Isn't roof pitch usually rise/run and the rise is stated in terms of 12" of run, so 12/16 could be described as 9/12?

So I'm an engineer, I can do geometry, let's see if I can walk ya through this.

Starting with your 9/12 pitch, if you start from a point at the top, then come down 9 then across 12, then you come across perpindicular to that by 9 (since your 12/12 pitch roof would have come down 9, it comes across 9). This gets you to the point at the top of your valley.

Taking your two legs where you came across, using pythagorean theorm 9^2+12^2=x^2. X=15. This is the amount that your valley comes across horizontally after 9 of rise. So that would figure into how your ends will be cut.

Viewing from the top, to get your bevel angle, you're going across in the direction the 9/12 roof slopes by 12 and in the direction the 12/12 roof slopes by 9. So your angle should equal inverse tangent (9/12) = 36.9 degrees.

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Please do NOT consider any "before" picture of my house as any kind of endorsement of any particular construction method. In fact, you should probably assume that if I post a "before" picture, I am posting it because I am soliciting advice on a proper replacement for one of MANY things done wrong by a previous owner.

Isn't roof pitch usually rise/run and the rise is stated in terms of 12" of run, so 12/16 could be described as 9/12?

So I'm an engineer, I can do geometry, let's see if I can walk ya through this.

Starting with your 9/12 pitch, if you start from a point at the top, then come down 9 then across 12, then you come across perpindicular to that by 9 (since your 12/12 pitch roof would have come down 9, it comes across 9). This gets you to the point at the top of your valley.

Taking your two legs where you came across, using pythagorean theorm 9^2+12^2=x^2. X=15. This is the amount that your valley comes across horizontally after 9 of rise. So that would figure into how your ends will be cut.

Viewing from the top, to get your bevel angle, you're going across in the direction the 9/12 roof slopes by 12 and in the direction the 12/12 roof slopes by 9. So your angle should equal inverse tangent (9/12) = 36.9 degrees.