clamped-on PEX, and sweated copper.

If your shut-off valves are the sweated copper type, which has a sautered joint, I highly recommend you call a professional plumber for this fix. Sweated copper is a repair for a professional.

When to Call a Professional

If you have attempted a DIY repair to fix a leak under your bathroom sink but are still experiencing issues, it may be time to call a professional plumber. 

Here are some situations where it is best to seek professional help:

  • If you are unsure about the cause of the leak.
  • If you do not have the necessary tools or experience to fix the problem.
  • If the leak persists even after you have attempted to repair it.
  • If there is significant damage to the sink or surrounding plumbing.
  • If the repair requires replacing sweated copper-style shut-off valves.

Remember, professional plumbers have the expertise to diagnose and fix any plumbing issues accurately, ensuring a long-lasting solution.

Jenny's Note Graphic

It’s a good idea to keep contact info for emergency plumbers on hand for serious plumbing problems that need immediate attention.  But if the issue is a leaking sink, I suggest shutting off the water supply for that sink and not using it until it can be fixed.  This will help you avoid a costly emergency plumbing call or give you time to research how to repair it yourself.

Final Thoughts on Leaky Sink Repair

Fixing a leak under your bathroom sink can be a manageable task with the right tools and knowledge. By understanding the different components of your sink and identifying the source of the leak, you can usually successfully repair it yourself.  

A small leak can quickly become a pool of water and cause serious water damage, so I encourage you to tackle leak repairs as soon as possible.  

If you have questions or successful repairs to share, please comment below, and check out the video for more hands-on help.

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