There are several products on the market specifically designed to remove algae stains from roofs such as wet forget outdoor.
How to remove stains off roof.
The second method is to spray on a coating of stain blocking solution defy stain blocker for roofs is one product but there are other brands as well a stain blocking product can buy you up to three years of protection from algae.
Rinse from the top of the roof down to avoid re staining the shingles with the dirty water.
If you decide to try it apply it shortly after you ve cleaned the roof.
If there are no more stains on the roof you re done.
A mixture of trisodium phosphate tsp bleach and water will also remove stains.
Allow your roof ample time to dry off and then check on it.
Set the nozzle at the highest pressure setting and hold it 12 inches from the shingles.
Only use a garden hose with normal pressure as using a high pressure washer will strip granules.
Allow the mixture to sit on the shingles for 15 minutes then rinse the roof well with water from the garden hose.
Use a cleaner specially made for killing algae or by spraying a mixture of one part bleach to four parts water with tsp trisodium phosphate or other detergent added in a garden pump up sprayer.