Quote Originally Posted by Frank S View Post
It it is thick enough it will make a fair heat barrier against the sun
I disagree with you there. Concrete has an atrocious r value. Poured concrete is closer thermally to steel than any kind of insulation. It's the sitting down on a cold toilet seat of building materials. Generally we pour 4" over q-deck which I think is industry standard in North America, so assuming this pour is roughly the same thickness that'll get you an r-value for that whole roof of around 0.3.

Normally around here concrete roofs are specified with 4" of iso insulation laid on top for an r-value of 25, then the water-tight roofing system is installed over top of that. One single inch of iso insulation will get you an r-value of over 6. So one inch of iso insulates as much as 75 inches of concrete.