Ponding instability describes what phenomenon?

• A. A simple static problem with no feedback
• B. A feedback loop where water causes deflection, increasing ponding, which leads to more deflection and possible structural failure
• C. A minor cosmetic issue
• D. A thermal expansion phenomenon

Answer: (B)

Ponding instability happens when water on a roof or deck creates a positive feedback loop between load and deformation. The weight of the water causes the surface to sag or deflect downward. That deflection reduces drainage efficiency and can deepen the pool of water, so more water accumulates. The increased water load then drives more deflection, which can continue until the structure can fail. It’s about the dynamic interaction between the water load and the structural response, not just a static condition. This isn’t a cosmetic issue or a thermal expansion effect, and it’s not a simple one-way load—it's the looping effect of deflection feeding more ponding that makes it a concern.