UPC 708.1 starts at 1/4 inch per foot for building drains and other horizontal drainage piping. Building drains 4 inches and larger may use 1/8 inch per foot where the baseline is impractical because of sewer depth, structural features or building arrangement, with prior approval from the authority having jurisdiction.
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. UPC Section 708 sets a uniform minimum fall for building drains and other horizontal drainage piping. It does not set a universal half-inch-per-foot maximum slope.
What this section says
The 2024 UPC starts with a uniform slope of at least 1/4 inch per foot, about 2%, for building drains and other horizontal drainage piping. Section 708.1 permits a reduced slope only under specific conditions.
Where sewer depth, structural features or the building arrangement make the baseline impractical, building drain piping 4 inches and larger may use at least 1/8 inch per foot with prior approval from the authority having jurisdiction. A 3-inch building drain does not qualify for that UPC exception. The IPC has a different pipe-size table, so do not apply its 3-inch threshold to a UPC design. "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. This explanation follows the 2024 UPC Section 708.1 published by IAPMO. Phoenix adopted the 2024 UPC with local amendments effective August 1, 2025; its amendments address building-sewer slopes separately in Section 718.1. Read the official code in IAPMO's viewer and the Phoenix 2024 UPC amendments.