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Streaks on roof surface??


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It's algae. It will usually appear on the side of a roof that slopes away from the sun. Here in Washington, that is the East and South sides, but we also get it on the other sides when they are shaded. UV kills algae, so it is rare to see it on the sunny sides of a roof. It's a fungal organism and secretes oxalic acid to eat, so it is mildly corrosive and can, in some instances, cause accelerated loss of granules.

Removal is simple. For every 10 squares that need cleaning mix up a batch of cleaning solution. On an overcast day mist the roof with clear water and then apply the solution and allow it to work about 30 minutes and then rinse it off with clear water at low pressure. Never use a pressure washer! Make sure to wet down and temporarily cover with plastic any vegetation near the eaves, because the overspray will kill it.

Solution (Can treat 1000 sf): 1 part liquid swimming pool chlorine (sodium hypochlorite), 4 parts water and 1/8 part non-ammoniated liquid dishwashing detergent.



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For awhile some were touting the use of zinc strips which were installed every so many (I don't remember how many) courses. Someone noticed the area downslope from the lead plumbing vent stack flashing was free of algae. By process and elimination they settled on zinc as the component of the flashing which was responsible.


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Originally posted by jak531

What are the streaks that I see quite often on a common aspalt shingled roof? I've asked roofers and got different answers. This can be in the woods or in an open field condition is the same. Just wondering if you fellas know what causes this. Thanks

It's algea. Harmless, and it does grow in the sun. It's more prevelant since the formulation of the minerals in the shingles was changed by adding more phosphate. Algea likes that. Some manufacturers are adding copper particles to the granules to prevent algea. Ownens Corning AR shingles are Algea Resistant.

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Originally posted by bradfeldt

Although shingle manufactures are coming out with algea resistant products, they are pretty much useless. Read the fine print on the warranty and you will find they only warranty the algae resistance for 10-12 years - about the same time you really start to see the staining.


If they're "coming out" with algae-resistant products, how do you know they're useless (regardless of the warranty, which doesn't time- test anything)?

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