A slow drip under the sink can turn into swollen cabinets, mold, and a bad smell before you even notice it. The good news: most minor sink leaks are fixable in an afternoon with basic tools and a bit of patience. This guide walks you through finding the source, fixing common issues, and putting everything back together so you can trust your plumbing again.
Understand Where Sink Leaks Usually Start
Before you grab a wrench, you need to know what you’re looking at. Under most kitchen and bathroom sinks, you’ll see:
- The drain assembly (the pipe coming down from the sink)
- A P-trap (the U-shaped pipe)
- Supply lines (flexible hoses bringing hot and cold water to the faucet)
- Shutoff valves (small knobs or levers on the wall side of each supply line)
- The garbage disposal or dishwasher connection (in many kitchens)
Leaks typically show up at:
- Joints and threaded connections (where pipes meet)
- Around the P-trap
- At the base of the faucet or drain
- Flexible supply lines (especially older rubber ones)
- Dishwasher or disposal hose connections
Getting familiar with these parts helps you narrow down what to fix instead of randomly tightening everything and hoping for the best.
Step 1: Safely Inspect and Confirm the Leak Source
Before you start turning anything, make sure you’re working safely and cleanly.
**Clear the cabinet**
Remove all cleaners, baskets, and anything else under the sink. You need room to move and a clear view of pipes.
**Protect the area**
Place an old towel or rag on the cabinet floor. Set a bucket or bowl under the P-trap and main joints to catch water.
**Dry everything off**
Use a towel or paper towels to dry the pipes and fittings completely. You can’t trace a leak through already-wet surfaces.
**Run water slowly**
Turn on the faucet at a low-to-medium flow. If it’s the kitchen sink, run both hot and cold for a bit (one at a time). For a bathroom sink, fill the basin halfway, then open the stopper so a lot of water goes through the drain at once.
**Trace with your fingers and a dry tissue**
Starting from the top (faucet base, drain connection) and moving down, run a dry fingertip or a dry tissue around each joint and connection. Watch and feel for: - Drips forming at a joint - Water wicking into the tissue - A steady “sweat” on the same spot
If you can’t see the leak but smell mustiness or see staining or bubbling on the cabinet floor, the leak may be slow or intermittent. Repeat this test a couple of times, including when using hot water and when using any sprayer or side attachments on your faucet.
Step 2: Tighten and Reseat Loose Drain and Trap Connections
Once you’ve found the area that’s leaking (or strongly suspect it), start with the simplest fixes.
**Check hand-tight fittings first**
Many sink drains and P-traps use plastic slip nuts. These should be snug but not overtightened. Try tightening them by hand a quarter-turn. If they’re already very tight, don’t force them further or you may crack the fitting.
**Use pliers gently on metal connections**
If you have metal nuts or compression fittings, use an adjustable wrench or channel lock pliers. Turn in small increments (about an eighth to a quarter turn). Stop as soon as you feel firm resistance.
**Test again with water**
Run the water through the system the same way you did before. Watch the previously leaking spot closely. If a slow drip continues, move to the next step.
**Disassemble the P-trap if needed**
If the leak is at the trap and tightening doesn’t help: - Place your bucket directly under the trap. - Unscrew the slip nuts by hand (or gently with pliers on metal). - Let the water drain into the bucket. - Inspect the washers (the tapered plastic or rubber rings inside). If they’re hard, cracked, misshapen, or gunked up with mineral buildup, they should be replaced.
**Clean and reassemble with new washers**
Wipe the mating surfaces of the pipes clean. Install new washers in the same orientation as the old ones (tapered side facing the fitting). Hand-tighten the slip nuts until snug, then give a slight additional turn if needed. Test with water again.
This alone fixes a large percentage of under-sink leaks caused by minor loosening, age, or dried-out washers.
Step 3: Repair a Leaking Sink Drain Flange at the Basin
If water appears around the top of the drain (inside the sink) or you see drips coming from just under the sink where the drain body meets the basin, the seal at the sink drain flange may be failing.
**Confirm it’s the drain seal**
- Plug the sink and fill it partially with water. - Look underneath: if water only drips when the basin is full and not when just a small flow is running, the drain seal is a strong suspect.
**Remove the drain assembly**
- Turn off the water at the faucet if there’s a chance of bumping the handles. - Put a bucket under the drain. - Disconnect the P-trap from the drain tailpiece. - Use pliers or a wrench to loosen the locknut holding the drain body to the sink. - Hold the flange (top piece in the sink) with one hand while you unscrew the nut from below.
**Clean off old putty or gasket**
- Lift the flange out from the top of the sink. - Scrape off old plumber’s putty or remove the old gasket from both the flange and sink surface with a plastic scraper or putty knife. Wipe everything clean and dry.
**Reseat with fresh seal**
- If your drain uses plumber’s putty: Roll a rope of putty in your hands and wrap it under the flange lip. Press it into place on the sink drain opening. - If your drain uses a rubber or foam gasket: Position the new gasket according to the manufacturer’s instructions.
**Reinstall and tighten evenly**
- Insert the flange back into the sink opening. - From below, slide the gasket (if used) and any friction rings or washers into place. - Thread the locknut on by hand and tighten gradually, checking from above that the flange stays centered. Wipe off any excess putty that squeezes out. - Reconnect the drain tailpiece and P-trap, then test by filling the sink again.
A freshly sealed drain flange stops leaks that slowly rot cabinets and stain ceilings below, all with a relatively accessible repair.
Step 4: Replace Dripping or Bulging Flexible Supply Lines
If the leak is on the pressurized side (before the faucet) at the flexible lines or shutoff valves, you need to take more care. Pressurized leaks can get worse quickly.
**Turn off the water at the shutoff valves**
- Turn the hot and cold shutoff valves clockwise until they stop. - Open the faucet to relieve pressure and confirm water is off.
**Inspect the supply lines closely**
Look for: - Corrosion or rust at the connections - Cracks, bulges, or damp spots on the hose - Green or white mineral buildup at joints (a sign of slow leaks)
If the hoses are old rubber, discolored, or suspect, replacing them is usually safer than trying to coax more life out of them.
**Remove the old supply lines**
- Use an adjustable wrench to loosen the nut at the faucet connection. - Loosen the nut at the shutoff valve. - Have a towel under the connections to catch residual water.
**Install new braided stainless lines**
- Measure or check the length of the old lines and buy the same length or slightly longer (but not excessively long). - Attach one end to the faucet first. Hand-tighten, then snug with a wrench (do not overtighten). - Attach the other end to the shutoff valve the same way.
**Turn water back on slowly and check for leaks**
- Turn the shutoff valves counterclockwise slowly. - Watch each connection while you do this. - If you see a slow seep at a connection, tighten that nut just a bit more.
Proactively replacing weak supply lines is one of the easiest ways to avoid a catastrophic under-sink flood later.
Step 5: Final Testing, Drying, and Preventive Checks
Once your fixes are in place, don’t skip a thorough final check. This is where you confirm all your work and set yourself up to catch future problems sooner.
**Run multiple test scenarios**
- Run cold water at full blast for a few minutes. - Run hot water at a good flow. - If there’s a sprayer or pull-down faucet, use it and let it retract several times. - For kitchen sinks with disposals or dishwashers: run the disposal briefly and, if possible, run a short dishwasher cycle to check that connection.
**Check every joint with a dry tissue**
Return to each fitting you touched and press a dry tissue or paper towel against it. Any damp spots show up clearly. It’s more sensitive than fingers alone.
**Look for slow-forming drips**
Sometimes a leak takes a minute or two to appear. Keep the area lit and watch for: - A drop forming and hanging from a fitting - A shiny “bead” forming around a connection - Water tracks on the exterior of the pipe
**Dry everything thoroughly**
Once you’re confident the leaks are fixed, dry the cabinet floor and pipes completely. This way, any new dampness you see later will be easy to identify as new, not leftover.
**Add preventive protection**
- Place a thin plastic tray, boot mat, or under-sink liner on the cabinet floor to catch any future leaks early. - Keep the area reasonably tidy so you can see pipes without removing a mountain of stuff. - Make it a habit to glance under the sink monthly, especially after guests, heavy use, or any work on nearby fixtures.
Taking the extra 10–15 minutes for thorough testing can be the difference between a successful fix and discovering the same leak a week later.
Conclusion
A leak under the sink doesn’t automatically mean calling a plumber or tearing out cabinets. With basic tools and a step-by-step approach, most DIYers can tighten loose fittings, refresh worn washers, reseal drains, and upgrade old supply lines in a single afternoon. The key is to work methodically: find the true source, fix one thing at a time, test thoroughly, and give yourself a clean, protected cabinet area so you’ll spot any future issues early.
Addressing a small leak now protects your cabinetry, prevents mold, and avoids bigger plumbing bills later—exactly the kind of practical, manageable home repair that keeps your place feeling solid and “fix ready” for whatever comes next.
Sources
- [U.S. Environmental Protection Agency – Fixing Leaks at Home](https://www.epa.gov/watersense/fix-leak-week) - Overview of common household leaks and why timely repair matters
- [Family Handyman – How to Fix a Leaky Sink Trap](https://www.familyhandyman.com/project/how-to-fix-a-leaky-sink-trap/) - Step-by-step visuals and tips for working with P-traps and slip joints
- [The Home Depot – How to Replace a Bathroom Faucet and Supply Lines](https://www.homedepot.com/c/ah/how-to-replace-a-bathroom-faucet/9ba683603be9fa5395fab9013f5b5a6) - Practical guidance on working with shutoff valves and flexible supply hoses
- [This Old House – How a Kitchen Sink Drain Works](https://www.thisoldhouse.com/plumbing/21016467/how-a-kitchen-sink-drain-works) - Helpful diagrams explaining the parts and connections under a sink
- [PF WaterWorks – Installing a Sink Drain Using Plumber’s Putty](https://www.pfwaterworks.com/using-plumbers-putty) - Manufacturer guidance on correct drain flange sealing techniques
Key Takeaway
The most important thing to remember from this article is that this information can change how you think about Home Repair.
