The stains on your roof mean something depending on the color they are. Recognizing which color matches which problem helps with how to solve it. Please don't apply the same solution to all different colors of stains on your roof.
The different colors of roof stains are: black or brown stains are caused by black algae. The black algae feed off of the calcium carbonate or powdered limestone that is in the asphalt shingles. Black algae growth used to only occur around warm and humid locations, but has spread north because the asphalt shingle manufacturers use more and more limestone as a filler material in asphalt shingles.
Limestone is cheaper than other materials and helps with asphalt shingle durability, but the calcium carbonate in the limestone multiplies the algae growth. If you have algae-resistant shingles, then they have zinc or copper granules mixed in. When it rains, it activates the algae resistant properties in the copper and zinc.
The green algae gets on the roof from the near-by tree branches. Keep trees trimmed away from your roof as they can cause the biggest risk of damage to your roof during storms.
To clean your roof, begin by mixing powdered oxygen bleach with water. Oxygen bleach is safe for your asphalt shingles because it doesn't discolor them.
You'll need a pump sprayer. Don't use high pressure on your roof because high pressure will create granule loss on your shingles. The granules are the first layer of defense and an important structure of your roof.
Keep spraying for about 10 to 15 minutes on each area of your roof. The oxygen bleach attacks the roof algae and will remove the stains. Get a scrub brush and lightly scrub the asphalt shingles. Don't press too hard on the brush because you want to keep the granules intact.
Once the algae on your asphalt shingles is gone, use plain water to hose your roof and remove the oxygen bleach. You might need multiple applications of this algae cleaning roof system before your roof is rid of all of the algae completely.
I would suggest to avoid the hottest part of the days to clean your roof. Excess heat will cause the oxygen bleach to evaporate. Early mornings or late evenings are the best time to clean your roof.
Cleaning your roof yourself is dangerous business. If you insist on DIY, then please be careful with your ladder placement. Your gutters are vulnerable to scratching (from the ladder) or breaking. One thing is for sure, don't climb a ladder without the Lock Jaw. Buy two (one for each side). The Lock Jaw holds the ladder in place so the ladder doesn't move side-to-side.
The Lock Jaw is an expensive gadget, but well worth it if you care about your safety. We ordered ours from Amazon, but you can buy them in so many more places.