I tried something on the house we bought up here, to see what it does and how it works out.
There is a company which 'cleans' shingle roofs with a bleach/water/degreaser solution, which is sprayed out at a low pressure (less than garden hose pressure) through a section of PVC with what looked like (and I am sure it was) a half circle sprinkler head on the end. The spray was minimal, yet you could see it building up on the shingles and running down the roof.
After this set and dried out, they went around and rinsed it off with the same sprayer and plain water from a garden hose (this time at garden hose pressure, they hooked it up to the garden hose).
I was surprised that the solution cleaned as thoroughly as it did (the roof looks great and has NO stains on it). They give a 5 year guaranty too, and will come back at anytime within that 5 years and retreat at no cost (no bull, one of my new neighbors had this done to his house and they were coming back to retreat his (after 3 years, there was limited staining re-appearing) for no charge, so I figured there was not much to lose.
I figure our roof has less than 5 years left on it (it's a 9:12 slope, so it drains very well), so what did I have to lose. The staining was algae - the bleach kills it, the water dilutes the mix, the degreaser is for longevity.
I asked about the solution and they were hesitant to tell me, but I did get out of them that it was not just bleach and water, but the degreaser is what allows them give the 5 year warranty. Don't know what the mix ratio was, couldn't get that out of them.
Talk about roof slope, one neighbor has a 14:12 slope and another has a 20:12 on part of their roof, and another is even steeper on the front section. Talk about a 'water slide' like those kids built.
