Horizontal Drainage Pipe Slope Standards

What is the proper slope of drain pipes? That depends on the pipe size. It is important that liquids flow smoothly, which helps transport solids away without clogging. A pipe that is too flat will prevent waste from flowing away.

The minimum velocity in a horizontal drain pipe is 2 feet per second (0.61 m/s).  This is called the “scouring velocity.”  The velocity is intended to keep solids in suspension. Pipes ≤ 2 ½ in. – ¼ in. per foot (2%) Pipes ≥ 3 in. – ⅛ in. per foot (1%)

improper slope of sewer line

improperly sloped drain

Improper support can create air pockets and clogged drains. This may lead to stress on the pipe and fittings causing leaks on both supply and DWV systems.  Poor support can also cause the system to be noisy.


JJ & Suzanne are both licensed, highly skilled inspectors and educators. We are the authors of our class curriculum, and truly enjoy sharing this with our students

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