The Safer moss and algae killer with the organic OMRI label listing. We have been using this stuff for years and it works better than anything else we have seen on the market so far…. Salts of fatty acids is the active ingredient and it has other ingredients that help it stick to the shingles and work longer. A safe approach, safe for our waterways, fish, gardens, plants and animals. I see too many other companies using high pressure to wash and most of them use zinc sulfate, which is very harmful to the environment. It is cheap though, which is why other companies use it. Kills fish and gets into our waterways, very toxic to breath, bad for plants and animals. We have a much safer and much more effective treatment option. Plus the Safer lasts much longer than the sulfates, because it has a surfactant to help it stick to the roofs and last longer. The zinc does not, it is just a fine powder that rinses off with the first rain.