A leak under the bathroom sink is more than just a nuisance. That constant drip, drip, drip can lead to water damage, mold growth, and frustratingly high water bills. It's a common household problem that often causes panic, but here's the good news: most bathroom sink drain leaks are fixable by a determined DIYer with some basic tools. The key is a methodical approach to correctly diagnose where the leak is coming from before you try to fix it. This guide will walk you through the most common causes of a leaking sink drain, from simple fixes to more involved repairs, and give you the confidence to tackle the job safely and effectively.
The Usual Suspects: Where Your Sink Drain is Most Likely Leaking
Before you start taking things apart, it's crucial to understand the anatomy of your sink drain. The main components are the strainer/pop-up assembly (the part inside the sink bowl), the tailpiece (the short vertical pipe directly underneath), the P-trap (the curved pipe that holds water to block sewer gases), and the drain arm that goes into the wall. Leaks can occur at the connections between any of these parts.
Most leaks are caused by one of four issues: a failed seal or gasket, a loose connecting nut, a crack in a pipe, or a blockage that creates pressure. Grab a flashlight and a paper towel to dry all the pipes and joints. Then, have a helper slowly pour water down the drain while you watch from below. The source of the leak will usually reveal itself.
Leaks from the Sink Strainer or Pop-Up Assembly
This is one of the most common leak sources. The metal strainer (or the housing for the pop-up stopper) is sealed to the sink basin with a plumber's putty or a rubber gasket. Over time, this seal can degrade, dry out, or crack, allowing water to seep down around the flange and drip into the cabinet below. Another spot here is the clevis strap and pivot rod that operate the pop-up stopper; if the connection is loose, water can drip from there.
How to Fix It: Fixing this requires removing the entire pop-up assembly from the sink basin. You'll need to disconnect the pivot rod and clevis strap from under the sink and unscrew the large locknut that holds the assembly in place. Once removed, you must thoroughly clean off the old putty or gasket from both the sink hole and the drain flange. Apply a fresh rope of plumber's putty (or use the supplied rubber gasket) and reassemble, tightening the locknut snugly from below. Be careful not to overtighten, as this can crack a porcelain sink.
Leaks at the Slip-Joint Connections
The pipes under your sink are almost certainly connected with slip-joint nuts and compression washers (usually plastic or rubber). These include the connections between the tailpiece and the P-trap, and between the P-trap and the drain arm. These connections can loosen due to vibration, temperature changes, or accidental bumps. The plastic compression washer inside can also become flattened, cracked, or misaligned.
How to Fix It: This is often the easiest fix. Place a small bucket under the connection. Using slip-joint pliers or an adjustable wrench, try to gently tighten the connecting nut. Turn it clockwise, but be gentle—often a quarter to a half turn is enough. Overtightening can crack the nut or damage the washer. If the leak persists, loosen the nut completely, remove the old washer, and replace it with an exact-size replacement from a hardware store. Ensure the washer is seated correctly before retightening.
Leaks from the P-Trap or Drain Pipes
The P-trap itself can develop leaks if it becomes cracked or damaged. This can happen from physical impact or if a frozen or overly hot substance is poured down the drain. More commonly, a clog located downstream from the P-trap can cause water to back up and create enough pressure to force leaks at the joints or even cause the trap to leak if it has a minor flaw.
How to Fix It: For a cracked pipe, replacement is the only option. P-traps and tailpieces are inexpensive. Simply take the old one to the store to ensure you get the correct size and shape. To check for a clog-related leak, first attempt to clear the drain. You can use a plunger (with water in the sink to create a seal) or a hand auger/drain snake. Removing the clog often relieves the pressure and stops the leak, but you should still inspect the washers at the joints afterward.
Leaks from the Drain Arm Wall Connection
Where the drain arm enters the wall is a more serious connection. It is typically a permanent threaded or glued fitting. Leaks here are less common but more problematic. They can indicate a failure of the sealant (like pipe dope or Teflon tape on threads) or a problem with the pipe inside the wall.
How to Fix It: If the leak is at the threaded joint, you can try to tighten it with a large wrench, but extreme care is needed to avoid damaging the pipe in the wall. If it is a glued joint (ABS or PVC), it cannot be tightened and must be cut out and replaced. This repair is more complex and may be a point where calling a professional plumber is the wisest and safest choice, as it can involve working inside the wall.
Step-by-Step Repair Guide for a Standard Leak
Let's walk through a complete repair for the most typical scenario: a leak from the sink strainer connection.
Tools & Materials Needed: Adjustable wrenches or slip-joint pliers, channel-lock pliers, a small bucket, plumber's putty or replacement rubber gasket, replacement compression washers (assorted pack), and rags.
Step 1: Prepare the Area. Clear everything out from under the sink. Place the bucket under the P-trap. Put on safety glasses.
Step 2: Disconnect the Drainage Pipes. Loosen the slip-joint nuts at both ends of the P-trap. You may need to gently twist and wiggle the trap to remove it. Empty any water into the bucket.
Step 3: Remove the Pop-Up Assembly. Disconnect the clevis strap from the pivot rod by removing the spring clip. Unscrew the locknut that secures the pop-up assembly to the sink. You may need to hold the strainer body from above to keep it from spinning. Push the entire assembly down and out from under the sink.
Step 4: Clean and Apply New Sealant. From above, scrape away all old plumber's putty from the sink hole and the bottom of the drain flange. If it used a rubber gasket, remove it. Roll a new strand of plumber's putty (about the thickness of a pencil) and apply it around the underside of the drain flange. Press the flange firmly into the drain hole.
Step 5: Reassemble from Below. From under the sink, slide the rubber gasket (if applicable) and the cardboard friction washer over the threads, followed by the locknut. Hand-tighten, then use tools to tighten snugly. Wipe away any excess putty that squeezes out from the top. Reconnect the pivot rod and clevis strap.
Step 6: Reconnect the Pipes. Reattach the tailpiece to the drain assembly. Reconnect the P-trap using the old or new compression washers. Hand-tighten all nuts first, then give each a final gentle snugness with pliers.
Step 7: The All-Important Test. Do not skip this. Turn on the faucet and let water run down the drain. Watch every single connection you touched—the strainer flange, the tailpiece connection, and both ends of the P-trap—for several minutes. Check for any drips. If you see a leak, try a slight tightening at that specific joint. If it persists, disassemble that connection and check the washer alignment.
When to Call a Professional Plumber
While DIY is rewarding, know your limits. Call a professional if:
- The leak is coming from the sealed wall connection and tightening doesn't help.
- You discover corroded, cracked, or lead pipes.
- The leak persists after you have meticulously reassembled everything with new parts.
- You are uncomfortable with any part of the process or lack the proper tools.
- The leak is accompanied by signs of major water damage or mold in the wall or floor.
Prevention Tips to Avoid Future Leaks
Avoid Overtightening: Snug is sufficient. Over-torquing plastic or chrome nuts can cause them to crack later.
Use Hair Catchers: The number one cause of clogs (which lead to pressure leaks) is hair and soap scum. A simple strainer can prevent most problems.
Mind What You Pour: Avoid pouring grease, coffee grounds, or chemical drain openers down the sink, as they can corrode pipes or create tough clogs.
Annual Check-Up: Once a year, quickly check under the sink for moisture, slow drips, or any signs of corrosion. A small fix then prevents a big problem later.
FAQs
Q1: Can I use tape or sealant from the outside to stop the leak?
A: No. Products like regular tape or external sealants are temporary fixes at best and will fail quickly. Water pressure will find its way out. A proper repair requires fixing the seal or connection from the inside.
Q2: Why does my sink only leak when I drain a full bowl of water, not when the faucet runs?
A: This points to a specific leak path that is only overwhelmed by a larger volume of water all at once. It's likely a partially failed gasket or a hairline crack. The test with a full bowl helps identify slower, smaller leaks.
Q3: Is plumber's putty or a rubber gasket better for the sink strainer?
A: Both work well. Plumber's putty is traditional and molds perfectly to imperfections, but shouldn't be used on certain synthetic stones. Rubber gaskets are cleaner and are often recommended by modern sink manufacturers. Check your sink's instructions or use the method that was originally installed.
Q4: My pipes are old and the nuts won't budge. What should I do?
A: Apply a penetrating oil (like liquid wrench) to the threads and let it sit for 15-30 minutes. Use proper fitting tools to avoid rounding the nuts. If they are truly seized, you may need to carefully cut the old assembly out, which is a good time to consider a full replacement with new PVC parts.
Q5: How tight should the slip-joint nuts be?
A: Tighten by hand first until the washer is compressed. Then, using tools, give them an additional quarter to half turn until they feel snug. You should not be straining or using excessive force. The seal is made by the washer, not extreme torque.
