If you live in hot and humid areas and/or your home doesn't get a lot of sunlight, your roof is more likely to develop algae. Algae growth is generally only a cosmetic issue; the streaks of discoloration on your roof typically won't do more than make your home look shabby and unkempt. However, the algae can attract other organisms that do harm your roof shingles, such as mold. Luckily, there are two fairly easy things you can do to prevent algae from growing on your roof.
Install Copper or Zinc Plating
Copper and zinc are two metals that are toxic to algae. The mineral binds to the algae and damages its cell walls, causing the algae to die as a result. This is why many shingles are manufactured with bits of copper embedded in them to help stave off both algae and mold. Unfortunately, the copper gradually dissipates over time (usually about a decade or so) due to environmental friction (e.g. rain, snow), until there is little to none left to protect your roof from algae growth.
If you notice algae growing on your roof, that means either the copper algaecide has worn off or your shingles never had it to begin with. One way to fix this issue is to install small strips of copper or zinc on the roof in strategic places. This will help sterilize your roof each time it rains by distributing the ions in the metals all over the surface and inhibiting algae growth.
Treat with Algaecide
The other option is to treat your roof with an algaecide. These chemical treatments kill existing algae and helps prevent its return for a period of time. Generally, you would attach a bottle to a regular hose or power washer and spray it onto the roof to apply.
This method has the benefit of easy application. Be aware, though, that you typically have to reapply the chemical on a regular basis to maintain the protection it provides. Additionally, the chemicals in the algaecide may be harmful to plants. So you may want to avoid using it if you have flowerbeds or other flora that sit directly underneath the roof because they may be negatively impacted by wate runoff.
There are other things you can do to prevent algae from forming on your roof. Contact a local company that provides roofing repair and cleaning services or discover more here.