Date: Oct 23, 2008 9:58 AM
Author: Steven
Subject: Intersecting surfaces

Imagine two roof surfaces of different pitches intersecting at right angles to each other to form a hip ( a common configuration for many buildings ). 
Now the interior angle of the intersection of these roofs can be found using the formula:

180 degrees - inv cos ( cos roof pitch A x cos roof pitch B ). Some kind person on here gave me this formula a couple of years ago :)

But this only tells you the interior angle and nothing of which roof "imparts" what percentage of this angle.

Understanding this will have a practical outlet in my current job as I need to fabricate an internal channel to carry water from the intersection to the gutter.

Taking the current job as an example it has one roof of 33 degree pitch and the other of only 15 degree pitch. I know using the above formula that the slopes intersect to form an interior angle of 144.105 degrees. So basically what I need to know is what part of this angle comes from the 33 degree roof and which comes from the 15 degree roof.

Thanks for your time