||10-09-2008 08:44 AM
Agreed, no issues with the application of rock. It is common for multiple layers of rock to be applied to the underside of I-joists in multifamily construction. If the floor is solid and properly spanned, the added dead weight shouldn't affect the floor's performance.
As for the comments about the integrity of I-joists, pooey. They're a tried and true product, and are every bit as structurally stable as dimensional lumber, if not more. Since they are thin, a catastrophic fire will normally compromise them much faster than dimensional lumber joists, but proper firestopping and a sheetrock ceiling will protect them from all but the worst fires.