A leaking kitchen sink almost always comes from one of four places: the faucet, the water supply lines, the drain pipe or P-trap, or the seal under the sink’s drain flange. Find the exact spot before you touch a wrench, since the fix and the parts you need are different for each one. Most of these repairs take under an hour with basic hand tools and less than 30 dollars in parts, and they don’t require a plumber. Here’s how to trace the leak to its source and seal it for good.
Where is the leak actually coming from?
Clear out the cabinet under the sink and lay down some old towels or newspaper. Dry every pipe, joint, and connection completely, then turn on the faucet and run water for a minute while you watch closely with a flashlight.
Check the faucet base first, since water pooling there points to a worn gasket or O-ring inside the faucet itself. Next, watch the P-trap, the curved pipe under the sink, along with the slip nuts that hold it together. Run your fingers along the drain pipe joints and the underside of the drain flange, where the strainer meets the sink basin. Finally, look at the two supply lines and the shutoff valves they connect to, even when the faucet is off, since a constant drip here means the leak has nothing to do with faucet use.
Leaks like this add up. EPA’s WaterSense program estimates that the average home loses more than 9,000 gallons of water a year to small, fixable leaks exactly like these, so tracking down the source is worth the ten minutes it takes.
If the source still isn’t obvious, dab a strip of dry tissue or a paper towel against each suspect joint one at a time and hold it there for a few seconds without touching anything else wet. A dry sink can still hide a slow drip, and a damp tissue will show you the exact joint that’s failing instead of leaving you to guess between three or four possible spots.
What tools and materials do you need?
Most kitchen sink repairs call for the same short list of supplies:
- An adjustable wrench and a pair of channel-lock pliers
- A flashlight and a small mirror for tight spaces
- A bucket and old towels
- Plumber’s putty or 100 percent silicone sealant
- PTFE (Teflon) tape for threaded connections
- Replacement O-rings, washers, or a cartridge that matches your faucet brand
- A new supply line if the old one is cracked or corroded
- A Phillips or flathead screwdriver
Bring the old part with you to the hardware store if you’re not sure of the exact size. Faucet cartridges especially vary a lot between brands, and a mismatched part will leak again within days.
How do you turn off the water before you start?
Reach under the sink and turn the two shutoff valves clockwise until they stop, one for hot and one for cold. Then open the faucet to release the pressure left in the lines and let any remaining water drain into a bucket. If a shutoff valve is stuck, corroded, or missing, close the main water valve for the house instead. Never work on a supply line or faucet with the water still under pressure.
How do you fix a leaking faucet?
A faucet dripping from the spout or leaking around the base is usually a worn internal part, not a sign the whole unit needs replacing.
- Turn off the water and plug the drain so small screws don’t fall in.
- Pop off the decorative cap on the handle and remove the screw underneath, then lift the handle away.
- Unscrew or unclip the cartridge, ball, or stem assembly, depending on your faucet type, and pull it out.
- Inspect the O-rings and rubber seals for cracks or flattening, then replace them, or replace the whole cartridge if it’s scored or loose.
- Reassemble in reverse order, turn the water back on slowly, and run the faucet for a minute to check the fix.
If water is dripping specifically from the spout with the handle off, the valve seat inside the faucet body may be corroded. A light cleaning sometimes helps, but a pitted valve seat usually calls for a new cartridge or faucet. It’s also worth unscrewing the aerator at the tip of the spout and rinsing out any mineral buildup, since a clogged aerator can sometimes be mistaken for a leak.
How do you fix a leaking P-trap or drain pipe?
Place a bucket under the trap to catch any standing water, then loosen the slip nuts at each end of the P-trap by hand or with channel-lock pliers. Slide the trap down and check the rubber or nylon washers inside each nut. A leak here is almost always a washer that’s cracked, twisted, or missing, or a nut that was never tightened enough in the first place.
Wipe the pipe ends clean, seat a fresh washer if the old one looks worn, and slide the joint back together with the washer facing the right direction. Hand-tighten the slip nuts, then give each one a quarter turn with pliers, no more. Overtightening plastic slip nuts is one of the most common causes of a cracked trap. Run water and check every joint again once it’s back together.
How do you reseal the sink drain flange?
If water is dripping from underneath the basket strainer, where the drain meets the sink bowl, the seal between them has failed. Loosen the locknut under the sink with pliers, then lift the old strainer assembly out from above and scrape away the old, dried putty or caulk from both the sink and the flange.
Roll a rope of plumber’s putty about as thick as a pencil and press it around the underside of the new or cleaned flange, then set it into the drain hole and press down firmly. From underneath, add the rubber gasket, friction ring, and locknut, then tighten. Oatey’s guide to using plumber’s putty walks through the right amount to use and how to wipe away the excess before it sets. Skip putty in favor of 100 percent silicone if your sink is natural stone or a stone composite, since oil-based putty can stain those surfaces permanently.
How do you fix leaking supply lines and shutoff valves?
A damp connection where the supply line meets the shutoff valve or the faucet is often just a loose nut. Tighten it a quarter turn with a wrench while holding the valve steady, then check again. If it still weeps, take the line off, wrap the threads with two or three turns of Thread seal tape, and reconnect.
Braided supply lines last for years but not forever. Replace one immediately if you see rust, a bulge, or a crack in the braiding, since those are signs it could burst rather than just drip. If the shutoff valve itself leaks around the stem when you turn it, or won’t fully stop the flow of water, the valve has failed internally and needs replacing, which usually means shutting off the main water supply first.
When should you call a plumber instead?
Some situations go beyond a straightforward DIY fix. Call a licensed plumber if the shutoff valves are corroded solid and won’t turn, if you find a leak inside a wall or under the floor rather than under the sink, if the pipes are old galvanized steel showing rust, or if a repair you’ve already made keeps failing.
A professional sink repair typically runs between 150 and 450 dollars depending on the cause, which is worth it next to the cost of water damage or mold from a leak left alone.
How do you prevent the leak from coming back?
Once the sink is dry, check on it again after a full day of normal use, since some fixes hold under a quick test but fail once the cabinet warms up or the pipes shift slightly with hot water. Give every slip nut, supply line connection, and the faucet base a light snug about once a year, and swap out rubber washers or O-rings as soon as they look flattened rather than waiting for a drip to show up.
Avoid pouring grease down the drain, since cooled grease narrows the pipe and puts extra pressure on the P-trap joints every time water backs up behind it. If your home has hard water, a light film of mineral scale can build up around the faucet cartridge and valve seat within a couple of years, so a quick rinse of the aerator every few months keeps that buildup from turning into a leak later.
Conclusion
Most kitchen sink leaks trace back to a worn washer, a dried-out seal, or a connection that simply worked itself loose, and each one has a specific, inexpensive fix. Shut off the water, find the exact source with a dry towel and a flashlight, then repair or reseal that one spot. A sink that stays bone dry under the cabinet after running water for a few minutes means the job is done right.
Frequently asked questions
Why is my kitchen sink leaking even when I’m not using it?
A leak that shows up with the faucet off usually comes from a supply line or shutoff valve under constant water pressure, not the faucet itself. Check those connections first and tighten or replace them as needed.
Should I use plumber’s putty or silicone under the sink?
Plumber’s putty works well for standard stainless steel or enamel sinks and stays removable for future repairs. Switch to 100 percent silicone for natural stone or composite sinks, since putty can stain those surfaces.
How much does it cost to fix a leaking kitchen sink?
A DIY repair with new washers, putty, or a supply line usually costs 10 to 30 dollars in parts. Hiring a plumber typically costs 150 to 450 dollars, depending on what’s actually causing the leak.
Do I need to replace the whole faucet if it keeps leaking?
Not usually. Most leaking faucets just need a new cartridge, O-ring, or washer. Replace the whole faucet only if the body itself is cracked, badly corroded, or a new cartridge doesn’t stop the drip.
How tight should slip nuts be on a P-trap?
Hand-tight, plus about a quarter turn with pliers. Overtightening plastic slip nuts is a common cause of cracks, which creates a bigger leak than the one you started with.
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