Moss and lichen tends to flourish in rural areas or where trees are nearby and where there are shady, damp conditions. Steeper pitched roofs are less likely to support moss and lichen growth as they shed water more quickly than low pitched roofs. By contrast, north facing and shallow roofs tend to remain damper longer and attract the growth of mosses and lichens.
In normal circumstances, the growths are not harmful to the tiles or slates. However, in some extreme cases this growth may affect the drainage of water down the roof. In such cases, they should be removed to avoid possible water ingress into the roof space. There are several methods for the removal of moss and lichen
If unsure, you should contact the Marley technical department for further details.