I need to attach a shed roof to an existing roof. My soffit is at 7' 6". The peak of my roof is at 15 feet. My roof is a 12/12 pitch. It is being built on a 12 foot by 16 foot deck. 16 foot will be the width of the deck and roof. I have a 2nd deck that is attached that is approximatly 19 inches above the the deck I am putting the shed roof on with a set of stairs in the middle of the deck. I am debating wether to go to the peak and bring the roof a total of 18 feet or star the roof half way up the 12/12 and do I tie the rafters into the existing rafters or just on top of the roof.
Not sure I'm precisely following what we're talking about... Is this an addition attached to the house over which you're building a shed roof? Some sort of dormer or something?
Whatever the case, I think the answer to your question at the end is that "just on top of the roof" is problematic because it's potentially a way for water to flow under this roof at the joint, and you might not be getting the ventilation in this new roof it needs to prevent moisture damage on the roof.
I've been through a dormer roof reconstruction in my own roofing project, and the way that you do that is with a header and doubling rafters on the ends of that header... Of course, that's because the space that the bottom of the rafters goes through is space you want to use for living in, I suppose you could probably do what you're saying with tieing into the rafters. I'd recommend looking for a book on framing, I got one that I found at a big box store.