Pictured is a moldy roof deck in a 10-year-old home. When warm, humid air leaks up into the attic and is cooled by the cold attic air, it condenses and causes condensation to accumulate on the bottom of the roof deck. Here you can see the resulting rusted nails which dripped the rusty water onto the attic floor. All of this wet wood can lead to mold and then rot and even mold spores being circulated throughout the house.