Originally Posted by TTMartin
This is a photo of a 9 degree angle which would be 10% of 90 degrees I guess.
That's not what "10%" means when describing slope. A 10% gradient means that the rise is 10% of the run, so for instance over ten feet of floor length one end is one foot higher than the other. That's a pretty steep slope, of 5.74 degrees. The example shown, of 9 degrees, is a 15.8% gradient... steeper than almost any highway grade.
If you remember some basic trigonometry, the tangent of the angle from horizontal is equal to the ratio of the opposite over adjacent sides of the triangle, which is the ratio of the rise over the run, which is the grade in percent multiplied by 100. So...
tan(5.74°) = 0.10 = 10%
tan(9°) = 0.158 = 15.8%