They aren't made fire proof becuase you reach a point of deminishing returns very quickly. It is an issue of weight and expense, but also a simple one of, if it strong enough, you just made an oven and everything inside will burn. Oops.

What you can do is not put ceder shakes on your roof. You want "quant" and "picturesque", "country-style" roofs, go with a metal one like the ones on half the barns I know.