Gasket Under Bathroom Sink Leaking (Troubleshooting Guide)

On this page

Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

Turn off the water supply, remove the sink's mounting nuts, and replace the damaged gasket with a new one. Ensure the new gasket fits properly and reassemble the sink securely.

Confirm the symptom

Water pooling under the sink or on the countertop
Dripping sound when the sink is not in use
Visible moisture around the base of the sink
Musty smell near the sink area
Stains or discoloration on the underside of the sink

Tools and prep

Adjustable wrench or pliers
Screwdriver (flathead or Phillips, depending on your sink)
Replacement gasket (rubber or silicone)
Plumber's putty or silicone sealant
Towel or rag to catch water
Bucket to collect any water
Safety gloves and goggles
Turn off the water supply to the sink before starting

Step-by-step troubleshooting

Turn Off Water Supply
Do: Locate and turn off the shut-off valves under the sink. If there are no valves, turn off the main water supply to the house.
Observe: Water should stop flowing from the sink faucet.
Means: The water supply is successfully turned off.
Next: Prepare tools and materials.
Remove Sink Mounting Nuts
Do: Use an adjustable wrench or pliers to loosen and remove the nuts securing the sink to the countertop. Place a towel underneath to catch any water that may leak out.
Observe: The sink is loose and can be slightly lifted.
Means: Mounting nuts are removed, and the gasket is accessible.
Next: Inspect the gasket.
Inspect the Gasket
Do: Look for signs of wear, cracks, or deterioration on the gasket. This is usually located between the sink and the countertop.
Observe: The gasket shows visible damage or is missing.
Means: The gasket is the source of the leak.
Next: Remove the old gasket.
Remove the Old Gasket
Do: Carefully remove the damaged gasket from the sink base and countertop. Clean the surfaces thoroughly with a dry cloth or towel to remove any debris or old sealant.
Observe: The sink and countertop surfaces are clean and dry.
Means: The area is ready for the new gasket.
Next: Install the new gasket.
Install the New Gasket
Do: Place the new gasket over the sink base, ensuring it aligns properly with the countertop. Apply a thin layer of plumber's putty or silicone sealant around the edges for a watertight seal.
Observe: The gasket is evenly seated and secure.
Means: The new gasket is correctly installed.
Next: Reattach the sink.
Reattach the Sink
Do: Position the sink back onto the countertop and secure it with the mounting nuts. Tighten the nuts evenly with the wrench or pliers to avoid warping the sink.
Observe: The sink is firmly attached and there are no gaps.
Means: The sink is properly reinstalled.
Next: Test for leaks.
Test for Leaks
Do: Turn the water supply back on and let the sink run for a few minutes. Check under the sink for any signs of water leakage.
Observe: No water is leaking from the gasket area.
Means: The repair was successful.
Next: Final inspection.
Final Inspection
Do: Inspect the sink and surrounding area for any signs of water damage or improper installation. Ensure all tools and materials are put away.
Observe: The sink is functioning properly and there are no leaks.
Means: The repair is complete and successful.
Next: End

Common causes

Cause	Likelihood	Difficulty	Part often needed
Worn or damaged gasket Over time, the gasket can degrade due to constant exposure to water and temperature changes.	Common	Easy	Replacement gasket
Improper installation If the gasket was not installed correctly during initial installation or a previous repair, it can lead to leaks.	Sometimes	Easy–Medium	Replacement gasket
Incorrect gasket size Using a gasket that does not fit properly can result in gaps and water leakage.	Sometimes	Medium	Correctly sized gasket
Loose mounting nuts If the nuts securing the sink to the countertop are not tightened properly, the gasket may not form a proper seal.	Sometimes	Easy	Mounting nuts
Corrosion or mineral buildup Mineral deposits or corrosion on the sink or countertop can prevent the gasket from sealing properly.	Rare	Medium	Cleaning tools or descaling solution
Damaged sink or countertop Cracks or warping in the sink or countertop can compromise the gasket's ability to create a watertight seal.	Rare	Hard	Sink or countertop repair or replacement

Parts matching

To ensure a proper fit, match the replacement gasket to your sink's specific model and size. Measure the diameter and thickness of the old gasket, and compare it to the new one before installation.

Measure the old gasket's diameter and thickness
Check the sink's model number for compatible gasket specifications
Compare the new gasket's dimensions to the old one
Ensure the new gasket is made of the same material (rubber or silicone)
Verify that the gasket fits snugly without stretching
Use a correctly sized gasket to avoid gaps and leaks

If you're unsure about the correct size or type of gasket, take the old one to a hardware store for a direct comparison.

When to call a professional

The leak persists after replacing the gasket
You notice water damage to the countertop or cabinetry
The sink is loose or wobbly after reinstallation
You suspect a damaged sink or countertop that requires replacement
You are unable to locate or access the gasket
You are not comfortable working under the sink or with plumbing

FAQ

How often should I replace the gasket under my bathroom sink?

It's recommended to inspect the gasket annually for signs of wear and replace it every 3–5 years, depending on usage and water quality.

Can I use any type of sealant with the new gasket?

Use plumber's putty or silicone sealant specifically designed for plumbing applications to ensure a watertight seal.

What if I don't have the exact replacement gasket size?

If you can't find the exact size, opt for a slightly larger gasket that can be trimmed to fit, ensuring it still forms a proper seal.

Why is my sink still leaking after replacing the gasket?

Check for loose mounting nuts, improper installation, or a damaged sink/countertop. If the issue persists, consult a professional plumber.

Is it normal for some water to pool under the sink after installation?

A small amount of water may be present initially, but it should dry up within a few hours. Persistent pooling indicates a problem.