Metal roof shell over concrete slab

I will open this up to see what ideas exist out there for you. All best.

Hello, I think I have a metal roof question that is of interest to anyone building strong in an area visited by hurricanes (or tornados). I am a few weeks off from pouring a five inch 2,300 sqft reinforced concrete gable roof slab (with a 1 vertical:3.25 horiz pitch). The positives of a concrete roof are several (resists hurricane wind uplift, termite proof, fire resistant...). However, I live in a hot & generally wet area with clay soils. Thus. despite our extensive foundations, I still need to consider the strong possibility of settlement cracks in said roof. I have researched several admixtures, chemical paints, and rubber coatings. In the end I believe a white metal Aluzinc roof "shell" is a good solution for waterproofing a concrete roof. From the estimates I've received, a metal roof shell would cost less than a chemical or rubber one. I dont wish to put a whole lot of expansive bolts or screws into my new roof slab in order to apply this metal shell. I was hoping to come up with something clever. Do you have any ideas???? I came up with the concept of casting 2x4 galvanized steel Z (or C) purlins in my slab. They would stick out about 1" to 1.5". A radiant bubble insulation foil would also be layed above the purlins (silmilar to this: http://www.ecofoil.com/All-Products/Double-Bubble-Insulation/Double-Bubble-Foil-Insulation-Foil-Both-Sides-4-x-125-500-sq-ft-). My builder has come up the concept of using cast-in 1"x2" retangular galvanized steel 'hollow' tubes (picture a square 'pipe,' sometimes used for metal pergolas and the such), placed every 4'. He thinks the Aluzinc roof should be flush to the concrete roof (& perhaps the same radiant bubble product directly inbetween). The idea is that the screws would penetrate into the surface-flush tube and cavity within - ie. also not into the concrete. He says the tubes would be easier to cast into the slab since they present a shallower profile. Would either of these ideas work? What about condensation issues? The steel z/c-purlins would have to run horizontal along the roof (right?) so any condensation may pond along the purlin (would some ventilation stop this?). Similarly, the square tubes are hollow and (even if they are capped) may also cause condensation and corrosion within the tube. Any bright ideas or heedings? I have researched this concept to a dead-end. In short I think a creative solution would benefit the metal roof industry. Gracias, PJ