What is the significance of primary and secondary drain elevation?

• A. Primary drain activation timing is more important than elevation.
• B. Water will rise to the elevation of the secondary drain before it activates, impacting structural load.
• C. Elevation has no effect on performance.
• D. Secondary drain elevation is irrelevant.

Answer: (B)

Understanding how primary and secondary drain elevations interact is key. The primary drain handles normal rainfall, but the backup (secondary) drain has a higher elevation. Water on the roof will continue to rise until it reaches the level of the secondary drain, at which point that backup drain begins to relieve the water. This means the maximum water height—and thus the hydrostatic load on the roof structure—is governed by the secondary drain’s elevation. In other words, the elevation of the secondary drain sets how high water can accumulate before relief occurs, which directly affects structural loads and the effectiveness of redundancy in drainage.

That’s why the significance is that water will rise to the elevation of the secondary drain before it activates, impacting the structural load.