This is hard to talk about without naming the actual product but I’ll do my best to describe this solution in broad terms. Several companies use a product that is promoted as completely bio-friendly and is activated by the rain. Once it’s applied it will slowly work over a period of time and will eventually clean your roof. The benefit from this rain activated process is that it has no negative impact on plants that is sometime associated with those who use a chlorine based process. The details of what’s in the product is normally hidden under the banner of a trade mark name but you can tell they are using it if they say “it’s activated by the rain and works over time”.