Very interesting. And counterintuitive in many ways. But I treated some shingles that were going black/green with this product and sure enough in 2 weeks it gradually disappeared. What that shows I'm not sure ! It's more a biocide maybe? It's also got detergent that helps it penetrate I think. Anyway I'll use up mine then I'll never waste money on it again(it's now over £40 a tin) should be called rot promoter.