If we discuss this as if it is a triangle then the "rise" is the vertical side and the "run" is the adjacent side. So the ratio of the rise to the run is 8 / 15 which equals
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0.5333333333
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To find the angle of this roof pitch we look up the arc tangent of
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<span>
<span>
0.5333333333
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</span>
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and we find that is 28.072 Degrees