What causes roof stains

I know you veterans will know this. I live in Florida and have wondered what causes the dark stains common on roofs. They are like streaks but are not visible under vents. It is like the vents shed what ever is causing the stains from getting on the shingles located down the slope from the vents.

Here’s my standard verbiage for this issue:
*One or more sections of the roof covering appear stained. An algae known as Gloeocapsa Magma is the most likely culprit and this algae affects nearly 80 percent of the homes across the United States. Gloeocapsa Magma is a species of algae that causes black streaking and discoloration on asphalt/fiberglass shingles. The black staining you see on many roofs is caused by the life cycle of algae and fungus spores that land on houses via wind or wildlife. While this algae can grow just about anywhere, it prefers humid environments. A preferred food source of this algae is limestone which is used as “filler material” by most shingle manufactures. Higher quality shingles are manufactured with preventative measures such as copper or zinc containing granules. Several methods exist to prevent and clean infected areas. Installing zinc or copper strips near the roof ridge can prevent further algae growth. Application of bleach (non-chlorine) can aid in removing the stains, as well as many available commercial cleaning products. Some products may harm vegetation beneath roof eaves or near downspout extensions. While many remedies can be performed by the home owner, we recommend the use of qualified professionals due to the extreme danger and risk of injury or death associated to roof repairs and cleaning. *

If the vents are made from Galvanised this will stop mould from growing . In sever areas here they put down a zink strip at the edghe of the first row of shingles and this stops it .
Lots of info here .http://www.ronhungarter.com/
Roy Cooke

Gloeocapsa magma (a very hardy algae). The zinc in the metal “through the roof” vents, etc kills the algae. The dark stains and streaks you are seeing is mostly the dead algae cells. Harmless for the most part except it darkens the roof, causing more absorbsion of heat, etc. etc. Durashield http://www.durashield.com/ is one product used to get rid of it and preventing its return for a short time.


Thanks folks,
That solves the mistry.


Went to the Hungarter site and was dismayed he has not been involved in upgrading his Building Science. Not one mention about airsealing the attic to stop air and water vapour leakage from the house.

Adding vents to take care of attic moisture, improve shingle life, reduce ice dams, reduce air conditioning bills are all old hat. Research from the last 20-30 years has dispelled some widely held “facts”.

For example: The 1 to 300 rule for attic vent requirements is a code regulation here but has been disproven by some historical and recent research. The rule came out of research on vapour barriers (not attic ventilation) at Penn State University in 1947-8. It was never tested in a scientific manner until recently and has been found to be lacking. As stated in a 1993 article titled “The Mysterious Origins of the 1:300 Rule” (in Energy Design Update- a pricey newsletter from Vermont; US $385/year or $1.50/page):

“After an extensive search for the roots of the 1:300 rule, (building science researcher Bill) Rose found it was hastily derived from a single lab experiment on a flat roof. The vent-to-roof ratio, 1:300, appears to be merely an incidental detail peculiar to the test setup. For want of a better number, the 1:300 ratio was subsequently adopted as good building practice and was eventually added to building codes.”

A good piece from the CMHC About Your House series is “Attic Venting, Attic Moisture and Ice Dams”
See http://www.cmhc.ca/en/co/maho/gemare/gemare_001.cfm

I’ve been working in energy efficiency and conservation since 1977 and feel that the boat is being missed regularly on the attic issues since most of the trades are not required to re-train or get CEU’s to maintian their licenses or contractor status. Many guys like Hungarter, Shel Busey in BC and Ren Molnar and Mike Holmes get on radio/TV shows not because of their in depth building knowledge but because of having a show idea first or for their personalities.

Brian MacNeish

Thanks Brian I have not been to his site in a while .
The reason I used him is because he was the first one I know of who talked about zink to stop moss .
Got to love NACHI so many great ideas come from all .
Thanks again . Roy Cooke

NO THOSE STRIPS DO NOT,NOT stop it!!! I nailed down my 1st shingles 41 years ago,as a child(age 6);wired my 1st 200amp home breaker panel @ age 12; And I will tell you NOW, the zinc strips,etc. do NOT WORK!!! Everything is fine at first(2,extreme maybe 4 years) then nature rules!! Cleansers are only temporary bandaids(NEVER powerwash roof shingles; u destroy granules),therefore, we need to find out what has changed in our atmosphere making this such a massive problem now, when it was all but a rarity just a few 20 years ago!!! What IS different insideour weather patterns?? THINK: is it logical that it’s the shingle manufacturers using too much limerisking reputation,hence killing sales??? I think we all as consumers deserve the right to know;mind you,we all breathe said air.

Zinc leaching out of the lead boots.

David Glad to see you visit NACHI , Please visit a little more often .
This string is 4 years old and you are the first to say ZINK does not work . Well sorry I disagree and from what I have seen ZINK does work .


Hmmm… seems someone has their own “agenda” here.

A “rarity” 20 years ago? Right. When’s the last time you heard the term “acid rain” on the news… 20-35 years ago!


Roy , I don’t if zink will work , but I’m sure zinc will . :D:D:D

David Atkins,

You’ve got to hit the books buddy as you’ve got some additional information that you’ve never learned.

Zinc absolutely works and I would like to show you a few pics where zinc flashing did not allow black lichen to build up on this particular roof…

Absolutely correct Mr. Valley. Zinc ions are quite effective at killing those little algae thingies. Some of the new roofing materials have zinc powder or salts and the “old” zinc strips that were nailed at the ridge work very nicely.

David Atkins,

You’re awful quite. What happened? No comeback?

You mean Like this? Zinc from flashing inhibits the growth of algae. If you want to learn your stuff, get ready for the release of the first of the InterNACHI Certified Roofing Contractor videos.

If you complete this roof instruction series, no one you deal with will know more than you.

OK, here is what happening with roofs stains. First, I am a certified roof cleaner and I am always searching online for companies and articles about roof cleaning. I saw this article and had to respond because of all the misconceptions associated with roof stains. The algae grows from a combination of moisture and a limestone filler found in most asphalt shingles. The reason it is predominately found on the north side of a roof is because after heavy rains or dew, that side of the roof does not get a chance to dry out as much as the other sides. Same reason you find moss growing on the north side of a tree. The moisture mixed with the limestone filler creates a perfect environment for mold and algae growth. Yes, zinc does help SLOW down the the mold and algae growth(gloeocapsa magma). But it does not completely stop the problem from occurring. The main reason that there are no stains underneath vents, ducts, chimneys, etc. is because when the water runs down the roof it is redirected around these objects. These sections of roof do not constantly have moisture on them, so they take a lot longer for these stains to appear. I have cleaned numerous roofs with expensive zinc strips on them. I have cleaned roof with 6 year old zinc strips and also ones that took 10 years to appear, but yes, eventually mother nature wins the war. I have even cleaned newer roofs with cooper and zinc granules, it is all a matter of time.
These stains are not caused from jet fuel(epa and faa would be all over that, and it would also be all over everything you owned that sat outside). They are caused by pollution or chemical factories or anything else you may have heard. Simply moisture and shade is the suspect. Not all the times is it a limestone filler, such as in tile roofs (mostly in Florida). I have cleaned an 80 year old tile roof that had moss and algae growth on it, mostly were under dormers and overhangs where there is always moisture and shade.
All cleaning is done by applying a solution recommended by ARMA and also GAF, and yes it is a chlorinated bleach solution. Using only NON pressure methods. This is what the companies who make and oversee the production of these shingles recommend, so I WILL NOT believe what anyone else says to clean these roofs with. Yes, this solution can damage plants but if it didn’t then it would not kill the living organism growing on the roof(just common sense) if it won’t kill your plants then it won’t kill a plant growing on your roof. These is where being a professional comes into play and knowing the correct procedures. Here is the link to the ARMA bulletin.

I am not trying to step on toes, disrespect, or argue with anyone here, just figured I put some myths to rest and try to get some facts to everyone so they can be further educated. Hope this helps.

Shawn Holderman Sr.
Prestige Pressure Washing & Roof Cleaning

IMHO, the main reason is that there is much more zinc flashing or copper vent stack exposed at these areas (than 1 to 2 inches of zinc/copper purposely installed and protruding from a ridge cap)…thus much more zinc/copper released to inhibit the growth. Rain does hit the areas below these penetrations and condensation as night dew also wets these areas, so there is lots of moisture available to encourage growth.

There’s enough area exposed below the vents that zinc from the flashing has to be a major contributor to inhibiting growth. Look below and beside the chimney. Those areas get as wet as and dry as fast as other areas on the same slope. The eves have more growth, so I expect that heat from the living space is helping to dry the main roof. This home is north of San Fransisco about 15 miles. It’s cool and foggy there in the summer.