A 4-inch building drain is laid out with a required 1/8 inch fall per foot. What fall is required across 40 feet?
Correct Answer
B) 5 inches.
At 1/8 inch per foot, a 40-foot run needs 5 inches of fall.
Why This Is the Correct Answer
At 1/8 inch per foot, a 40-foot run needs 5 inches of fall.
Why the Other Options Are Wrong
Option A: 2 1/2 inches.
2 1/2 inches. is below the supported result for plumbing drainage slope; it likely omits allowance, conversion, coverage, or final round-up.
Option C: 10 inches.
10 inches. is above the supported result for plumbing drainage slope; it likely double-counts allowance, uses the wrong unit, or rounds too early.
Option D: 20 inches.
20 inches. is above the supported result for plumbing drainage slope; it likely double-counts allowance, uses the wrong unit, or rounds too early.
Memory Technique
Measure to inch, adjust for foot, then let Reviewing Under-Slab set the finish line.
Reference Hint
Study anchor (closed-book): memorize Plumbing drainage slope from 2025 California Plumbing Code, California Code of Regulations Title 24 / Title 24 California Plumbing Code horizontal drainage piping slope; know the rule or calculation trigger without relying on exam-room lookup.
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