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This infographic illustrates one of the most critical physics-based rules in plumbing: Scouring Velocity. Many people assume that a steeper pipe is always better because water moves faster, but in the world of Drain-Waste-Vent (DWV) systems, that is a dangerous misconception. The Science of the Slope A horizontal drainage pipe is designed to carry a combination of liquids (water) and solids (waste and debris). For the system to work, these two must move together. 1. 0% Slope: The Standing Pool When a pipe has no slope, gravity cannot pull the waste forward. * The Result: Solids settle to the bottom and "agglomerate" (stick together). Over time, this creates a hard dam that leads to a total backup. * The Visual: Notice the rocks sitting still at the bottom of the first pipe. 2. The "Sweet Spot": 0.8% to 1.5% Plumbing codes (like the IPC or UPC) generally mandate a slope of 1/4 inch per foot for pipes 2" or smaller, and 1/8 inch per foot for larger pipes. * Scouring Action: At these specific angles, the water moves at a velocity of approximately 2 feet per second. * The Physics: This speed creates enough "buoyancy" and friction to keep solids suspended in the middle of the water stream. The water essentially "scours" the pipe clean as it flows. * Pipe Diameter Matters: As seen in the chart, larger pipes (8") require less slope (0.8%) because the sheer volume of water generates enough force to move solids even at a shallower angle. 3. More than 2%: The "Separation" Failure This is the most common mistake made by DIYers or inexperienced installers. They think, "If a little slope is good, a lot of slope is better!" * The Failure: If the pipe is too steep, the water gains too much speed. Because water is a liquid, it races ahead, leaving the heavier solids behind on the dry pipe wall. * The Result: The solids dry out, stick to the pipe, and eventually create a "soft clog" that is incredibly difficult to clear with a standard snake. Summary for Your Class * Too Flat: Solids don't move. * Too Steep: Water outruns the solids. * Just Right: Water carries the solids to the sewer. Follow me for a daily plumbing teaching. If you want to master the math behind these

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Comments (8)

@robsather Z😮😮😮😮

@zerobonden Why don’t they build pipes with some internals that move with the force of liquids, pushing any solids/other materials? 🤔 Maybe even use it to generate electricity ⚡️

@nadex_nick the 2”-3” pipe looks perfect. the others are too large…

@damirbek_axi Lmmmm..m bmm

@damirbek_axi Okmmm.

@j_wil_g Just say 1/4” per foot

@flwkmp 1 meter is not 16cm… Trust me I am an engineer…

@syepye It’s called McGuire’s Law.

Posted06 Apr 2026, 10:05

Pin added19 May 2026, 17:10

This infographic illustrates one of the… | PinThis