The slip-joint nuts on drain trap connections are the threaded plastic or chrome fittings that hold the P-trap sections together. They’re the most common source of under-sink drips — and usually the simplest to fix. Here’s what causes them to leak and how to stop it.

What Are Drain Trap Nuts?

Slip-joint nuts are the threaded couplings that connect P-trap sections.

A standard under-sink P-trap assembly has two or three slip-joint connections:
– Where the drain basket tailpiece connects to the top of the P-trap
– Where the P-trap connects to the horizontal trap arm going into the wall
– At the wall drain stub-out (if using an extension)

Each connection uses a slip-joint nut — a threaded fitting that slides over the pipe and screws onto the adjacent fitting. Inside each nut is a rubber or plastic washer that creates the watertight seal when compressed.

Why Drain Trap Nuts Leak

Three main causes:

1. Loose nut: The nut has backed off from vibration, thermal cycling, or slight movement when someone bumps or pulls on the drain pipe. This is the most common cause.

2. Worn washer: The rubber or plastic washer inside the nut has hardened, flattened, or deformed over years of use. A worn washer can’t create a full seal even when the nut is tightened.

3. Cracked nut: Plastic slip-joint nuts crack when overtightened, subjected to physical stress, or simply aged. A cracked nut leaks regardless of how tight it is.

Fix 1: Tighten the Nut

Most leaks stop with simple tightening.

  1. Locate the leaking connection (dry everything and run water to identify which nut drips)
  2. Hand-tighten the nut clockwise as far as possible
  3. Use channel-lock pliers or slip-joint pliers for an additional 1/4 turn
  4. Run water and check

Important: Don’t overtighten. Plastic slip-joint nuts crack if forced past hand-tight plus 1/4 turn. The washer should seal the connection — more torque doesn’t help if the washer is worn.

Fix 2: Replace the Washer

When tightening doesn’t stop the drip, replace the washer inside the nut.

  1. Put a bucket under the trap
  2. Unscrew the leaking nut by hand (counterclockwise)
  3. Slide the nut back on the pipe to expose the old washer
  4. Remove the old washer — it may fall out on its own or need to be pried out
  5. Take the washer to the hardware store to match the size (they’re cheap — buy a few)
  6. Place new washer in the nut, reassemble, hand-tighten plus 1/4 turn
  7. Test

Washer cost: $0.50–$2.00 each. Available in a variety pack for under $5.

Fix 3: Replace the P-Trap Assembly

When the nut is cracked, or the trap itself is damaged, replace the whole assembly.

A complete PVC P-trap kit for 1-1/4″ or 1-1/2″ drain (the two standard sizes) costs $5–$15 and includes the curved trap section, trap arm, and new slip-joint nuts with washers. Installing a fresh assembly is often faster than sourcing individual replacement parts.

Sizing: Bathroom sinks typically use 1-1/4″ trap; kitchen sinks use 1-1/2″. Verify before purchasing.

Steps:

  1. Put bucket under trap
  2. Unscrew both nuts, remove the old trap (water will drain into bucket)
  3. Slide new nuts onto pipe ends, insert new washers
  4. Connect the new trap sections, hand-tighten all nuts plus 1/4 turn
  5. Run water and verify no drips

Avoiding the Most Common Mistakes

Overtightening: The most frequent DIY error. Plastic slip-joint nuts don’t need much torque. Forcing them past 1/4 turn past hand-tight cracks the nut and creates a new leak.

Skipping the washer check: If tightening didn’t work the last time you tried, the washer needs replacement — more tightening won’t help.

Using Teflon tape on slip-joint threads: Teflon tape is for tapered NPT threads, not slip-joint connections. The washer creates the seal on slip joints — tape doesn’t help and makes disassembly harder.

FAQ

Q: Why is my P-trap nut leaking?
A: Most likely it’s loose (vibration or minor movement backed it off) or the washer inside is worn. Tighten by hand plus 1/4 turn with pliers. If that doesn’t stop it, replace the washer inside the nut.

Q: How do I tighten drain trap connections?
A: Hand-tighten the slip-joint nut clockwise as far as possible, then use pliers for an additional 1/4 turn. Don’t force it further — overtightening cracks plastic nuts.

Q: Can I use Teflon tape on drain trap connections?
A: No. Teflon tape is for tapered threaded connections (NPT), not slip-joint drain fittings. The slip-joint nut seals via the rubber washer inside. If the washer is worn, replace it — tape won’t fix the problem.

Q: How much does it cost to fix leaking drain trap nuts?
A: A replacement washer costs $1–$2. A complete P-trap assembly (if the nuts or trap are cracked) costs $5–$15. If a plumber handles it, expect $75–$100 for a simple trap connection repair.

Q: Do plastic drain trap nuts need to be replaced periodically?
A: Not on a schedule — replace when leaking or visibly cracked. PVC slip-joint nuts can last 10–20 years. Chrome nuts corrode faster. Washers inside the nuts typically wear out before the nuts themselves.