According to Oregon plumbing codes, what is the minimum slope requirement for horizontal drainage piping?
Correct Answer
A) 1/4 inch per foot
Oregon plumbing codes require a minimum slope of 1/4 inch per foot (2% grade) for horizontal drainage piping.
Why This Is the Correct Answer
Option A is correct. Oregon plumbing codes require horizontal drainage piping to slope at a minimum of 1/4 inch per foot (equivalent to a 2% grade). This slope ensures adequate flow velocity to carry waste and prevent solids from settling in the pipe.
Why the Other Options Are Wrong
Option B: 1/8 inch per foot
1/8 inch per foot is below the code minimum and provides insufficient flow velocity. At this slope, solids can settle out and cause blockages. Oregon code does not permit this as a minimum for drainage piping.
Option C: 1/2 inch per foot
1/2 inch per foot exceeds the minimum and would actually be too steep for larger diameter pipes—it can cause the liquid to race ahead of solids, leaving solids behind. The 1/4 inch minimum is the standard that balances flow velocity with solid transport.
Option D: 1 inch per foot
1 inch per foot is far too steep for standard horizontal drain lines. Such a steep grade would cause velocity separation issues and is not a recognized standard slope for drainage piping. This figure is a distractor.
Memory Technique
Quarter-inch rule: 'A quarter inch a foot keeps the drain clean.' Visualize a ruler lying flat—every foot, the pipe drops just one quarter-inch. That's enough fall to keep water and waste moving without causing separation issues.
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