Black streaks?

always look at other roofs when passenger in a car. and notice that some have black streaks going down them,even with archs. So why is it that this happens?


algae, is it from trees or improper ventilation.

It does come from the trees and air born from other sources.
It’s not from ventilation or lack of.

its no different from anything growing anywhere.

moisture, and shade are usually the two factors. you wont see much of it on a house in the middle of the desert.

It’s on about 8/10 houses here humidity pays a big roll.

thanks guys was curious about that