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Use a long-handled brush to scour the surface of the shingles and loosen stuck-on mold while it's dry and brittle. Focus your attention on the heaviest growth and hard-to-reach areas where the shingles overlap—the chemical treatment will take care of the rest. Make sure you’re wearing your respirator during this phase to avoid breathing the mold particles that will be floating around. To get rid of mold and other types of growth for good, it’s best to use a powerful chemical agent like chlorine bleach. You can whip up your own basic cleaner by combining one quart of bleach, one gallon of water, and ¼ cup (60ml) of trisodium phosphate (TSP). For maximum coverage, use a pump sprayer to disperse the solution.  The chlorine will kill the mold-causing bacteria while the TSP will help remove stains and discoloration. Never combine bleach with any product containing ammonia. This can produce toxic chlorine gas which can be extremely hazardous to breathe.  If you’d rather minimize your handling of chemicals, keep an eye out for preformulated treatments such as Wet & Forget or Moss Out at your local home improvement center. Start on lowest row of shingles and work your way backwards to the upper part of the roof. This will prevent the solution from draining in your direction. Continue spraying until you see runoff, then let the chemicals sit for 15-20 minutes to soak in and begin attacking the mold.  Though it may seem like common sense, using a pressure washer is not advisable. The force they generate is enough to damage delicate shingles.  The best time to clean your roof is during cool or cloudy conditions, when the solution won’t evaporate before it’s had a chance to be effective. Just make sure the forecast doesn't call for rain—a heavy downpour could wash away all your hard work. Empty the sprayer and fill it back up with fresh water, or snake the garden hose around to where you are. Sweep the stream over every part of the roof that you just sprayed to wash away the concentrated chemicals. Any remaining traces will dry naturally in a few hours. If not properly rinsed, chemicals like bleach and lye can cause permanent damage or discoloration.

Summary:
Scrub away the thickest buildup. Mix up your cleaning solution. Spray the entire roof thoroughly. Rinse the shingles.