For many years I have conducted surveys of roofing systems. There are many different roof types and the most successful are flat or low sloped roofing systems. Some roofs like inverted roofing systems and green/living roofs gain almost nothing from a survey using thermography.
The basics of roof surveys are that a roof leak will soak into the insulation material and this will change the thermal capacity of the insulation material itself. This change in thermal capacity mens that the wet insulation will change temperature more slowly than the dry insulation. For the survey to be most successful, the insulation material that is used in the roof structure should be of an open cell structure, thus allowing for good soakage of the water. Closes cell insulation materials like polyiso do not soak water very well and are more difficult to do.
Some membranes are more difficult to survey than others. Felt is an excellent material to work with in infrared as it has a high emissivity and is a very diffuse surface. PVC on the other hand does have a high emissivity, but it is more specular in nature and this means that clouds may be visible in the images (particularly with a LW camera).