When secondary drainage is required
IBC 2021 Section 1502.2 reads: "Where roof drains are required, secondary (emergency overflow) roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof in such a manner that water will be entrapped if the primary drains allow buildup for any reason."
Two conditions:
- Roof drains are required, and
- the perimeter construction extends above the roof (parapets, for example) so water would be trapped if the primary drains back up.
When both are true, secondary drains or scuppers are required. They are sized per Section 1611 and the IPC (Section 1502.1 sends drainage design to Section 1611 and IPC Chapter 11).
"For any reason" is the point
The trigger is not that the primary drains are undersized. The rule covers buildup for any reason, including a correctly sized drain that is clogged. That is why sizing the primary drains to the IPC does not remove the requirement.
The trap: treating secondary drainage as optional whenever primary drains are provided.
Scenario: parapets on all sides
A flat roof has parapets on all sides and interior drains sized to the IPC. The owner's representative wants to omit secondary drainage. Parapets are perimeter construction above the roof that would trap water, so Section 1502.2 requires secondary drains or scuppers, sized per Section 1611 and the IPC.
Things that are not the trigger:
- Roof slope. Figures like ¼:12 and ⅛:12 are roof covering rules from Section 1507; ⅛:12 is the coal-tar built-up minimum.
- Coal-tar roofing. Also a Section 1507 covering rule.
- Underlayment. It is part of the roof covering and does nothing for ponding water.
Scupper size: 4 inches
Section 1502.3 requires scuppers to have an opening of at least 4 inches. 2 in and 6 in are not the code minimum.
The trap: 12 in is a different rule entirely. It is how far foundation waterproofing must extend above the maximum ground-water table under Section 1805.3, not a scupper dimension.