How to Calculate Average Butterfly Roof Angles

A butterfly roof is easily identifiable by its two rooflines that slope toward the center of the home to form a valley. This V-shaped configuration resembles the wings of a butterfly when viewed from a distance. The angle of a roof is referred to as slope. Slope is expressed as a ratio of rise to run. Run is the 12-inch horizontal extension of the roof. Rise is the number of inches the roof rises along the 12-inch run. Although butterfly roofs often feature identical slopes, it is wise to check both portions of the roof.

Carpenter’s Square
Level

Instructions

- 1
  Position a carpenter’s square against the roofline with the 12-inch mark noted on the horizontal portion of the square touching the top of the roofline to find the “run.”
- 2
  Put a level against the top of the horizontal portion of the carpenter’s square to ensure that the square is level. The square is level when the air bubble inside the level’s liquid-filled indicator rests between the indicator’s two lines.
- 3
  Count the number of inches from the top of the horizontal portion of the carpenter’s square to the point at which the vertical side of the square meets the roofline to determine the “rise.” If the rise is 5 inches, for example, express the slope as 5-in-12, or 5 inches of rise for each 12-inch run.

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