I have a question, and a suggestion;
Seeing as this is a “courtesy scan”, why don’t you get your camera out and go spend the day scanning these roofs?
This will help you understand what you’re seeing, why you’re seeing it, and when you will see it. This will help you understand the question you just posted.
There are numerous opportunities to do roofing scans throughout the day. The results will be different. The most important thing is **not **to know “what time to do it” but what causes the conditions that make it feasible.
I’ve done some roof jobs so big that I’ve had to invert the palette colors so the anomalies all look the same in the report! This is because at one time of the day the wet spots were cold and other times they were hot.
Cameras in the “automatic mode” cannot make the correct assumptions, you have to.
If you do this, by the time you get to infrared training you will have some experience that you can relate to and will help you get more out of the class.
The one hour after sunset time frame is taught in school because it’s the easiest for you to achieve and understand without knowing all of the complexities involved in scanning the roof. Even though this is the best time under that standard, it doesn’t mean it will necessarily work for you.
I have done dozens of thermal scans on buildings when I should not have been doing them according to the schools. I run my camera and take pictures regardless of these “Windows of Opportunity”. By doing this (and following exactly what I’m suggesting you do here) I have learned that what causes exceptions to be observed with thermal imaging is much more complex than we can hope to perceive in all cases. We try to take the most opportune times and control the conditions as much is possible by understanding physics, thermodynamics, fluid dynamics, psychrometrics etc. but things often do not appear as they seem!
Infrared training is a progression. The further you go, the more understanding you will have as to what the camera is registering and what you must do to obtain useful data. Some infrared trainers believe that this information and education is totally unnecessary because were just collecting pretty pictures. Well, pretty pictures show up under different circumstances which may be beyond your control or initial observations.
When you see a spot on your thermal scan, you should always be asking yourself what caused this to appear? If you can’t answer this question than you do not have sufficient information to write a report. Just because there is a warm spot on the roof one hour after sunset, it doesn’t mean you have a leaking roof!
“a warm anomaly was apparent on the roof, recommend further evaluation by a roofer” is insufficient information (even though it seems to be sufficient and home inspections). In commercial/infrared inspections your scope of inspection is not the same nonsensical limited inspection as a home inspection. You cannot drag thermal imaging backwards into the home inspection industry. You’re going beyond the limitations of a home inspection and you are no longer protected by these assumed limitations of liability.
I highly recommend that you discuss what you use your camera for with your insurance provider, because at some point in time they may feel that you are not performing a home inspection and are not covered under your insurance. I have this conversation each year with my provider, and just received an email from them yesterday to discuss it again for this year.