Plumb Pak 1-1/2 in. washers
$6.05 at Amazon
Check it on AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Kitchen sink leaking underneath
Dry it, find which side it’s on, then fix that one joint.
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Quick answer
Dry everything, then run water. A drip only while water drains is the drain side: a slip nut, washer or the basket strainer. A drip with the faucet off is the supply side: a stop or supply line.
Most common fix: a new 1-1/2 in. slip joint washer$6.05 at AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Check it on AmazonWhere is your sink leaking from?
Answer two questions. The locator names the likely joint and the fix.
The question that cuts the list in half is pressure. Supply pipes are always under pressure, so they drip whether or not you use the sink. Drain pipes only carry water when the sink empties, so a drain leak stops when the sink is dry.
If the disposal itself is wet, go straight to garbage disposal leaking. If the drip is at a compression nut on a stop, compression fitting leaking under the sink has the full fix.
Washers, a drain kit, putty and an alarm
Most under-sink leaks need a few dollars of parts and your hands. I’ve used every item in this kit.
PlumbCraft 27-piece kit
$27.86 at Amazon
Check it on AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Oatey 31177 putty
$4.54 at Amazon
Check it on AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Resideo RWD21 alarm
$10.99 at Amazon
Check it on AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
| Item | Key specs | Pick it if | Skip it if |
|---|---|---|---|
| Plumb Pak PP25519-20 | 1-1/2 in. beveled polyethylene slip joint washers, 20-pack | A trap nut drips even after a snug hand-tightening. | Your drain is 1-1/4 in. (common on bathroom sinks). Buy that size. |
| PlumbCraft 7027010N kit | 27 pieces for 1-1/2 in. drains, center or end outlet; thermoplastic pipe and poly washers | A trap, tee or arm is cracked, or the old metal trap has corroded. | Only a washer is bad. A whole kit is more than you need. |
| Oatey 31177 putty | Non-staining; Oatey lists granite, marble, quartz and Corian; stays flexible | The basket strainer leaks and needs resealing. | Your strainer uses a rubber gasket only; follow its instructions. |
| Resideo RWD21 | Reusable spot alarm; detects water 1/16 in. deep; audible alert | You want a warning before the cabinet floor swells. | You want phone alerts. This one only sounds on site. |
Check your drain size before buying washers. Kitchen sinks almost always use 1-1/2 in. tubular drains; bathroom sinks are often 1-1/4 in. Mixing sizes with a stack of reducing washers is a common cause of slow weeps. The trap size by fixture guide has the full table.
A PlumbCraft kit’s parts list, as printed by retailers for the 27-piece kitchen kit, includes a center-outlet tee, end-outlet tee, J-bends, waste arms, a wall tube, a floor tube, tailpieces and seven slip nuts and washers. Check the box against your sink before you start.
How to find the leak in five minutes
A dry cabinet and a paper towel beat guessing.
- 1Empty the cabinet
Pull everything out and lay down a towel so new drips show.
- 2Dry every pipe
Wipe all pipes, nuts and valves completely dry with a rag.
- 3Test the supply side first
Leave the faucet off for 10 minutes. Wrap a dry paper towel around each stop and supply nut and look for a damp spot.
- 4Test the drain side
Fill the sink a few inches with the stopper in, then pull it. Hold a dry towel under each joint as the water drains.
- 5Follow the first drop
Water runs downhill along pipes. The highest wet spot is the leak; everything below is runoff.
Filling the bowl matters. A running faucet sends a trickle down the drain; a full bowl pushes real volume and pressure through every joint, the way a sink of dishwater does. If you have a double bowl, test each side separately.
Some people add a few drops of food coloring to the water. It makes a slow weep easy to see on a white paper towel.
Check the dishwasher too. Run a short cycle and watch the drain hose connection at the disposal or the sink tailpiece while it pumps out. A dishwasher pushes water through that joint under pump pressure, so it can leak during a cycle and look dry the rest of the day.
Fixing a leak on the drain side
Most under-sink leaks are here, at a joint you can fix by hand.
| Leak at | Cause | Fix |
|---|---|---|
| Slip nut on the trap | Loose nut or a hardened, cracked or backward washer | Snug by hand; replace the washer bevel-first into the joint |
| Trap or J-bend body | Crack, or corrosion on a metal trap | Replace the trap; swap metal for plastic |
| Under the bowl at the strainer | Dried putty or a loose locknut | Tighten the locknut, or reseal with putty |
| Tailpiece joint | Washer or nut at the strainer’s tailpiece | Re-seat the washer and snug the nut |
| Wall connection | Arm too short or crooked in the wall fitting | Replace the arm so it enters straight |
- 1Put a bucket under the trap
The trap holds water; it will spill when you open it.
- 2Loosen the slip nuts by hand
Plastic nuts turn by hand. Use channel locks gently on metal ones.
- 3Check each washer
The bevel (tapered side) faces into the joint, toward the pipe it seals onto. A flat, hard or cracked washer gets replaced.
- 4Line the pipes up
Joints should slide together straight. Forcing a crooked joint closed is the usual cause of a repeat leak.
- 5Hand-tighten, then a quarter turn
Snug plastic nuts by hand. Overtightening cracks them.
- 6Retest with a full bowl
Fill, drain and check with a dry towel again.
That beveled washer is where most repeat leaks come from. Installed flat side in, it never seals; reused after years, it’s hard as plastic. A 20-pack costs little enough to replace every washer you touch.
A corroded chrome trap that leaks at the bend can’t be fixed with a washer. Replace it with a plastic trap from a repair kit, and replace any brittle metal tailpiece while you’re there.
Resealing a leaking basket strainer
If water appears right under the bowl, the strainer’s seal has failed.
- 1Disconnect below
Loosen the tailpiece slip nut and move it aside.
- 2Remove the locknut
Unscrew the large locknut under the sink. A strainer wrench or channel locks helps; have someone hold the strainer from above.
- 3Lift the strainer out
Scrape all old putty off the strainer and the sink opening, then clean and dry both.
- 4Roll a rope of putty
Press a pencil-thick rope of putty around the underside of the strainer’s rim.
- 5Set it and tighten
Press the strainer into the opening, then from below fit the rubber gasket, fiber washer and locknut, and tighten until the putty squeezes out evenly.
- 6Wipe and reconnect
Wipe away the squeezed-out putty, reconnect the tailpiece and test with a full bowl.
On a granite or quartz counter, regular putty can stain. Oatey sells 31177 as non-staining and lists granite, marble, quartz and Corian. Some strainers ship with a foam or rubber gasket in place of putty; follow the strainer’s own sheet if so.
If the strainer body is cracked or its threads are worn, replace the strainer. Our kitchen sink basket strainer guide compares them.
Fixing a leak on the supply side
Supply leaks run all the time, so they do the most damage. Shut the water off first.
| Leak at | Cause | Fix |
|---|---|---|
| Compression nut on the stop | Nut loose or ferrule not seated | Snug the nut; see our compression fitting guide |
| The stop’s handle stem | Worn packing | Snug the packing nut a little, or replace the stop |
| Supply line nut | Loose nut or failed seal inside | Snug as the maker says; replace the line if it still drips |
| Supply line body | Bulge, kink, rust on braid or crack | Replace the line now |
| Faucet inlet above | Loose connection at the faucet | Snug the connection at the faucet’s shank or hose |
For tightness, follow the connector maker. Fluidmaster’s sheet tells you to thread a metal nut onto the shutoff valve and tighten it 1/2 turn beyond hand-tight, and if a leak appears, to tighten just enough for a leak-free joint without overtightening. Cranking a compression nut can crush the ferrule and make it worse.
A braided line that’s bulging, frayed or rusty is near the end. Replace it rather than tightening. The braided faucet supply lines guide covers sizes, and how to install a faucet shows the connections.
When the water comes from above the cabinet
Wet cabinet, dry pipes? Look up.
- 1Faucet base
Water pooling on the deck can run down the faucet shank. Check the base seal and that the faucet doesn’t rock.
- 2Pull-down or side spray hose
A worn hose can split where it rubs on the cabinet or sink edge. Run the sprayer and feel along the hose.
- 3Sink rim
Failed caulk where the sink meets the counter lets splash water run under the rim.
- 4Dishwasher connections
Check the drain hose clamp and the hot-water feed to the dishwasher.
- 5Soap pump or filter faucet
Extra deck fixtures each have their own seal and line.
A faucet that rocks has a loose mounting nut, and the movement can break the base seal. How to tighten a loose faucet base covers that fix. A pull-down hose leak usually means a new hose from the faucet maker; the hose isn’t a generic part.
After it’s fixed: dry out and watch
- 1Dry the cabinet floor
Leave the doors open with a fan on it. Particleboard swells if it stays wet.
- 2Check for soft spots
Press the cabinet floor. Soft or swollen board means water sat there a while.
- 3Recheck tomorrow
Run the test again the next day; slow weeps can take hours to show.
- 4Leave an alarm
A spot alarm under the trap warns you next time, before the floor soaks.
The Resideo RWD21 is a reusable spot alarm. Resideo’s listing says it detects water only 1/16 in. deep and sounds an audible alert. For whole-house protection with phone alerts and an automatic shutoff, see smart water leak detectors with shutoff.
When to call a plumber: a leak inside the wall behind the sink, a stop that won’t shut off, corroded pipe that crumbles, or mold in the cabinet. Those go past a washer swap.
Questions people ask
Why is my kitchen sink leaking underneath?
Most often a loose slip nut or worn washer on the trap, or a failed basket strainer seal. A drip with the faucet off points to the supply side instead.
Why does my sink only leak when water drains?
Drain pipes only hold water while the sink empties. Check the strainer, the slip nuts and the trap with a full bowl draining.
How tight should a P-trap nut be?
Plastic slip nuts go hand-tight, plus a little more. Overtightening cracks them; a bad washer won’t seal no matter how tight.
Which way does a slip joint washer go?
The beveled, tapered side faces into the joint, toward the pipe the nut is sealing onto.
Can I use plumber’s putty on a P-trap?
No. Slip joints seal with their washers. Putty is for the basket strainer and faucet bases.
Is a slow drip under the sink an emergency?
Not usually, but supply-side drips run all the time. Shut that stop and fix it soon; cabinet floors swell fast.
Why is water under my sink but the pipes are dry?
It’s coming from above: the faucet base, a sprayer hose, the sink rim caulk or a dishwasher line.
Should I replace a metal trap with plastic?
If it’s corroded, yes. A plastic 1-1/2 in. trap from a repair kit won’t rust.
How I put this together
Why trust me
I’m Isaac Daniels, a licensed journeyman plumber in Georgia (license #JP507711), and I’ve used every item in the kit. The steps are my trade order, with tightening figures from Fluidmaster’s sheet and product facts from each maker’s listing, read on Oct 23, 2026.
How I pick
- One part for each common leak: trap washer, drain parts, strainer seal, and an alarm.
- Real brands with model numbers that match the maker.
- New and in stock on Amazon on the day of the update.
- No scores: each item does a different job, so they’re listed as a kit.
- Prices come from Amazon’s API with the time they were read.
Sources
- Fluidmaster, Click Seal connector installation sheet FM91233 Rev. 2.
- Oatey, 31177 stain-free plumber’s putty product details.
- Resideo, RWD21 reusable water leak alarm product details.
- PlumbCraft (Waxman), 27-piece complete kitchen sink drain repair kit 7027010N; parts list as printed by retailers.
- Amazon product data for B005BIG7TE, B00TSJR3PY, B006E356S8 and B00BIBD4EY, read through the Creators API on Oct 23, 2026.