What rainfall rate does Phoenix use to size roof drains and storm piping?

The model IPC 1106.1 sizes vertical conductors and leaders, building storm drains, building storm sewers and their horizontal branches from the 100-year hourly rainfall rate shown in its Figures 1106.1(1) through 1106.1(5), or from other rates based on approved local weather data. Phoenix replaces that. Its amended section reads that the size of those conductors, leaders, building storm drains, building storm sewers and horizontal branches "shall be based on an hourly rainfall rate of three (3) inches per hour."

The city struck the 100-year figure and the local weather data option. The justification says the 2024 UPC and the 2024 IPC list rainfall rates for Phoenix as 2.2 and 2.5 inches per hour, respectively, and that three inches per hour is recommended "to remain consistent with previous amendments and for ease of using the sizing tables." The cost note says the impact is minimal, "Due to drain and pipe size increase."

The city marks the amendment as approved in an earlier code cycle, and it is part of the 2024 code that Ordinance G-7397 put in force on August 1, 2025.

• Conductor or leader: the vertical pipe that carries roof water down from a roof drain.
• Building storm drain and storm sewer: the horizontal piping that carries storm water from the leaders to the point where it leaves the building.
• Hourly rainfall rate: the design rain in inches per hour that the drain and pipe sizes must be able to carry.

Designers, engineers and plumbing contractors sizing storm drainage on a Phoenix building, and the owners who pay for it. Roof drains on flat commercial roofs are the common case, since a large roof collects a lot of water into a few drains.

The city's own justification says the IPC lists Phoenix at 2.5 inches per hour, so 3 inches per hour means 20 percent more design flow into the same roof. Roof flow scales with area and rain rate. Using the standard conversion of about 0.0104 gallons per minute per square foot for each inch per hour, a 10,000 square foot roof produces about 312 gallons per minute at 3 inches per hour, against about 260 at 2.5. That figure is our arithmetic, not city text, and a design uses the code's sizing tables.

• A new retail building draws its roof drainage on a sizing table. The designer starts from 3 inches per hour, not from the rainfall figures in the model code, and sizes each drain and leader from that.
• A tenant remodel adds a roof drain to an existing flat roof. The added drain and any new leader or storm branch use the 3 inch rate.
• The same firm designs a building in Chandler. The answer flips: Chandler has no local rate, so the model code's 100-year rate applies there, and the design cannot carry a Phoenix assumption across the city line.
• A design already uses 3 inches per hour for a Mesa project. Mesa writes the same 3 inches per hour into its amendment, so the assumption carries over.

1. 1Put 3 inches per hour at the top of the storm drainage calculation for any Phoenix building.
2. 2Size roof drains, conductors, leaders, storm drains and storm sewer branches from it.
3. 3Show the design rate on the plumbing plans so the reviewer sees it. Submit them through the city's current SHAPE PHX permitting process.
4. 4Check how the code sizes secondary (overflow) drains for the same roof, and confirm with Planning and Development that the 3 inch rate carries over to them.
5. 5For a project in another city, look up that city's rate before reusing the calculation.

• "The model code figures apply." Phoenix replaced them with a fixed 3 inches per hour.
• "3 inches is the 100-year storm." The Phoenix text states a rate of three inches per hour with no return period. Tempe's amendment ties its 3 inches to the 100-year rate.
• "It only affects the roof drain." The section covers vertical conductors and leaders, building storm drains, building storm sewers and their horizontal branches.