P-Trap Leaking Under a Commercial Sink: How to Find and Fix the Leak
A leaking sink trap is usually caused by a loose slip joint, crooked washer, cracked nut, damaged tubular piping, or a partial clog that makes joints seep. Dry everything first, then run water and identify the first point that gets wet.
Last reviewed 2026-09-12 · General facility-maintenance information, not a substitute for site procedures, manufacturer instructions, codes, or qualified professional judgment.
Safety boundary
Symptom → cause → first move
Leaks only while draining
Check: Slip joint or trap crack
First move: Dry and watch joints during flow.
Drips constantly
Check: Supply leak may be above trap
First move: Trace upward first.
Leaks after basin dump
Check: Loose joint or restriction
First move: Check alignment and blockage.
Tight nut still leaks
Check: Washer damaged/backwards
First move: Reassemble correctly.
Cracked/stained trap
Check: Aged piping
First move: Replace damaged section.
What to check first
Bring these
- Bucket
- Rags
- Channel-lock pliers
- Slip-joint washers/nuts
- Small brush
Before touching anything
- 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.
Step-by-step procedure
- 1Clear the cabinet and place a bucketProtect contents and give yourself space.
- 2Dry every pipe and jointDiagnosis is much easier on a completely dry assembly.
- 3Run water and watch top-downFind the first joint that becomes wet.
- 4Snug slip nuts by hand firstCompression washers seal better when straight, not overtightened.
- 5Re-seat the leaking jointLoosen, align, inspect washer, and reassemble.
- 6Clean the trap if slowRemove over a bucket, clean debris, inspect cracks.
- 7Test normal flow and full-basin dumpA joint may leak only under high discharge.
Stop and call a licensed professional when
- 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
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
FAQ
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.
Parts for this repair
Gasket and O-ring assortment
Common seals for compatible faucets, valves, and mechanical repairs.
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