roofs

I have a question (as opposed to a problem, for now anyway)...
Here in the lovely state of Virginia...specifically the "Tidewater" area because that is where I currently live...the majority of the roofs have this streaked look...I thought it was due to the gravelly finish being washed off the asphalt shingles. My father-in-law feels it is due to poor ventilation.
My question to you roofing people out there...could poor ventilation (lack/low count of soffet vents and/or ridge vents) cause the streaking I see on 75+% of the roofs in my area?
AND...if vents (soffet/ridge) are critical/important to overall roof life...then WHY don't the local builders install what is best? I've never seen this same type of streaking on roofs up north. I orignally thought it was rain washing off the gravel layer, but now I'm not so sure.
Lynn
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Why do you suppose trees cause this "streaking"? >ransley wrote:

Someday, it'll all be over....
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LFR wrote:

if you are talking about the black roofs with a gray streak on it then its mold on the majority of the roof and the gray steak it the mold that came off the roof due to the rain water picking up some chemicals from the metal fixtures on the roof and killing the mold... my neighbor, well the old neighbor had put a medium gray roof on his house.. later these new neighbors bought the house and their roof it black, with the exception of one spot on the front with is gray due to the metal vent pipe sticking out the roof and the rain water picks up chemicals and it kills the mold in that area and runs down to the gutters and leaves a streak... hope this helps??? if that is what you are talking about...
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Ventilation is very important, but I doubt that's your issue. Algae or mold will grow on roofs. If you notice roofs with metal flashing, you'll see that the mold won't grow in the area underneath the metal due to the ions released by the metal. I installed zinc strips at the peak of my new roof--no streaks after 10 years.
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