How to Measure a Roof Angle

Knowing the angle of a roof is important when building a house, re-roofing or trying to decide whether it's safe to walk on. Roof angles are generally expressed as a fraction, such as 7/12 or 3/12. The measurement reflects the roof's rise in inches for every foot of its horizontal run. Calculating this is very easy, requiring only a level and a tape measure.

Level
Tape measure

Instructions

- 1
  Find a good location to perform the measurement. If your roofing material doesn't have an uneven slope caused by the edges of shingles, you can do it on top of the roof. Otherwise, the bottom edge of a gable or a rafter in an unfinished attic will suffice.
- 2
  Hold the level perfectly horizontal, with one end resting on the slope.
- 3
  Measure the distance between the 12-inch point on the level and the slope directly above or beneath it, depending on whether you're doing the calculation on top of the roof or underneath. Use this number as the numerator of your angle fraction, with 12 as the denominator.

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