Those black streaks running down your roof do not mean you need a new roof or that it is getting ready to fail, rather you have Algae on your roof.
Algae produce a dark pigment acting like sunscreen protecting the cells from the sun. As this algae accumulates on your roof shingles, the roof becomes less reflective, causing your house to become hotter and cooling bills to go up.
This organism comes in either spores or clumps of cells on your roof top. The north side of your roof gets less sun and more moisture, which makes it a great place for algae to multiply and grow faster on your roof. Asphalt shingles contain limestone, which makes them heavy, durable and reflective. Roof algae over time will consume the limestone and weaken the shingles.
Roof algae is the first wave of attack on your roof, Algae eventually teams up with fungus to form Lichen. Moss follows next building layers of soil that catches seeds, and bird droppings. By the time grass is growing on your roof, it’s time to get a new roof. Pressure washing can save you money by extending the life of your roof and retaining the beauty and cleanliness of your home.