The first thing you can do is - - DO NOT pressure wash the shingles at any time. This will remove the protective granules leaving the underlying membrane exposed to UV Rays which will damage them faster than the algae growing on them.
Now the way to make anything last longer is to implement proper preventive and corrective maintenance procedures. Changing the oil in your car is a preventive maintenance procedure. Getting a tune up could be a corrective maintenance procedure. So it is the same with your home and with the most expensive part of that home, the roof. Proper preventive and corrective measures can increase or at least maintain the lifespan of your roof.
Those black streaks on your roof are often mistaken for mold or fungus but are actually a very hardy algae called Gloeocapsa Magma. The process starts with spores carried by the wind currents being deposited on a roof surface. The algae then attaches itself to your roof and begins feeding. Even though you only have streaks on one side of the house, usually the North side, they are all over the roof surface. They just grow slower on southern facing surfaces. This is why when you have your roof cleaned you need to clean the entire surface.
Without interdiction on your part the algae will eventually start to degrade the integrity of your shingles. About 20 years ago manufacturers started using crushed limestone in the makeup of the shingles and this species of algae actually feed on the limestone. The limestone is a food source allowing the algae to grow unchecked causing the dark streaks that you see.
Take my advice and have your shingles cleaned by a professional. Don't risk your health or worse your life trying to do this on your own, not to mention possible damage to the shingles themselves. Too strong of a chemical mix can do as much damage as pressure washing and too weak is ineffective. With the cost of roof replacement being somewhere in the neighborhood of $5,000 to $15,000 it only make sense to keep it in tip top shape. And what about the danger of working on a roof? Wet roofs become slippery and an inexperienced roof cleaner, such as a typical home owner, are taking unnecessary risks. Before you decide to tackle this problem on your own talk to a roof cleaning professional. They will know exactly what it takes to get your roof clean and have the experience to get the job done.
Visit Roof Cleaning of Clarksville and Nashville for more information