The two main culprits are: moisture and warmth.
Mould, lichen, and moss all love to breed in warm moist environments, and unfortunately your roof makes the ideal breeding ground.
Although the roof is designed to repel water, your roof can become the reason that the moisture is hanging around.
This can be caused by a variety of factors.
Most commonly, foliage in your gutter system.
Likewise, if debris, such as branches fall on your roof and isn’t removed for a long period of time, it can slow the drainage process down.
This in turn can cause excess moisture build up on your roof and this coupled with hot summer days, and you’ve got a recipe for unwelcome guests.
How often you clean your roof will all depend on your surroundings and environment.
A key thing to remember is as soon as it appears, that will be the best time to act on it before it gets too out of hand. The sooner you get on top of removing it, the happier, healthier and safer you and your surfaces will be.