ok so this was a commercial flat roof, big HVAC units all over the place, maybe 40,000 sq ft. client wanted a full condition report with thermal to check for moisture intrusion under the membrane. ive done maybe a dozen of these before so i wasnt going in blind.

problem was i flew it about 2 hours after sunrise and the thermal data was basically useless. everything was already heating up unevenly because of the HVAC units throwing heat around and some sections of the roof had been in shade from a neighboring building. my color gradient was all over the place and i couldnt confidently call anything as a wet spot versus just a shadow heating artifact.

i know the golden window for flat roof thermal is supposed to be either early morning before solar loading kicks in or like an hour or two after sunset when wet areas retain heat and dry areas cool down faster. i just misread how fast that roof was going to heat up given the equipment on it. ended up having to reschedule which the client was not thrilled about.

anyone have a reliable way to pre-plan the timing for a specific site or do you just kind of learn the hard way like i did here. feels like there should be a better process.

yeah the post-sunset window is way more reliable for membrane moisture work in my experience, the differential is just cleaner. i usually aim for 90 minutes to maybe 2.5 hours after sunset depending on how warm the day was. the HVAC situation you described would have screwed up morning timing on any roof honestly, those units create their own little microclimates and it throws everything off.

for pre-planning i started checking the hourly cloud cover and wind forecasts the night before, not just for flying conditions but because cloud cover during the day actually helps with evening thermal because the roof doesnt get as unevenly baked. clear hot sunny days followed by a calm evening are ideal. theres no magic app for this really, you just build intuition after a while. your instinct to reschedule was right though, bad thermal data is worse than no data.

this is sort of unrelated but did the client understand why you had to reschedule or did they just think you messed up the flight? asking because ive had a hard time explaining thermal timing windows to people who arent technical and it always comes across like im making excuses even when im right lol