Roof stains usually first appear on the northern-facing areas of a roof surface, as you’ve no doubt noticed. This is because these areas receive less sunlight in the Northern Hemisphere (it’s the other way around in the Southern Hemisphere, as roof cleaners in Australia could attest to). Sunlight aids in drying out the southern-facing slopes much more quickly after a rain, and since roof algae growth is accelerated by moisture, you can see how sunlight plays a role. Direct UV rays from the sun also aid in retarding the growth of algae on these surfaces.
However, it’s important to understand that even though the sun helps to slow the growth of algae on southern-facing surfaces, it does not stop it. If you have stains beginning to develop on the northern-facing surfaces of your roof, it means that it’s just a matter of time before the entire roof is covered with algae stains. You might not yet see black stains on the southern-facing surfaces, but that doesn’t mean that algae hasn’t already taken root and begun to multiply in these areas.
That’s why it’s a good idea to have the whole roof surface cleaned, even though it may appear that stains are confined to one side of the roof. If you can see it somewhere on the roof, that means it’s everywhere on the roof. You can demand that your roof cleaning contractor only cleans the area where you can actually see black stains, but don’t be surprised when, 12 months later, that area is still clean but the rest of the roof is suddenly covered in now visible black algae.