What rainfall rate does Mesa use to size roof drains and storm drains? IPC 1106.1

Mesa's amendments to the 2024 International Plumbing Code are in City Code 4-5-2. Section 1106.1 now reads that the size of the vertical conductors and leaders, building storm drains, building storm sewers "and any horizontal branches of such drains or sewers shall be based on an hourly rainfall rate of 3 inches per hour."

That gives the sizing input. The city's plumbing plan checklist adds the arithmetic and the details a reviewer looks for under Roof Drainage:

• Sizing calculations. Provide roof drain or scupper sizing calculations. The equation is area of roof times 0.0104 times inches of rain equals gallons per minute (the checklist cites IPC Table 1106.2.1).
• Vertical walls. Vertical walls must comply with IPC 1106.4.
• Scuppers. Openings must be at least 4 inches high, and at least as wide as the circumference of a roof drain sized for the same roof area (IPC 1106.5).
• Secondary drains. Provide a separate endpoint discharge for the primary and secondary roof drain systems (IPC 1108.2).
• Public walkways. Drainage shall not flow over public walking surfaces (IBC 3201.4).

One flag for readers who open the PDF: the worked sample on its last page is set out as a graphic and can use a different rainfall figure than the code text. The 3 inches per hour in 1106.1 is the code requirement, so use the adopted 3-inch rate rather than copying a conflicting sample assumption.

• Conductor or leader. The vertical pipe that carries roof water down.
• Building storm drain. The horizontal pipe that carries storm water inside or under the building to the storm sewer.
• Scupper. An opening through a parapet wall that lets roof water drain out.
• Secondary roof drain. The backup drain or overflow path that works if the primary drain is blocked.

Every pipe size in the storm drainage chapter starts from the design rainfall. Phoenix explained its own identical amendment by noting that the base code lists lower figures for Phoenix, and it chose 3 inches per hour to stay consistent with earlier amendments and the sizing tables. Mesa's text does not state a reason, but the practical effect is the same: a single, higher design number that every plan set is checked against.

Designers and contractors on commercial, multifamily and other permitted buildings with roof drainage. A single-family house is under the residential code, which this page does not cover.

The equation is the city's. The roof areas below are examples, not project data.

The flow scales in a straight line with the rainfall figure, so a design done at a lower rate than 3 inches per hour produces a proportionally smaller number than the one the city's equation gives.

A new retail building. The engineer sizes the roof drains on 3 inches per hour, puts the calculation on the plans and shows a separate discharge point for the overflow drains. That covers the items the checklist asks for.

A tenant improvement. The tenant only alters interior plumbing, so the roof is untouched. This section becomes relevant if the project adds roof equipment that changes the drainage.

The edge case. A parapet building with scuppers only. Scuppers still need capacity calculations for the design flow; the 4-inch opening height is a minimum detail, not a replacement for sizing. Show the roof area, rate and approved overflow arrangement on the plans.

1. 1Get the roof area for each drainage zone from the architectural plans.
2. 2Run the checklist equation at 3 inches per hour and record the result on the plumbing sheets.
3. 3Size leaders, storm drains and horizontal branches from that flow using the IPC tables.
4. 4Show the secondary drain or scupper overflow with its own discharge point.
5. 5Confirm where the storm drain discharges, including any retention basin or street connection, and whether that needs a separate permit. The valley retention basin guide compares how the cities treat on-site storm water storage.

• "The base IPC rainfall figure applies." Mesa amended 1106.1 to 3 inches per hour.
• "Primary and secondary drains can share one outlet." The checklist asks for separate endpoint discharge.
• "Scuppers are exempt from sizing." The checklist gives a minimum opening height and width.
• "Storm water can run across the sidewalk." The checklist says drainage shall not flow over public walking surfaces.