Things You'll Need
Chlorine bleach or copper sulfate crystals
Ladders or portable scaffolding
Hose and fresh water
Black stains on your shingles could be mildew or just dirt. One way to determine if it's mildew is to put a few drops of chlorine bleach on to the black mark. If it's dirt, the dark stain will remain, but if it's mildew the stain will go away. A few years ago, commercial roof-cleaning products containing hydrogen peroxide (lye) were commonly recommended to remove mildew. These products have been shown to damage shingles.
Don't use a pressure washer to clean or rinse your roof. It will remove surface granules from asphalt shingles, shortening their useful life (and may even void your shingle warranty). Be careful. Working on or near a roof is dangerous, particularity when it's wet and slippery.
How to Clean Mold Off a Roof. Mold or mildew makes black ugly streaks on a roof. Not only is it unattractive, it can actually shorten the life of your shingles, so you really should get rid of it. There are a number of commercial products available that will get rid of mold, but if you don't mind working at heights, getting mold off roof shingles is really something you can do yourself.
Chlorine Bleach Solution
Spray all the plants growing near your house with water, and this will help protect them from any damage caused by the chlorine bleach solution.
Mix 1 part bleach to 3 parts water and apply to the mildew-covered area with a hand pump sprayer. Adding some tri-sodium phosphate (TSP) will help clean the roof at the same time.
Let the solution sit for 15 minutes, then rinse it off with a hose. Check to see if any mildew remains on the roof. If so, it will feel slimy when you run your hand over the surface). If mildew is still present, allow the roof to dry and repeat the process.
Always rinse the roof after applying the chlorine bleach solution. If you leave the bleach solution on the roof, it could damage the shingles.
Copper Sulfate Solution (Alternative)
Spray all the plants growing near your house with water, as this will help protect them from any damage caused by the copper sulfate solution.
Prepare a mixture of copper sulfate and water, using a ratio of 12 ounces (dry measure) of copper sulfate to 1 gallon of warm water.
Apply the solution to the mildew-covered area and wait for the mildew to turn brown.
Sweep the mildew off the roof using a push broom.
Rinse the roof with a hose and clean water.
Preventing Mildew From Forming
Removing overhanging tree branches and allowing sunshine and air movement over your roof. This can eliminate the mildew-friendly environment and stop it from growing.
Installing zinc strips under a row of shingles near the ridge of the roof will prevent mildew from forming. The mild chemical reaction caused by rainwater coming in contact with the zinc inhibits the growth of mold and mildew.
Applying asphalt roofing shingles containing a copper additive in their surface granules is an option. The copper granules in combination with rainwater have a mild chemical reaction that prevents mold and mildew.
Spray-on commercial products that you apply and leave on are also available. Annual application reportedly blocks the growth of mildew.