It seems like there is some overlap between this topic and the other one we have going….
Here is a drawing of a scenario where you extend the facing significantly above the core, in some cases to the ceiling.….
In your case, @dclaire, it sounds like you are thinking of actually tilting the facing capping slab to match the pitch of the roof. And, as you are considering, it may be necessary to create additional structure in order to extend the single wythe of masonry up. You would have to remove anything combustible. I have considered a steel stud and cement board form above that I could leave in place and build the facing against but have never had a project where it was called for thus far.
Hope that helps!