Why is secondary drainage required by code?

• A. It reduces the weight of the roof.
• B. It protects the structure if primary drainage fails.
• C. It increases roof slope efficiency.
• D. It improves thermal performance.

Answer: (B)

Water on a roof is normally removed by the primary drainage system, but codes require a secondary drainage path to prevent ponding if the primary system becomes blocked or fails. This backup outlet provides an overflow route so rainwater can still escape, reducing hydrostatic loads on the roof, protecting the membrane and deck, and helping prevent leaks or structural damage when the main drains are clogged or otherwise unavailable. The other options don’t fit because drainage’s purpose isn’t to reduce roof weight, boost slope efficiency, or improve thermal performance; it’s to provide redundancy and safeguard the roof system against failure of the primary drainage.