Quick answer
An under-sink leak has three suspect sources: supply lines (hot and cold shutoffs to faucet), drain P-trap (the U-shaped pipe), or the basket strainer (metal drain fitting in the sink). Find the source by drying everything, running water, and watching where drips first appear. Most repairs are $5–$25 in parts and 30 minutes of work. Ignoring the leak destroys cabinet bottoms and can cause mold.
Key facts
| Common causes | P-trap loose slip nuts; Basket strainer leak; Supply line failure; Faucet base leak |
|---|---|
| DIY difficulty | Easy |
| Time to fix | 45 min |
| Parts cost | $5–$25 parts |
| Tools needed | Flashlight, adjustable pliers, bucket, plumber's putty |
| When to call a pro | If leak traces to inside the wall |
What's happening
Every kitchen sink has three separate water systems under it. Supply lines bring pressurized hot and cold water to the faucet through braided or copper tubes with compression fittings. Drain lines carry wastewater from the sink through a P-trap. The basket strainer is the metal fitting where the drain connects to the sink itself, sealed with plumber's putty. Each has typical failure modes: supply lines fail at fittings; P-traps loosen at slip nuts; basket strainers loose their putty seal over years.
Common causes (ranked by frequency)
- P-trap loose slip nuts 35% of cases
The plastic slip nuts holding the P-trap together loosen from vibration and wear. Hand-tighten first. If dripping continues, replace washers ($1) or the whole trap ($10).
- Basket strainer leak 25%
The plumber's putty seal between the sink and drain fitting deteriorates over 10–20 years. Fix requires removing the drain fitting and reinstalling with fresh putty.
- Supply line failure 20%
Braided supply lines fail after 10–15 years — corrosion or braided sheath damage. Copper supply lines fail at compression fittings. $10–$15 replacement.
- Faucet base leak 10%
Water seeps down through the faucet's mounting hole in the sink. Cartridge O-ring failure — replace faucet cartridge (see kitchen faucet dripping).
- Garbage disposal leak 10%
Disposal housing leaks from the top mount, side, or bottom. Top mount = reseal to sink. Bottom = disposal is failing, replace.
How to diagnose
- Empty the cabinet and dry every surface with towels.
- Line the cabinet floor with paper towels or dry cardboard. This shows where drips land.
- Run cold water for 30 seconds, hot for 30 seconds, then fill the sink and drain.
- Check paper towels for wet spots. Wet spot directly under P-trap = trap leak. Wet under faucet base = supply line or basket strainer. Wet at disposal seam = disposal issue.
- With a flashlight, watch the suspected component while running water. You'll see the drip source directly.
The fixes
Fix #1 — Tighten P-trap slip nuts Try first
Free. Sixty seconds. Fixes ~35% of under-sink leaks.
Tools
- Channel-lock pliers (optional — hand-tight often works)
Materials
- Place a small bucket under the P-trap.
- Hand-tighten both slip nuts (top of trap where it meets tailpiece, bottom of trap where it meets drain line).
- Only use pliers if the nuts are stuck. Wrap in rag to protect finish.
- Do not overtighten — cracks plastic slip nuts.
- Run water for 60 seconds and check for drips.
Fix #2 — Replace supply line
Braided stainless supply lines fail from age. $10 replacement, 10-minute swap.
Tools
- Adjustable wrench
- Bucket
- Towel
Materials
- New braided stainless supply line — match length and connector type
- Turn off the shutoff valve for that supply line (hot or cold, whichever is leaking).
- Place bucket and towel below.
- Disconnect old supply line from shutoff (bottom) and faucet (top). Small amount of water will drip.
- Install new supply line hand-tight, then quarter-turn with wrench. Do not overtighten — crushes seals.
- Turn shutoff back on slowly. Check for drips at both ends.
Fix #3 — Replace basket strainer plumber's putty
For a leak at the drain fitting inside the sink. Requires removing the P-trap and disposal (if present), then unscrewing the basket strainer from below.
Tools
- Adjustable pliers
- Screwdriver
- Putty knife
Materials
- Plumber's putty
- New basket strainer if old one is corroded ($10)
- Disconnect P-trap and disposal from the strainer tailpiece.
- Unscrew the large slip nut holding the strainer to the sink (from below).
- Push strainer up and out from above.
- Scrape old putty off the sink hole.
- Roll fresh plumber's putty into a rope, place around the strainer's flange, and press into the hole from above.
- Reinstall the slip nut and gasket from below, hand-tight plus quarter-turn.
- Wipe excess putty from inside the sink.
- Reconnect P-trap and disposal.
Prevention
Check under the sink monthly — 30-second visual inspection catches small drips before they damage the cabinet. Replace braided supply lines every 10–15 years as preventive maintenance. Line the cabinet floor with a plastic tray to catch small leaks and protect the wood. Consider a water leak detector alarm ($15) — sounds when it detects moisture, catches slow leaks early.
When to call a pro
When to stop and call a pro
- Leak source appears to be inside the wall — supply lines coming out of the wall are leaking at the wall.
- Repeated fixes at the same location — indicates underlying issue (bad connection, corroded threads).
- Cabinet has visible water damage — mold assessment may be needed.
- Slow leak has caused floor damage below the cabinet — flooring repair beyond DIY.
Plumber service call for under-sink leak: $150–$350. Faucet replacement labor: $150–$300. Cabinet repair from water damage: $300–$1,500. Water damage restoration: $1,000+.
Cost estimate
Cost estimate
Frequently asked questions
Supply-line leaks happen constantly (pressurized water is always trying to escape). Drain leaks only happen when water is running through the drain. Run only cold water to test cold supply; only hot for hot supply; fill and drain the sink for drain testing.
Depends on damage. Small area of surface water damage — dry thoroughly, apply water-resistant coating. Warped, delaminated, or moldy — replace the cabinet floor (a lightweight piece of 1/2 inch plywood works). Mold present = professional remediation for anything larger than 10 sq ft.
For basket strainers specifically, plumber's putty is standard because it's removable for future repair. Silicone works but permanently bonds — a nightmare if you ever replace the sink or strainer. Follow the drain fitting manufacturer's recommendations if in doubt.
A small battery-powered alarm ($15) that sounds when moisture touches its sensor. Place one under the sink, one behind the toilet, one under the water heater. Catches slow leaks in weeks rather than months — pays for itself many times over.