By Michael Chotiner of The Home Depot Stains on asphalt roofing shingles make a house look shabby, which detracts from its value. In some cases, stains are merely a cosmetic issue.  But sometimes they’re symptomatic of a problem that, if left unchecked, can lead to more serious damage and, eventually, roof failure. It's not always hard to distinguish the causes of stains, nor, in most cases, to get rid of them and prevent the stains from recurring. Common Causes of Staining D