Which statement correctly describes the required relationship of secondary drainage to structural capacity?

• A. It must be independent from structural capacity.
• B. It must be connected to primary drainage.
• C. It should never be functional.
• D. It should be the same material as primary.

Answer: (A)

Redundancy and independence of drainage paths from the structure’s load-carrying capacity are essential. Secondary drainage must operate independently of the structural capacity so it can remove water even if the roof deck experiences deflection, settlement, or that the primary drainage path becomes blocked or overloaded. This separation ensures a back-up route that remains functional under conditions that could compromise the primary path, reducing the risk of ponding and potential structural or membrane damage. If secondary drainage were tied to the primary or relied on the same structural pathway, a failure or blockage in the primary could cascade and disable both paths. It isn’t about using the same material or about a path never being functional; the key is independence from the structural system to provide reliable redundancy.