When I talk to people about roof cleaning chemicals it seems like most of them tune me out, their eyes glaze over, and their mind wanders to other things. I really wish these folks would pay closer attention because there's a right way and a wrong way to clean a roof, and a lot of it has to do with the kind of chemicals you're using.
Before I even get to talking about solutions and mixtures and ratios let's get something straight. You should never use high pressure to clean a roof. Whenever I see some weekend warrior up on his roof blasting away at his shingles with a rented power washer from Home Depot I just shake my head in disbelief. If a roof can be damaged by hail, wind, and driving wind then what makes you think that it will survive the pounding that a pressure washer will administer? That said, you should ideally find a roof cleaner that cleans without the use of pressure.
Now there are a number of different roof cleaning chemicals that you can buy on the internet and some of them might be worth looking into if you have no other options (just make sure they don't require the use of a power washing wand). Ideally, however, you would use a cleaning mix that is bleach-based. Perhaps you've heard somewhere that bleach is "bad" or will hurt your roof. What a bunch of garbage. You go swimming in your pool, don't you? If you're willing to swim in it and get it in your eyes and on your skin then why are you so scared of getting it on your roof? Roof cleaning bleach is the same chemical as swimming pool bleach and is perfectly safe for your roof and home if handled by a professional. Anybody that tells you otherwise is probably selling you a fad product that requires pressure!
As for the nitty gritty, mix equal parts bleach and water, then throw in a cup of liquid laundry detergent if the roof is steep. Apply to the roof surface, wait several minutes, and rinse gently with the garden hose. That's it! If there are stubborn spots you simply increase the ratio of bleach to water and re-apply. The results should be instant and you've got yourself a brand new looking roof. Just make sure any runoff is diluted and the shrubs and plants will be fine.
As you can see it's not rocket science to clean a roof. If you're using the right chemicals then it's a snap. Just be careful, take your time, and make sure to have a helper. Wet roofs are extremely slippery so take appropriate precautions.
You know I think a lot of people freak out when they hear the term "chemical cleaning", but the fact of the matter is that chemicals are your friend if used intelligently and safely. There are chemicals in your tap water, there are chemicals in your makeup, there are chemicals in your eye drops, and there are chemicals in your skin lotion, so why are you so darn scared to put chemicals on your roof? Trust me that using the right roof cleaning chemicals instead of a pressure washer is the only way to go when it comes to roof stain removal.