The steeper the roof, the better GutterShutter performs. The funny thing about roofs is that the steeper they get, the less water they catch. A flat roof catches all of the rain that falls over it. As you begin to tilt a flat roof upward, angling its surface to the rain, you reduce the surface area that can catch rain and reduce the total amount of rain to be handled by the gutter. Granted, the water that is caught will flow faster but, fast water flow has no effect on how GutterShutter performs.