Home owners may not realize the stains on their roof are actually reducing the life expectancy of it. Moss, lichen and algae can be seen on roofs that do not get enough sunlight to dry them out.
Moss needs shaded, damp areas and only a small amount of sunlight in order to reproduce. While the appearance of moss may seem merely to be a cosmetic issue, it can also be damaging and eventually separate your roof shingles. This can then create moisture under the shingles and work its way through the sheathing and compromise your roof's structure i.e: plywood rotted as seen in attic space.
FYI: If you install a new roof over an existing roof that has moss, lichen or algae on it this can trap in moisture and rot out the sheathing below costing you thousands to correct.
Lichen is a fast damaging combination of algae and fungus that can be extremely successful at growing in difficult conditions. It grows in small areas all over your roof, making it not only look bad, but damaging the condition of your roofing material through the acid it releases, improvements /cleaning needed. Lichen starts off as small circles and spreads, damage can be extensive in a short amount of time.
Algae shows up as a black or green discoloration on the roof shingles usually found with moss or lichen. If left untreated it to will it to eventually destroy the roofing material.
Photos below are on my house. I used a product called "Spray & Forget" which will stop lichen, algae and moss growth. It must be applied when the roof is wet. I’ve found Spray & Forget to be very effective on moss and lichen, algae not so much. Algae takes some time to kill and prevent staining. However, I’ve had good but success with the zinc strips I installed along the ridge, see photo one year later.
If you have these issues consider having a roofer install zinc strips which is a lichen, moss, and algae inhibitor. It works with any wood or composition roof. The natural action of rain water time-releases zinc carbonate, a proven fungistat, which then washes down the roof preventing destructive roof moss, algae and fungus growth.
Be advised, copper and zinc dripping from your roof is bad for the environment. The runoff will find its way into streams and ponds where it is toxic to fish and other wildlife. Metal-infused rainwater is also unsuitable for landscape use.