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.
How to remove stains off roof.
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.
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.
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.
Allow the mixture to sit on the shingles for 15 minutes then rinse the roof well with water from the garden hose.
Only use a garden hose with normal pressure as using a high pressure washer will strip granules.
Unless you are on scaffolding or a walkway near the roof you should wash the solution off from the ground.
Step 7 let dry.