A building official is reviewing the roof drainage plan for a 20,000 sq ft commercial building with a relatively flat roof in a region with a design rainfall rate of 6 inches per hour. The primary roof drains are sized for this rainfall rate. According to IBC Section 1502.1, what is the minimum number of secondary (emergency) roof drains or overflow scuppers required for this roof area?
- A1
- B3
- C4
- D2
Show answer & explanationAnswer & explanation
Correct answer: D. 2
IBC Section 1502.1 requires secondary (emergency) roof drainage systems for all roofs, except where the roof construction is such that the design provides for water accumulation. The code also requires that 'each roof area' shall have at least two primary drains and two secondary drains, or a combination of primary drains and scuppers, or two scuppers where secondary drainage is provided by scuppers. The question specifically asks for 'secondary roof drains or overflow scuppers'. The intent is that for any given roof area, there must be at least two separate and independent emergency drainage points to prevent ponding and collapse, regardless of the overall roof size. Therefore, a minimum of two secondary drains or scuppers are required for this single roof area.
Why the other options are wrong
- A. One secondary drain is insufficient; the code requires redundancy for emergency drainage.
- B. Three might be used in practice for larger roofs but is not the minimum specified by code for a single roof area.
- C. Four would provide ample drainage but is not the minimum required for a single roof area.
Secondary Roof Drainage
An independent emergency system, such as overflow drains or scuppers, designed to prevent water accumulation on a roof if the primary drainage system fails or is overwhelmed.
- Required by IBC Section 1502.1 for most roofs.
- Must be independent of the primary system.
- Protects against excessive ponding loads and structural collapse.
Memory trick: Drainage systems are like backup plans for rain: primary first, secondary for emergencies.