|Ideally roofing inspections should be performed starting around sunset to an hour or so after sunset. The weather conditions should be a clear warm day and cool clear evening. These conditions maximize the radiation heat transfer from the roof to the cold sky and the dry areas and wet areas of the roofing surface will have a greater temperature differential and be easier to see. There is an ASTM standard for performing thermal inspections of roofs, ASTM C1153 97(2003). The roof should be dry, free of snow, ice and ponding water. It should have rained no earlier than 24 hours before the proposed inspection or no later than 1 week prior to the inspection.
If the weather conditions are not ideal the temperature differentials between the moisture saturated insulation and the dry roof areas will be less. Also keep in mind that the type of roof that is to be inspected, single ply membrane, ballasted, modified bitumen, built up etc. have unique chacteristics and can yield false indications of moisture due to multiple plys or thick ballast. It is difficult to perform an inspection on a roof that has large stone, greater than 1/2" diameter as the ballast will retain heat longer. If some areas of a ballasted roof are considerably thicker than others this will also yield a false indication of moisture.