Many (but not all) low-slope roofs are fairly simple to inspect. If you are dealing with a built-up roof (BUR) or "bald" single-ply membrane that has been in use for many years, you should have success using one of two basic strategies in which wet insulation due to its own Thermal performance and self-display higher heat capacity or higher thermal conductivity. Both ballasted single-ply roofs and newer roofs utilizing low-absorbency insulation are difficult to inspect. Also, many bald monolayers are very reflective at long wavelengths.
In any case, you'll be working on the roof - so safety is a top priority. Plan an initial walk in daylight to help you identify potential hazards and visually inspect the roof. The importance of having a qualified escort on the roof at all times, day or night, cannot be overemphasized! Especially when working near roof edges, take precautions to prevent falls. When working at night, go slowly as your eyes adapt to the imager's viewing screen, making you "night blind!"
Desirable work is done on a quiet, clear night after a sunny day, but it is possible to do it any evening or in overcast weather when the outside temperature is 50°F or lower. The roof surface, including any gravel, needs to dry. It is important to know what kind of insulation is used, how many layers of insulation there are, and to know where the leak is occurring.
Walk slowly and systematically to the roof until the patterns start to stand out. Seen from the roof, the thermal mode of wet insulation will be higher than that of dry insulation. Absorbent insulation will have the typical right-angle "sheet" pattern (see below), while low-absorbency insulation will display a "picture frame" pattern.
Of course, there may be many other thermal modes unrelated to wet insulation, including sprinklers, drain pans, gravel piles, exhaust from HVAC units, and structural elements in roof systems, among others. Once you've identified a specific pattern that indicates wet insulation (and a hygrometer is recommended for testing), you can continue looking for similar patterns until you've covered the entire roof.
Don't worry about finding an actual leak during inspection, though. It is usually located within the boundaries of wet insulation and is usually easier to find during daytime follow-up. Ideally, you'll find hot indicators for every known leak. Use high-quality aerosol marking paint to mark thermal patterns as accurately as possible, and number each section for easy reference. If the conditions are right and the roof is not too complicated, you can usually measure 100-200 square feet at night!
