Moss growth often occurs on roof slopes, particularly where they may be shaded by a tree or another property meaning they do not dry out fully in the sunshine.
It is difficult to eradicate moss fully and you should be aware that it does tend to fall away and block valleys, gutters and downpipes, thus increasing the maintenance requirements to these areas. It should be cleared with a soft brush periodically to allow better rainwater drainage, although it does not in itself cause much damage.