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.
Match the symptom before you touch anything
Good maintenance starts by describing what the system is actually doing. Use the pattern below to decide where to look first.
| 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.
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
BuildingWrench provides general educational information for commercial-facility maintenance. Manufacturer instructions, applicable codes, site procedures, and required trade qualifications always take priority.