Chlorine Bleach

• Chlorine bleach removes algae discolorations from the surface of the roof and rinses quickly. Bleach will lighten a stain enough to make it invisible, but it will not remove the root of the algae. One must be very careful when using chlorine, as it can cause damage if caution is not used.
  
  Place bleach in a garden pump sprayer and apply only to the areas affected with stains. Be certain it is not spilled onto surrounding vegetation, as it will kill instantly. Let it sit for about 10 minutes, then rinse with a water hose. If stains are not removed, let the area dry completely and repeat the process. Using chlorine bleach is a quick, temporary method to remove stains but is not permanent. Most stains will reappear in six to 12 months.
  
  

Sodium Hydroxide

• Sodium hydroxide tends to be safer than bleach when used properly and removes algae colonies. The likelihood of vegetation being ruined after being washed off the roof is not as great as with bleach, but the product is harder to rinse and requires more setting time. Sodium hydroxide is applied with a garden pump sprayer and rinsed with a water hose.
  
  As with bleach, apply only to stained areas, and let this sit for about 10 minutes, then rinse. This roof cleaner can be used on metal, tile, concrete or slate roofs. Be careful when dealing with asphalt shingle roofs, as sodium hydroxide can cause serious damage.
  
  

Hire A Pro

• If the thought of walking around on the roof of your home frightens you, you're in good company. Many homeowners choose to hire a professional roof cleaner to get rid of unsightly stains. First, you need to look for certain things. One, is the cleaner insured? The company must have sufficient liability coverage because roofs are not cheap to replace if something goes wrong. You may end up stuck with the cost of a roof replacement or in court if your roof cleaner has little or no insurance. Check the Better Business Bureau website to see if the company is listed and what its rating is. Also, check to see if it has a website and testimonials available.
  
  Be certain it uses chemical treatments instead of just power washing. The high-pressure nature of power washing can cause severe damage to shingles.
  
  Also find out if the contractor does this full or part time. Be wary if this is only a side business for someone part time. A full-time roof cleaner will likely have more experience and be more inclined to do a better job.