This is a spirit level to test whether a surface is flat or not. It acts as a tool like a carpenters level, with the bubble resting in the center if a surface is flat. Aside from horizontal testing, you can do plumb (vertical) measurements as well. You can calibrate your device from a flat surface as well, and the calibration can be saved.
This level app has options and settings to show the roof pitch, as well as the angle and inclination of a surface.