How to Determine Roof Slope Factor

Roof slope, which refers to the rate at which a roof descends, is expressed as a ratio of rise to run. Rise is the vertical measurement, and run is the horizontal measurement. Finding the slope of a roof is useful to determine how much roofing material is necessary to cover the roof. Slope can also determine whether you can use a particular roofing material, as some materials are not appropriate for steep slopes. The most challenging aspect of determining slope is accessing the roof, but you only have to take one measurement once there.

Extension ladder
12-inch level
Tape measure

Instructions

- 1
  Place an extension ladder against the edge of one side of the roof, then climb the ladder to access the top of the roof. You don't need to walk onto the roof, but doing so is a good idea if you're not comfortable standing on a ladder without holding the sides.
- 2
  Position a 12-inch level onto the roof, then lift the end of the level until the two ends are level. The tool is level when the air bubble in the liquid-filled leveling indicator rests between the two vertical lines of the indicator.
- 3
  Position a tape measure at a right angle against the bottom of the raised end of the level, then measure the distance between the bottom of the raised end of the level and the roof to determine the slope. For example, if the measured distance is 5 inches, and because the length of the level is 12 inches, the slope is 5:12, meaning that the roof gains five inches of height for every foot.

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