Diagnose slow drains, blockages, and root intrusion with cameras and snakes. Know when the problem is grease, roots, hard water, or a blocked vent.
Key Takeaways
Why This Matters on the Exam
Drain problems are the bread and butter of service plumbing. The C-36 exam loves drain questions because they test whether you can diagnose and repair the most common calls you'll ever get. Understanding root causes—not just symptoms—is what separates professionals from DIY YouTube fixers. This knowledge protects your customer's investment and your reputation.
What You Must Know
Most drain complaints fall into five categories. Learn to recognize these and you'll diagnose fast:
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Slow drains in one fixture are typically grease, hair, or mineral deposits; diagnose cause before mechanical repair.
Drain line slope minimum: 1/4 inch per foot for pipes ≤2½ inches, or 1/8 inch per foot for pipes ≥3 inches.
Root intrusion enters through cracks in clay, cast iron, or concrete pipe; mechanical cutting is temporary; permanent fix requires chemical treatment or replacement.
Slow drains after heavy rain affecting multiple fixtures indicates root intrusion; camera inspection confirms location and extent.
Gurgling drains signal vent blockage trapping air; inspect roof vent opening for leaves, debris, ice, or improper pitch.
P-trap drying from unused fixtures allows sewer gas escape; refill trap with water or install trap seal valve.
Camera inspection shows cracks, root intrusion, blockage type, and belly sags but cannot assess pressure or see through standing water.
Complete blockage affecting all house drains indicates main line or vent obstruction; camera inspection required before mechanical clearing or replacement.
Sewer gas odor stems from dry p-trap, clogged vent, cracked trap, or unsealed cleanout; identify source before repair.
Intermittent blockage with wet-soil correlation suggests root intrusion; repeated mechanical clearing without addressing roots wastes customer money.
Slow drains – water takes 10+ seconds to empty
Complete blockages – no water flows through
Root intrusion – tree roots penetrating sewer line
Vent-related gurgling – air trapped by blocked vent
Sewer gas odor – p-trap, vent, or cleanout issue
Each has a different cause and solution. Diagnosis prevents wasting time on wrong repairs.