An untreated wood shake roof doesn't even meet a “Class C” rating. A pressure impregnated, exterior rated fire retardant treated shake roof has a “Class B” fire rating by itself (considering only the wood shake covering) or a “Class A” rating “by assembly” (covering plus underlying material). An example is shown here - FR treated shake, plus the underlying gypsum-fiberglass product (the white material) and plywood roof sheathing that provide the Class A rating ‘by assembly’.