UPC Section 708 sets the minimum fall for a horizontal drain pipe so waste flows by gravity without clogging. Pipes 2.5 inches and smaller need at least 1/4 inch of slope per foot. Pipes 3 inches and larger may run at 1/8 inch per foot where the code official approves it.
The Uniform Plumbing Code is published and copyrighted by IAPMO. This page explains the section in our own words with a short excerpt only. Read the full official text at the source.
A drain works on gravity, and gravity needs a slope. If a horizontal drain pipe runs too flat, waste and water slow down and solids drop out and build up. If it runs too steep, the water can race ahead and leave the solids behind. UPC Section 708 sets the minimum fall a horizontal drain has to have so it clears itself and keeps flowing.
What this section says
The rule ties the required slope to the pipe's size. Smaller pipe needs more fall per foot. The code puts it this way:
Horizontal drainage piping shall be run in uniform alignment at a uniform slope of not less than 1/4 inch per foot (2%).
That 1/4 inch per foot is the baseline for pipe 2.5 inches and smaller, which covers most residential drains like sinks, tubs, and branch lines. For larger pipe, 3 inches and larger, the code allows a gentler minimum of 1/8 inch per foot where the code official approves it, because a wider pipe carries enough water to keep solids moving at a lower grade. "Slope" here just means how much the pipe drops over each foot of horizontal run. A quarter inch per foot means the pipe falls one inch for every four feet it travels.
When this comes into play
This rule governs any time a drain line is installed or moved. Picture a kitchen remodel that relocates the sink 10 feet along a wall. At 1/4 inch per foot, that new drain run has to drop about 2.5 inches from the sink to where it meets the stack. If the framing or the slab does not leave room for that fall, the layout has to change, because an inspector will check the grade before the wall closes. The same math decides whether a long basement or island drain can reach its connection with enough fall.
What this means for you
If a drain in your home is chronically slow even though it is clear, the slope may be wrong. It may be too flat to self-clean, or bellied so a low spot traps water. A belly often traces back to poor pipe support spacing, not just bad grade. Either way it is a pipe-layout problem, not a clog you can plunge away, so routine drain cleaning will not fix it. For the full breakdown of the slope rule and how it is measured, see what drain pipe slope does code require. Because fixing bad slope means re-hanging or re-running pipe, it is far cheaper to get right at rough-in than to chase later.
Full text and source
UPC Section 708 is part of the Uniform Plumbing Code. IAPMO publishes and holds the copyright on it. The excerpt above reflects the rule as adopted; Phoenix enforces the 2024 UPC with local amendments. Read the current section on UpCodes, or confirm local amendments through the City of Phoenix Planning & Development Department: phoenix.gov/pdd.