Yes l agree the roof is almost certainly unpainted galvanized metal, it has lost the polished silver look of new galvanized metal, it looks the same shade as the galvanized bucket l have. The bucket is over 60 years old but seen relatively little use and has been kept in a shed or garage most of its life. It is almost exactly that grey. Galvanised roofing nails weather to a lighter shade for some reason, in my experience.