First of all, you should be aware that the black stains you're seeing all over your roof do not consist of dirt, grime, tar, or oil. They actually consist of a hardy type of blue-green algae called Gloeocapsa Magma. If you have any familiarity with algae at all then you know it's a living and feeding organism that only gets worse with time. Ignoring it will not solve the problem. This algae uses the limestone filler in your shingles to feed off of and expand, and eventually your entire roof will be covered with the black stuff unless it is cleaned. Not only are the streaks unsightly, but they will ultimately shorten the lifespan of your roof by hastening the process of granule deterioration. So a professional roof cleaning will boost your curb appeal while saving you money in the long run.
Once you've decided to clean your roof then you'll need to understand the best way to do it. You may have been told by someone in your town or read somewhere on the internet that it's perfectly okay to clean asphalt shingles with high pressure, but this is just plain wrong. I can virtually guarantee that if you climb up on your roof with a power washing wand and blast away at your shingles from close range that you will dislodge countless shingle granules in the process. Yes, you might also remove the algae stains, but if you cut five years off the life of your roof in the process then what's the point? Not only can it cause real damage to your shingles, but it can lead to water penetration in places that water isn't supposed to be. I'm talking about high pressure water intrusion around skylights, chimneys, and flashing. In other words, high pressure roof cleaning can turn into an absolute nightmare.
The fact of the matter is that if you're cleaning a roof the right way then a pressure washer should not even be required! Using the proper mixture of cleaning chemicals will completely eliminate the need for pressure. But not just any chemical solution will do. Only a bleach-based concoction will do the trick the way it needs to be done. Now, someone may have tried to convince you that using chlorine bleach for roof cleaning is bad or will harm your roof, but these are usually the same folks who think it's a good idea to clean a fragile asphalt shingle with 3000 PSI of water pressure! Not exactly the most reliable source, if you ask me. I've actually cleaned hundreds of roofs with a bleach-based solution and I've never once seen any sort of roof or gutter damage as a result. In other words, don't believe the naysayers - they're just trying to sell you something.
The mixture itself is actually very simple. Go to your local pool supply store and purchase some liquid pool shock (10% sodium hypochlorite). Mix this in a large tank with an equal amount of water and add a half cup of liquid laundry detergent if the roof is very steep. Be very careful that the shock does not come into contact with your skin or mucus membranes and if it does rinse immediately. Then simply apply the mixture to your roof and wait. After several minutes you should literally see the black algae melting away before your eyes. After about ten minutes give it a good rinse with the garden hose and then stand back to enjoy your new-looking roof. If there are any stubborn spots you can always give them a second hit of chemical. The only other thing to keep in mind is that any grass or vegetation around the perimeter of the home should be rinsed very well during the cleaning to ensure that the chlorine only kills the algae and not your surrounding landscaping. Be particularly mindful with exotic plants and where downspouts dump out. Keep everything rinsed down and you shouldn't have a problem.
The most important aspect of all this is safety. If you have any doubts at all about your ability to safely navigate your roof or handle the chemicals then please contact a local professional. Almost every major US city now has a few non-pressure professional roof cleaners who'd be happy to take care of your roof stains for a reasonable price. If you absolutely must do it yourself then your list of safety items should include (but is not limited to) gloves, a mask, goggles, high-grip roof shoes, long sleeves and pants, a harness system, and a non-conducting ladder. Roof algae and moss is extremely slippery when wet, so consider yourself warned. A few people die trying to clean their roof every single year - please don't be one of them!
So that's your basic outline for the cleaning of a roof and what all is involved. The science behind it really isn't all that complex, but hopefully now you can see that there is definitely a right way and a wrong way to clean a roof. Do it the right way and you'll be improving your home's beauty while extending the life of your roof. Do it the wrong way and you could blast the life right out of your shingles in a single afternoon of pressure washing. Most importantly, I hope that you'll heed my advice and lay out a safety gameplan before attempting a roof cleaning.