How can I prevent black mold from forming on my composition shingles?

Commonly found predominately on tree covered or the west and northern slopes of roofs along the gulf coast, blue green algae is very tough to treat.  During a new roof installation it is important to select an Algae/Fungus guard feature that is factory applied. The manufacturer will install a variable ratio of zinc or copper coated granules in the color granule blend.  Most manufacturers will warrant against algae/fungus growth for 10 years as it is a living organism that adapts to its environment. Some manufacturers, have a coating designed to weather over time that will change the ratio of zinc/copper exposure and will warrant against algae growth for an extended period of time.  This new technology is relatively new (5-6 years) and the verdict is still out.

Zinc strips installed under the top course of shingles is a time tested method as well. This will typically lead to streaking at the lower sections of the roof.  Due to these inconsistencies we do not install.

Treating after the fact can be done with a 3-1 water to 3% bleach ratio. This can be sprayed on the roof with a manual pump sprayer.  It is important to wet plants surrounding the area to be treated.  We recommend this be done by a roofing professional as bleach can be very slippery and when combined with the height of most roofs can propose a serious fall hazard.  This is a temporary treatment that will kill the blue green algae for a couple of years. There are other products available on the market that we do not have adequate history with to comfortably recommend.