UPC Section 1101: Storm Drainage and Roof Runoff

The first rule is separation. Storm water and sanitary sewage must travel in separate systems. The code is direct about it:

Rainwater piping shall not be used as soil, waste, or vent pipe, and no soil, waste, or vent pipe shall be used as rainwater piping.

Mixing the two would overwhelm the sanitary sewer during a storm and push sewage backward, so code keeps roof runoff on its own path to an approved point of disposal. The second theme is sizing. Roof drains, gutters, downspouts (called leaders), and horizontal storm drains are sized from two numbers: the area that drains to them, and the local rainfall rate, the peak intensity a design storm can produce. A larger roof or a heavier local downpour needs bigger drains and pipe. The code publishes tables that turn square footage and rainfall into a required drain and leader size, so the system can carry the worst expected rain.

This governs commercial roofs, large paved areas, and any building where roof water is piped rather than just spilling off an eave. Picture a flat-roofed commercial building in Phoenix during a monsoon burst: its roof drains and leaders have to be sized for that short, intense rainfall, or water ponds on the roof and overloads the drains. See how monsoon season affects plumbing for more. The code's area-and-rainfall method is what sets those sizes. It also requires overflow protection so a blocked primary drain cannot trap water on a roof.

If a commercial roof drains slowly or ponds water after a storm, the drainage may be undersized or partly blocked, which is both a code and a structural concern, since standing water is heavy. Storm drainage is also why roof downspouts must not simply be tied into the sanitary sewer. That separated roof runoff is also the water some homeowners capture on purpose, as covered in rainwater harvesting in Phoenix. When a storm overwhelms a system, drains can back up, so see why drains back up during monsoon. For any new roof drainage or a re-roof that changes the drainage, the sizing should be checked against the local rainfall rate by a licensed plumber or engineer. Our commercial plumbing service handles storm drainage on larger buildings.

UPC Section 1101 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, and local rainfall rates drive the sizing, so verify the current tables and section numbers. Read the storm-drainage rules on UpCodes, or confirm local amendments and the Phoenix rainfall rate through the City of Phoenix: phoenix.gov/pdd.