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Bathroom Sink Leaking from Underneath: Causes and Fixes

No one loves waking up to a puddle of water under their sink. Both bathroom and kitchen sink leakage are unpleasant.

Even so, it is common for such issues to occur.

Fortunately, sink leakages are easy to handle, even without professional help.

Bathroom Sink Leaking from Underneath: Causes and Fixes 1

Defective P-traps, clogged supply lines, and poor drain pipe connections often cause leaks under the bathroom sink. The most important thing is to know how to respond when this happens. 

I once had a leaking bathroom sink. I called a plumber, and he quoted a $350 price to me with a $50 call-out fee. I thought his charges were too steep, and I decided to fix the issue myself.

My approach is simple. First, I diagnose the issue to identify potential causes. I then come up with potential remedies and apply the most appropriate ones. I will teach you how to do it too. 

In my experience, the right solution to a leaking sink depends on where it is leaking. I will consider the most common causes and solutions. 

Only call a professional plumber when everything you try fails to yield results.

Assessing Leaking Sinks

Assessing the leak will help you decide on a good fix. It doesn’t matter how small or minor the leak is; assess it first.

The first sign is a water-soaked floor/ cabinet. Some leaks are not so obvious. For example, leaks from pressurized inlets are clear. Yet, problems emanating from drain lines aren’t as obvious.

The type of under-sink leak can inform the solution one uses. Check common leak areas such as p-traps, water supply leaks, and faulty drain seals.

Water Supply Leaks

Bathroom Sink Leaking from Underneath: Causes and Fixes 2

This appears as sprays or continuous drips. Shut off faucets and valve connections. Tighten loose compression fittings too. Faucet connection leaks need locking pliers to access.

Faulty Seals

Bathroom Sink Leaking from Underneath: Causes and Fixes 3

Seals deteriorate with time. Remove the drain and wrap it with plumber tape before reinstalling.

P-trap Leaks

Bathroom Sink Leaking from Underneath: Causes and Fixes 4

The P-trap is under the sink. Leaks in this section result from loose connections, corrosion, or clogging. Fix is by hand-tightening loose connections. Also, clean the clogged pumps and replace them if the corrosion is too severe.

Fix the specific issues according to the severity and location of the leak. To do this, one needs a few tools. I have highlighted some of the tools and a few of the frequently occurring under-sink leaks.

Tools Needed to Fix an Underneath Bathroom Sink Leakage

To fix a leaking sink, the following are essential tools that any DIYer requires:

  • Teflon tape and plumber’s putty
  • Slip joint pliers/pipe wrench
  • Old rug/ bucket
  • Long nose pliers
  • Replacement parts

Once you have all the crucial tools, tackle each issue uniquely.

When the Bathroom Sink Leaks Underneath at the Drain Body

  • If your bathroom sink links at the point where the pivot rod enters the body, then fix it as follows.
  • Use an adjustable wrench and tighten the retaining nut. You can also use slip-joint pliers instead.
  • If the leak persists, unscrew the retaining nut and slide the pivot rod out of the drain body. Go on to replace the gasket or washer inside the retaining nut.
  • If this fails to solve the issue and the leak continues, check the lift mechanism for total wear and tear. If so, replace it with the drain body and the pop-up assembly.

Fixing a Sink that Leaks Underneath the Trap

  • Make sure to tighten the lock nuts and slip nuts. Make sure these are firm without using too much aggression.
  • Wipe the trap assembly using a dry piece of cloth. Next, run a whole basin of water through the drain to test and confirm the leak.
  • Check to see where the leak is. If it comes from the slip nuts, remove the trap bend. Follow up by removing the trap arm and finally replacing all the washers.
  • Check for damage or corrosion and replace the bend of the arm.
  • If the sink flange or the tailpiece leaks, make sure to fix the tailpiece first.
  • In case of a long trap arm that doesn’t fit into the sink’s drain pipe, use a hacksaw to trim off the excess. Ensure the trap arm extends only more than 1.5 inches into the stub-out or adapter.
  • Be sure to replace a damaged or corroding arm/ bend

Fixing a Bathroom Sink that Leaks Underneath at the Tailpiece

  • Use a wrench to unscrew the tailpiece except for the brass tailpiece. These easily crush, thus unscrewing them with your hand.
  • Slip a wide rubber band into the tailpiece to improve grip. This prevents your hand from slipping when attempting to loosen the tailpiece.
  • In case of a stubborn tailpiece, use a pipe wrench to remove it. You can replace this part with a similar-sized piece.
  • Expect old tailpieces to crumble. I advise using long-nose pliers to loosen them. Twist them out of the remaining drain body threads.
  • Wrap some Teflon tape/plumber’s tape/thread seal tape around the old thread. You can then replace the worn-out parts.

Fixing a Bathroom Sink Leaking Underneath at the Flange

Water leaks underneath the sink at the flange when a washer wears off. It also happens when the putty under the sink cracks. These two scenarios cause water to seep below the sink area.

You can fix this as follows:

  • Start by removing the trap bend if you haven’t already
  • Next, remove the drain body to free the flange under the sink
  • Proceed to remove, then drain the body. The goal is to free the flange from the leaking sink
  • Get some of the plumber’s putty and apply a thin strip under the flange’s lip.
  • Proceed to reinstall the drain body you removed earlier. Finish with the pop-up assembly.
  • Wipe all excess putty around the flange
  • If this does not solve the leaking problem, consider tightening the connections. Turn each connection ¼ way, taking each connection at a time.


What causes a bathroom sink to leak underneath?

Leaking bathroom sinks happen because of various issues, including faulty parts. Others are improper connections, supply lines and clogging, and faulty p-traps. Always check to ensure you know what you are fixing.

How do I fix a leak under my bathroom sink?

Under the sink, leakage issues are easy to fix. It depends on the damage’s extent and the leak’s exact location. There are instances where replacing the damaged parts works. Other times simple repairs work. Assess the issue first before proffering a solution.

Can you use a leaking sink as before after fixing it? 

If you address the issue properly, then the sink is good to use as before. If you need help with the solution, call a professional plumber to help. The bottom line is to have a functional sink.