Diagnose a Slow Drain

Goal

Identify the cause of a customer-reported slow drain quickly and correctly the first time. Slow drains range from a hair clog you can solve with a 10-minute hand snake to a collapsed branch line that needs camera inspection and a remediation quote. A clean diagnosis prevents callbacks and protects the price quote.

Prerequisites

Before opening any drain:

• Plunger (sink + toilet, both shapes)
• Hand auger (25 ft drum, ¼" cable) for branch lines
• Drop cloth, towels, gloves, eye protection
• Inspection light or headlamp
• Snake camera (deferred until step 4 - most drains never need it)

Confirm the customer's symptom in person - "slow drain" sometimes means "gurgles when the washer runs" which is a vent problem, not a drain problem.

Steps

1. Run the affected fixture for 30 seconds. Time how long water takes to clear. If water rises before fully draining, the line is blocked, not slow.
2. Check the trap. Place a pan, remove the P-trap, inspect for soft clogs (hair, grease, food). Clearing a trap clog resolves ~60 % of single-fixture slow drains.
3. Run an adjacent fixture. If both fixtures slow, the blockage is downstream - branch line or main. If only one slows, the blockage is in that fixture's branch.
4. Hand-snake the branch line through the cleanout or the disconnected trap arm. Feel for resistance changes - soft (clog), hard (root or scale), or none (line is clear and the issue is upstream venting).
5. If the snake clears, run hot water at full flow for 90 seconds. Watch for backup at the customer's other drains. If anything backs up elsewhere, escalate to a main-line inspection.
6. If the snake doesn't clear or hits a hard stop, document the depth and run the snake camera to identify root intrusion, broken pipe, or collapsed line. This shifts the job from a service call to a quoted repair.

Verification

• Water clears the affected fixture within 5 seconds at full flow
• No backup at adjacent fixtures during a 90-second flush
• No gurgle from other traps (gurgle = a vent issue, separate ticket)

Troubleshooting

• Drain clears but reopens within a week: likely grease deposit in the branch line. Recommend an enzymatic monthly maintenance treatment or a hydro-jet service.
• Multiple fixtures slow simultaneously: not a fixture problem. Pull the main cleanout cap (carefully - there may be backed-up water behind it) and proceed to main-line snake or camera.
• Gurgling from other traps without water flow: vent stack obstruction. Inspect the roof vent for nests or ice; do not "fix" the drain.

References

• IAPMO Uniform Plumbing Code, Chapter 7 (Sanitary Drainage)
• Drain & Sewer Cleaners Association (NASSCO) best practices