Algae can be killed and the stains removed using chemical cleaners though they usually return over time.
How to remove stains off roof.
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.
Step 7 let dry.
Allow your roof ample time to dry off and then check on it.
Allow the mixture to sit on the shingles for 15 minutes then rinse the roof well with water from the garden hose.
Unless you are on scaffolding or a walkway near the roof you should wash the solution off from the ground.
Rinse from the top of the roof down to avoid re staining the shingles with the dirty water.
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.
There are several products on the market specifically designed to remove algae stains from roofs such as wet forget outdoor.
If you decide to try it apply it shortly after you ve cleaned the roof.