I have as simple a roof as I can have.
A flat roof.
Which technically means.
The way it is made is typical.
Regards
Marc
This might work:
Draw the bases for all your slabs at their actual falls, apply the constructions attribute so you can keep them but easily turn off.
Having set the appropriate part/family (and the active element class attribute back to Primary!) use Form Modelling > Extrude Shape to Slab Form set with all angles to Vertical, I've just tried this and got a form of constant thickness with vertical sides.