Which statement about ponding depth and roof load is correct?

Which statement about ponding depth and roof load is correct?

• A. Ponding has no effect on structural load.
• B. Ponding increases the roof load as water depth grows.
• C. Ponding reduces load on the roof.
• D. Ponding eliminates load from the roof.

Answer: (B)

The key idea is that water adds weight to the roof, and that weight grows as ponded depth increases. The load from ponded water is the weight of the water itself, which per unit area is the unit weight of water times the depth: w = γ × h. For freshwater in US units, γ is about 62.4 lb/ft^3, so the load per square foot is roughly 62.4 × h pounds. As the depth h grows, more water sits on the roof, so the total vertical load increases correspondingly. This is why ponding never reduces load; it always adds weight, and deeper ponding means a larger load on the roof structure.

The key idea is that water adds weight to the roof, and that weight grows as ponded depth increases. The load from ponded water is the weight of the water itself, which per unit area is the unit weight of water times the depth: w = γ × h. For freshwater in US units, γ is about 62.4 lb/ft^3, so the load per square foot is roughly 62.4 × h pounds. As the depth h grows, more water sits on the roof, so the total vertical load increases correspondingly. This is why ponding never reduces load; it always adds weight, and deeper ponding means a larger load on the roof structure.