Section 1101.1/Storm Drainage System Design
IPC Chapter 11 covers roof drain sizing, conductor sizing, rainfall rates, and separation of storm and sanitary drainage.
Storm drainage systems must be designed to handle the maximum rainfall rate for the project location based on a 100-year, 1-hour storm event (or as specified by the local jurisdiction). Roof drains and leaders (vertical conductors) must be sized per Table 1106.2 based on the horizontal roof area served and the rainfall rate. Horizontal storm drain piping must be sized per Table 1106.3 with a minimum slope of 1/8 inch per foot for pipes 6 inches and larger and 1/4 inch per foot for smaller pipes. Storm drainage must be kept separate from the sanitary drainage system unless a combined sewer system is approved by the local authority.
Why this section exists
Inadequate storm drainage causes roof ponding (which can lead to structural failure), water intrusion into the building, and flooding of the site. The design must handle the maximum expected rainfall without overflow. Separation of storm and sanitary drainage prevents stormwater from overwhelming the wastewater treatment plant and prevents sewage from backing up through roof drains during heavy rain events. Secondary (overflow) drainage is required to provide a backup path if the primary drains become blocked.
What plan reviewers look for
Plan reviewers check the roof plan for drain locations and the drainage area served by each drain. They verify drain and leader sizing against Table 1106.2. They check horizontal storm drain sizing against Table 1106.3. They verify the rainfall rate used matches the local design requirements. They check for secondary (overflow) drainage per IBC Section 1502. They verify storm drainage is separated from sanitary drainage.