What is the load logic for ponding?

• A. 1 inch of water equals approximately 5.2 psf; as ponding depth increases, load increases linearly.
• B. 1 inch of water equals approximately 1 psf; load is constant.
• C. The load decreases as depth increases.
• D. The load is independent of depth.

Answer: (A)

Ponding creates a hydrostatic load that grows with the depth of water. Water weighs about 62.4 pounds per cubic foot, so the load per square foot on a roof is 62.4 times the water depth in feet. For each inch of ponding, that depth is 1/12 of a foot, giving 62.4 × 1/12 ≈ 5.2 psf. Consequently, as ponding depth increases, the load increases in direct proportion (a linear relationship). This makes the statement that 1 inch of water equals about 5.2 psf and that the load increases linearly with depth the correct one. The other options imply the load is constant, decreases, or is independent of depth, which contradicts the way hydrostatic pressure from water behaves.