The answer is: "forms an oxide film over its surface which prevents corrosion". This is because when aluminium is exposed to air, it reacts with oxygen to form a thin layer of aluminium oxide on its surface. This oxide layer is tightly bound to the aluminium and acts as a barrier, protecting the underlying metal from further oxidation or corrosion. This makes aluminium a good roofing material, as it can resist the effects of weather and other corrosive elements over time.