Bathroom Sink Leaking From Underneath? 4 Common Causes and Solutions

It can be frustrating and worrying when you notice your bathroom sink is leaking water from underneath. A leak indicates there is a problem somewhere in the plumbing that needs to be addressed. Identifying why your sink is leaking is the first step to fixing the issue.

We'll also provide tips on how to inspect your sink to pinpoint the source of the leak. With the right diagnosis, you can repair your leaky sink using simple DIY methods.

Loose Supply Line Connections

Supply lines carry water from your home's main water lines up to the faucet and sink basin. Flexible braided stainless steel is the most common material used for bathroom sink supply lines today.

These supply lines connect to shutoff valves that control water flow. They also connect to copper pipes or flexible plastic tubes that run up into the faucet assembly. The compression fittings joining the supply lines to the shutoff valve and faucet can loosen over time.

Signs of a leaky connection include water dripping from the supply line itself or wetness at the point it connects to the shutoff valve or faucet. You may also notice the leak originates near the point the supply line runs up to the faucet base.

To inspect for loose fittings, begin by shutting off the water supply valves below the sink. Then check each compression nut connection point by attempting to tighten by hand or with a wrench. Check the supply line's connection to the faucet base as well.

If you find a loose supply line fitting, tighten it to eliminate the leak. Be careful not to overtighten. If the fittings are severely corroded, you may need to replace supply lines.

Tips for Fixing Loose Supply Line Connections

• Shut off the water before working
• Protect cabinet with towels to absorb drips
• Inspect each fitting connection point
• Tighten loose compression nuts by hand or with wrench
• Replace corroded supply lines if fittings don't tighten properly

Faulty Drain Seals

The drain seal provides a watertight barrier between the sink's strainer assembly and drain tailpiece underneath. This seal prevents water from leaking through the gap between the sink and drain.

Sink drain seals are usually made of plumber's putty, rubber gaskets, or wax ring materials. Over time, these seals can crack, disintegrate, or become displaced. Faulty seals allow water to leak from around the drain as you use the sink.

To check for a drain seal leak, look for water droplets or moisture around the drain tailpiece and connection point to the sink. You may need to remove the curved p-trap piping from below to view the drain seal area.

Fixing this type of leak involves taking apart the drain assembly to replace the seal. Start by disconnecting and removing the p-trap from under the sink if necessary. Then loosen the drain tailpiece from the sink and remove it to access the seal.

Clean away any old putty or gasket material before applying a new seal. You can use plumber's putty or substitute a rubber gasket seal. Make sure to tighten all drain connections properly to prevent further leaking.

Tips for Fixing a Leaky Drain Seal

• Inspect around drain tailpiece for moisture
• Remove p-trap if needed for access
• Take apart drain assembly to replace seal
• Apply new plumber's putty or rubber gasket seal
• Tighten drain connections securely

Damaged P-Trap Pipe

The p-trap is the curved section of piping that connects the sink tailpiece to the drain line below. Its purpose is to hold water in the curve, creating a trap that prevents sewer gases from entering through the sink.

Like other pipes, p-traps are vulnerable to damage over time. The constant flow of water takes a toll, eventually causing leaks due to corrosion, loose fittings, and debris clogs forcing water out through seals.

Inspect the p-trap if you notice water dripping near the curve or moisture further down the pipe. Removing the p-trap may help spot specific damage points. Signs include corrosion pits, cracks, disconnected fittings, and obstructions blocking water flow.

To fix a leaky p-trap, begin by tightening any loose slip-joint compression fittings with an adjustable wrench. Clear any stuck debris in the trap using azip-it drain cleaner or bent wire. Replace severely corroded sections with new piping.

When re-installing the p-trap, apply teflon tape to threaded fittings to form a tighter seal. Make sure all slip joints are aligned properly and their compression nuts tightened firmly.

Tips for Fixing a Leaky P-Trap

• Inspect p-trap for corrosion damage or clogs
• Tighten any loose compression slip joints
• Clear any obstructions in the piping
• Replace severely corroded sections
• Use teflon tape on threads when re-installing

Worn Out Faucet Washers

Faucet washers are small rubber discs inside the faucet assembly that regulate water flow and pressure. When the valve is open, water flows through holes in the washer plates.

These washers deteriorate over time from the constant force of water pressure and friction wear. Cracked, torn, and flattened washers result in water leaking past the valve seat when the faucet is turned on.

Signs that worn washers are causing leaks include water dripping from the faucet spout when shut off. Leaks originating at the faucet base also indicate worn internal washers.

To replace the washers, start by shutting off the water lines under the sink. Pry off the faucet handle with a screwdriver to access the interior valve components. Locate the washers, remove the retaining nut, and pull them off the threaded valve body.

Examine the old rubber washers and replace them if they appear cracked, flattened, or irregularly shaped. Install new replacement washers of the same size and type according to manufacturer specifications.

Tips for Replacing Worn Faucet Washers

• Shut off water supply before starting
• Remove faucet handle to access washers
• Inspect washers and replace if worn out
• Install new replacement washers
• Make sure they are the correct size

Inspecting Under the Sink to Find Leak Causes

Finding the origin of the leak is the first step in any sink repair project. Begin your inspection by shutting off the hot and cold water supply valves below the sink.

Have a bucket and some towels on hand to catch dripping water. A flashlight can help illuminate hard-to-see areas under the sink. Carefully run your hand along pipes and connections, feeling for moisture.

Check supply lines first, inspecting all compression fittings for drips. Then examine the drain at the back of the sink cabinet for leaks. Remove the p-trap if necessary to check seals. Inspect the faucet base and valve interior for damage.

Finding the leak source will determine which repairs are needed. For instance, moisture at a compression fitting indicates a loose supply line. Wetness near the p-trap or drain seal shows faulty seals. Leaks originating at the faucet base mean worn washers.

Taking the time to properly diagnose the issue will get your bathroom sink leak fixed faster. Once you've completed repairs, slowly turn the water supply back on. Check for additional drips or leaks under the sink to confirm you fixed the problem.

How to Repair Leaky Bathroom Sinks

Once you've identified the cause of the leak, you can move on to making repairs. Follow safety precautions like turning off the water supply and draining any remaining water from pipes.

Assemble the necessary tools and replacement parts like wrenches, tubing cutters, seals, washers, etc. Have towels and a bucket ready to control water. Wear eye protection when soldering or using harsh chemicals.

Work carefully when taking fittings apart and re-assembling. Getting help from a second person can make the job easier. Make sure any new seals or washers are the correct size and type.

Go slowly and test your work as you go while making repairs. Check for additional leaks before turning the main water supply back on. Wipe up any spilled water and allow the area to fully dry to prevent water damage.

Finally, clean up the tools and materials used to make your repairs. Consider applying a fresh bead of plumber's putty around the sink edge for increased leakage protection.

Tips for DIY Bathroom Sink Leak Repairs

• Turn off water supply before starting
• Have replacement seals/washers on hand
• Follow safety precautions like eye protection
• Disassemble fittings carefully to avoid damage
• Ensure parts are aligned correctly when re-assembling
• Turn water supply on slowly to check for leaks
• Wipe up spilled water immediately
• Consider adding fresh plumber's putty around sink

Preventing Future Bathroom Sink Leaks

Completing repairs will stop your bathroom sink from leaking temporarily. But you'll also want to take steps to prevent future leaks and extend the life of your plumbing.

Check sink fittings periodically and tighten any that have become loose. Every few months, remove p-traps and drain assemblies to clean out debris. Replace any visibly corroded parts.

Use strainers in drains to catch hair and particles that can lead to clogs. Avoid pouring grease or harsh chemicals down drains. Take care not to place excess weight on sink basins that could damage seals.

Replacing old metal supply lines with newer braided steel lines can prevent leaks. Consider installing leak-proof flexible PEX piping when remodeling. Routinely using sink drain treatments helps keep water flowing freely.

By taking proactive maintenance steps, you can add years of leak-free life to your bathroom sink plumbing. Quickly addressing any minor drips that do arise will also help prevent major leakage disasters.

Dealing with a leaky bathroom sink may seem like a hassle, but arming yourself with the right knowledge makes fixing it a painless DIY project. Focus your troubleshooting on the four common sources of leaks under the sink - supply lines, drain seals, p-traps, and faucet washers.

Thoroughly inspecting the plumbing and connections will reveal the leak point. Making spot repairs with new seals, tightening fittings, or replacing worn parts will get your sink working properly again. Combine repairs with preventive sink maintenance to avoid future leakage headaches.