Estimate Roof Size

How do you calculate the fall on a flat roof?

How to Calculate Roof Pitch in Degrees
  1. First, you need to measure the run of your roof. …
  2. Next, you need to figure out the rise. …
  3. Now, divide the rise by the run. …
  4. Then, divide 1 by your tangent.
  5. Finally, multiply this result by 180/π and you’ve calculated your roof pitch!
A flat roof is generally any roof with a pitch of 1-10 degrees. That means flat roofs aren’t actually flat. It may look horizontal but often has a minimum slope of 1/4 inch per foot. This allows water to run off the roof to prevent damage.
Generally, roof pitch is calculated from the same formula that describes a right triangle. You can use the following equations to find the length of the rafter and the slope: rafter² = rise² + run² (from the Pythagorean theorem); rise / run = pitch , where pitch is expressed in percent; and.
 
To calculate percent slope, divide the difference between the elevations of two points by the distance between them, then multiply the quotient by 100. The difference in elevation between points is called the rise. The distance between the points is called the run. Thus, percent slope equals (rise / run) x 100.
Pick two points on the line and determine their coordinates. Determine the difference in y-coordinates of these two points (rise). Determine the difference in x-coordinates for these two points (run). Divide the difference in y-coordinates by the difference in x-coordinates (rise/run or slope).

 

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