Roofing repair contractors and installation professionals use the format Y/X to describe the steepness of a roof. Y is called the “rise”, and it’s how much a roof rises in inches while X describes the “run” or the distance over which the Y rises. Residential homes have steepness numbers that range from 3/12 to 20/12. Any roof with a steepness of 8/12 or lower is considered “walkable”, that is the roofer can safely walk over the roof.