A 4 in. horizontal building drain is installed below a slab. Which minimum gravity slope is commonly used for 3 in. through 6 in. drainage pipe?
Correct Answer
C) 1/8 in. per ft
For 3 in. through 6 in. horizontal drainage piping, 1/8 in. per ft is a common minimum slope. Other choices are too flat, are the small-pipe slope, or are unnecessarily steep for the stated pipe.
Why This Is the Correct Answer
For 3 in. through 6 in. horizontal drainage piping, 1/8 in. per ft is a common minimum slope. Other choices are too flat, are the small-pipe slope, or are unnecessarily steep for the stated pipe.
Why the Other Options Are Wrong
Option A: 1/16 in. per ft
In mechanical and plumbing, 1/16 in. per ft is the wrong selection for Drainage slope: 1/16 in. per ft is a different drainage slope than the pipe size in the item; the accepted slope is 1/8 in. per ft.
Option B: 1/4 in. per ft
The trap in B is 1/4 in. per ft. That selection is wrong because 1/4 in. per ft is a different drainage slope than the pipe size in the item; the accepted slope is 1/8 in. per ft; A 3 in. through 6 in. horizontal drain is commonly sloped at 1/8 in. per ft.
Option D: 1/2 in. per ft
Choice D (1/2 in. per ft) is rejected because 1/2 in. per ft is a different drainage slope than the pipe size in the item; the accepted slope is 1/8 in.
Memory Technique
File drainage slope under a ledger line: condition first, number or duty second, answer 1/8 in. per ft.
Reference Hint
Open Pipe and Excavation Contracting, 2011; use the index entry or section for "Gravity drainage pipe slope for 3 in. through 6 in. pipe" to confirm that For 3 in.
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