What happens when water rises 2 inches in a drainage system?

• A. Secondary drainage activates.
• B. Primary drainage activates.
• C. No action; system remains idle.
• D. The roof is considered failed.

Answer: (A)

When a roof drainage system has a secondary (backup) path, it is designed to take over once water reaches a predefined level. Reaching two inches indicates the primary drainage is at or near its capacity, so the secondary drainage activates to carry away the extra water and prevent ponding on the roof surface. This redundancy helps protect the roof by maintaining a safe water balance and reducing hydrostatic loads; the primary can still be in use, but the backup starts operating to relieve the excess. The roof isn’t failed simply because water rises to this level—activation of the secondary is an intentional protective function designed to keep drainage effective even under heavy flow.