What is the purpose of a secondary (overflow) roof drainage system on a roof with a parapet wall?
Not quite. The answer is D: To provide a backup drainage path if the primary drains are blocked, preventing dangerous water accumulation and structural overload
A parapet wall turns a flat roof into a basin — without a second escape path, a blocked primary drain lets rainfall pond faster than the roof structure was designed to carry, which is exactly the failure a secondary drain or scupper, set slightly higher, exists to prevent.
Checked against the 2021 UPC and Washington's 2021-edition amendments. The 2015 edition's secondary-drainage text was not read; confirm against your 2015 book.
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