Why do the galvanized iron roofs houses become rusty?

Some metals such as steel, galvalume, and galvanized roofing materials are prone to rust when exposed to moisture. Preventing rust is understandably a big concern for homeowners considering metal roofing

here is the answer

water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. weathering is the breaking down or dissolving of rocks and minerals on earths surface. once a rock has been broken down, a process called erosion transports the bits of rock and minerals away.