Thermography is an extremely useful tool for the investigation of roofing problems, such as wet or missing insulation, which manifest themselves as thermal anomalies. However, there are many conditions which can significantly affect the pattern of thermal anomalies on the roof. In fact, under certain conditions, the images of the thermal anomalies may disappear or become indistinct and roof scanning cannot produce any useful information. The problem that the thermographer faces is that these conditions can change, sometimes radically, over the course of a single evening. It is, therefore, essential that the thermographer understand under what conditions the "scanning window" will be open. The expression "scanning window" is a convenient term which refers to the cumulative effect of a number of conditions which affect thermal images. When the scanning window is open, the thermal images from the scan generate useful information for roofing analysis. When the scanning window is closed, it is impossible to obtain any useful information from the thermographic scan. The factors that affect the opening and closing of the scanning window for roofing applications will be discussed in this paper.