Chlorine Bleach

• Chlorine bleach, another name for household bleach, has highly corrosive properties, so it needs to be handled cautiously and reserved for emergency situations such as especially difficult stains. This Old House recommends mixing 1 part bleach with 2 parts water. Spray or pour the bleach conservatively onto the stain, wait 10 to 15 minutes, scrub the stain with a firm brush and rinse. If you have plants or grass below, water them first to protect them in the event that bleach drips onto them.
  
  

Oxygen Bleach

• Oxygen bleach is a safer, natural chlorine bleach alternative. While chlorine bleach contains the active ingredient sodium hypochlorite (corrosive and potentially toxic), oxygen bleach contains sodium perborate (natural and color-safe). Home improvement expert Tim Carter highly recommends the use of oxygen bleach on stained roofs because it won't discolor the shingles. Since oxygen bleach is not as powerful as chlorine bleach, it may not work as effectively on stubborn stains but will work wonders on mild to moderate stains. Spray it or pour it onto the stain, allow it to sit for about 30 minutes, scrub with a firm brush and rinse.
  
  

Pressure-Washing Detergent

• A pressure washer filled with a compatible pressure-washing detergent can also remove stains from roofs. Exercise extreme caution; if the spray is too strong, you can chip or damage your roof shingles. For this reason, some experts advise against pressure washing altogether. Nevertheless, it can prove an effective stain removal method as long as you apply the detergent using a light rinse. Only attempt this method if you have extensive knowledge and experience with pressure washing.
  
  

White Vinegar

• White vinegar functions as an effective household cleaner because the acetic acid dissolves stains and disinfects. For this reason, vinegar can prove especially useful on a roof plagued by mold, mildew, moss or algae stains. White vinegar can break down all of these organisms. Just pour it undiluted into a spray bottle and spray the vinegar directly onto the stain. After 15 minutes, scrub the stain with a firm brush and rinse.
  
  

Commercial Roof Cleaners

• If you visit your local home improvement store, you can find a variety of liquid products specially designated for cleaning roof shingles. If you want to find a product that will effectively rid your roof of difficult stains, note the ingredients on the label and look for a roof-cleaning product that contains bleach. Apply it to your roof, scrub and and rinse according to the manufacturer's instructions.