How safe is it to use wood shavings as a roof insulation covered with white corrugated iron?

I personally wouldn't use shavings in case of a fire. Unless they are coated in some sort of fire retardant like Ammonium Polyphosphate. Besides, unless it is really densely packed, it's not the best insulator. I'd go with blown cellulose like they put in attics or the sheets of fiberglass insulation, the pink fluffy stuff.