P-Trap Leaking from the Top? Find the Joint
A P-trap leaking from the top often has a misaligned slip joint, a damaged washer or a loose nut at the sink tailpiece. Water can also run down from the sink drain or supply above, making a sound trap look like the source.
Start dry and work downward. Place a bucket, dry accessible joints and observe a small controlled flow only if it is safe to use the fixture. Find the first point that becomes wet, not the lowest drip.
For a serviceable tubular slip joint, alignment and the correct washer create the seal. Follow its instructions; extra tightening or thread tape on a washer-sealed joint does not correct a crooked or damaged washer.
Match the symptom before you touch anything
| What you see | What it points toward | First move |
|---|---|---|
| Leaks only while draining | Slip joint or trap crack | Dry and watch joints during flow. |
| Drips constantly | Supply leak may be above trap | Trace upward first. |
| Leaks after basin dump | Loose joint or restriction | Check alignment and blockage. |
| Tight nut still leaks | Washer damaged/backwards | Reassemble correctly. |
| Cracked/stained trap | Aged piping | Replace damaged section. |
Tools and preparation
What to bring
- Bucket
- Rags
- Channel-lock pliers
- Slip-joint washers/nuts
- Small brush
Before you start
- Confirm the exact location and complaint.
- Photograph labels, settings, and error codes before changing anything.
- Know how to isolate water, power, or the affected area if needed.
- Follow site-specific PPE, lockout/tagout, ladder, roof, and access procedures.
Which “top” joint is leaking?
Identify the connection before undoing it: the sink drain-to-tailpiece connection, the trap inlet slip nut or a supply connection higher in the cabinet. A drip that continues when the sink is unused may be from the supply side or retained water; trace it rather than assuming.
Before trap removal, stop fixture use, make sure no hazardous drain chemicals are present and collect retained water. Inspect the washer orientation specified for that fitting, straight engagement and cracked/corroded parts. Do not force aged metal piping or a loose wall connection.
If drainage is slow, use sink restriction checks. If the issue is odor without a visible leak, compare water-seal and drain-odor diagnosis.
Step-by-step
Clear the cabinet and place a bucket
Protect contents and give yourself space.
Dry every pipe and joint
Diagnosis is much easier on a completely dry assembly.
Run water and watch top-down
Find the first joint that becomes wet.
Snug slip nuts by hand first
Compression washers seal better when straight, not overtightened.
Re-seat the leaking joint
Loosen, align, inspect washer, and reassemble.
Clean the trap if slow
Remove over a bucket, clean debris, inspect cracks.
Test normal flow and full-basin dump
A joint may leak only under high discharge.
Common mistakes
- Tightening every nut without finding source
- Overtightening plastic nuts
- Installing washers backwards
- Ignoring a partial clog
- Assuming the trap is the source because it is wet
When to stop and hand it off
- Wall connection is loose inside wall
- Corroded metal piping breaks
- Supply hose/faucet is actual source
- Sewage backs up from wall
- Active leak cannot be isolated
Frequently asked questions
Do slip joints need thread tape?
Typical tubular slip joints seal with compression washers, not thread tape.
How tight should plastic nuts be?
Usually hand-tight plus only a small additional turn if needed.
Why only during fast drainage?
Higher flow exposes loose joints and restrictions.
Replace stained plastic?
Staining alone is not failure; cracks, brittleness, or damaged threads are.
Related BuildingWrench guides
What to document
Record which joint first became wet, whether leakage occurred at rest or during drainage, pipe material, washer/fitting replaced and any restriction. Verify modest flow first, then a controlled basin discharge only when the drain is clear and overflow is prevented.
BuildingWrench provides general educational information for commercial-facility maintenance. Manufacturer instructions, applicable codes, site procedures, and required trade qualifications always take priority.
Tools that fit this task
Use what you already own first. These affiliate links may earn a commission at no extra cost to you. Confirm model and compatibility before buying.
Contain a small, safe clean-water release; empty it before moving and keep exits clear.
For suitable trap fittings after isolation; do not crush plastic or grip chrome finishes.
Dry accessible surfaces so fresh leakage is easier to identify; not for hazardous spill cleanup.