I would think a regular $500 NVR surveillance camera type system could also be used for this. These have the ability to detect motion based on how many pixels change. It is designed to show live video and store it. They are typically wifi based and are setup to allow phone access. My house camera system works this way, but I don't use phone access or its notification features. I get false alarms from trees and bushes that blow in the wind, but on a rooftop you shouldn't have this issue. It also has a masking feature so I can mask out most of the bushes but keep the walking paths and driveway monitored. Only other thing that will trip it is the steam from my furnace exhaust (a 95% condensing furnace that vents through a PVC pipe) that wafts across the front porch. The cloud of vapor triggers motion video recording on cold mornings. It has also happened occasionally with morning fog banks wafting in -- the camera image looks like you are in a huge rain storm when the fog droplets are very close to the camera lens.
So I'm not sure what technology the motion detector is using -- ultrasonic, video, or what. They can all give false positives under certain conditions. The plus with a camera based system, or one employing camera plus a different type or multiple types of motion sensing, is if you can access them via a phone app you can quickly see what the intruder is -- a person, a large bird or rat, something loose on the building blowing in the wind, or a fog cloud.
A friend also has a NVR system on his house and his phone is dinging every 5 minutes with motion notifications. He needs to go through and turn the sensitivity down on his...
Final thought would be if this is deployed and left long term, you need a way to deal with service people. Either someone is going to get an alarm when there is a legit person on the roof, or there needs to be a way to turn it off or notify someone before they enter the roof.
