1. Check the specs; papers, please
Read any paperwork that came with your shingles concerning their warranty. You want to know how the manufacturer recommends that you clean its product. If you don't have the paperwork, you can frequently get it online at the manufacturer's website.
2. Careful with that pressure washer
Most manufacturers will void the warranty on their shingles if they discover you used a pressure washer on your roof. They're afraid that if an untrained person gets the cleaning wand too close to the surface, it will blast away the ceramic granules that provide protection to the tender shingle below.
3. Stay away from chlorine cleaners
Clean your roof with just regular soap and water and a scrub brush. Some people use chlorine bleach and water, but chlorine bleach is highly toxic to any vegetation that surrounds the home. Any chlorine solution that drips or that is rinsed off the roof can kill landscaping.
4. Overcast days best for cleaning
It's best to clean a roof when it's cool and the sky is overcast. Working on a sunny day will cause the cleaning solution to evaporate in seconds or a few minutes. Spray solution on the roof working in strips from top to bottom of the roof. Each strip you clean should only be 4 feet wide. Do not stand on a wet part of the roof as you clean. Always be on a dry section, as the cleaning solution is slippery. Consider wearing a fall-protection harness in case something goes wrong.
Once the roof is clean, you can prevent the algae from coming back by installing a 3-inch strip of copper at the top of the roof. Every time it rains, a tiny amount of copper will wash down on the roof. Copper is a natural biocide, and algae will not grow in its presence. Zinc works, too, as does lead, but the copper is more powerful.