Dear Joe:
A structural fascia (SF)is one where it acts as a beam spanning between the hip and a designated rafter. This rafter is usually the first one that has a back span bigger than the cantilever (or is propped up by a bracket.) The connections can be hard to develop as the overhang becomes bigger. The SF supports the outboard end of the hip jacks that have little or no backspan. To overcome the connection problems, one can drop the hip below the plane of the rafters or use a hip much deeper than the rafters. The SF can then be supported from below at the hip.
Keep in mind that on a square hipped roof, when the over hang length exceeds 1/5 of the side of the supporting square, there is more weight outside the square than inside the square. The result of this is that without braces or skyhooks to support the overhang, you have uplift at the peak (this is where a brick cupola would come in handy.) As I mentioned before, on your proposed design, 3/4 of your roof area is outboard of your supporting square. Your structure is 3/4 of the the way to resembling the structure of an umbrella.
Regards,
Chris
