Which option is NOT listed as influencing water movement on low-slope roofs?

• A. Roof slope
• B. Long-term creep of structural members
• C. Thermal conductivity of insulation
• D. Deck deflection

Answer: (C)

Water movement on low-slope roofs is driven by how water can shed or pond on the surface, which is controlled by the surface geometry and its changes over time. The roof slope sets the baseline drainage path; a steeper slope sheds water more quickly, while a very gentle slope can allow more ponding. Long-term creep of structural members can cause the deck to sag, creating low spots where water tends to collect and slow drainage, altering the expected flow-path. Deck deflection similarly changes the flatness of the surface and can form depressions that trap water. Thermal conductivity of insulation is a property that describes heat transfer through the roof assembly, not the paths water follows or where it pools, so it does not influence water movement on the roof surface in the context of drainage behavior.