Re: Walk-In Safe/Basement/Building House Questions?
As BH said the ICF's (insulated concrete forms) are a good alternative for added insulation. However, IMO they wouldn't be a good choice for an interior wall due to the fact they have insulation around them. If I had an ICF home and wanted to put a safe room in it, I'd use either corefilled block or a standard poured wall to build a interior safe room. Just my opinion.
I would discuss the roof cap with your contractor in depth. When spanning with concrete one can use judgment and experience to design small spans. However, If I was asked to span something 15' wide, I would flat refuse to do so without professionally designed structural specs. Concrete weighs 4000 pounds per yard, and at 8" thick a cap on a 15 x 15" Inside dimension room (17' x 17' outside dimension) is about 28,900 pounds!
Even though I'd be confident that a double rebar mat would do the trick, I'm not taking a chance with that kind of weight OVERHEAD.
I used to re-load but now I "hand-load".
-- Well, at least I try