When planning a thermal roof survey, what timing is recommended?

• A. Midday
• B. One hour after sunset
• C. One hour before sunrise
• D. Midnight

Answer: (B)

Timing for a roof thermography is about maximizing the temperature contrast between the roof surface and the surrounding environment. After sunset, the sun’s heat has already been absorbed and begins to dissipate from the roof, but the surface still remains warmer than the cooler night air. This difference in cooling rates—often influenced by moisture, insulation gaps, and material conditions—creates clearer temperature anomalies that the infrared camera can detect.

Choosing a window like one hour after sunset provides a practical balance: enough time has passed for solar heating to drive thermal patterns, yet the roof is still warm enough relative to the air to produce strong contrast. Imaging at midday would wash out these contrasts due to continuous solar heating, while imaging very late at night or before sunrise might reduce the overall temperature differences, making anomalies harder to spot.