How to Angle a Roof

Measure the horizontal distance across the building from the outside of the wall to a point directly under the highest part of the roof. For a hip roof, this is usually the center of the building; and for a single span roof, it is the outer edge of the opposite wall. Record the distance.

Measure the vertical distance from the top of the building walls to the highest part of the roof. For a hip roof, this is the vertical distance from a horizontal line between the tops of the walls to the top of the ridge beam. For a single span roof, it is the vertical distance between the lower and higher ends of the span.

Substitute the horizontal and vertical distances into the equation Tan a = vertical distance / horizontal distance, where "a" is the roof angle.

For example, if the roof width is 10 feet and the height is 6 feet, the equation becomes Tan a = 6/10, or Tan a = 0.6.

Determine the arc tangent of 0.6 using a calculator. The result is the precise angle of the roof. For example, the arc tangent of 0.6 is 30.96, so the angle of the roof is 30.96 degrees. You cannot measure fractions of a degree with normal builder's equipment, so round the value to the nearest whole integer. To conclude the example, 30.96 is rounded up to 31, so angle the roof at 31 degrees.