There is no way to calculate this, but it is possible to display it via the element info palette. (its the plan area of the roof)
This is another reason we need calculation fields in the interactive schedules, all you would have to do is have a field that does the simple calc - roof area / cos(roofpitch)
This is something I was discussing with GS Australia tech support just last weekend. David if your listening...................
Sorry for the bad news