I am going to explain things here you are not likely to find in one place about roof cleaning. I am going to explain the reasons why a roof gets dirty, how Moss and Lichens form and how to stop it from happening! Firstly let's talk about habitat. It's very important for any living organism to find a place to thrive and survive getting the right amount of water and light and your roof is perfect which is why biocide is needed to help prevent it from coming back!. Moss is not Lichen but they both dwell in similar places. A roof tile is a perfect home for them both, they will grow best on concrete tiles due to its soft nature and porous surface being eaten away by the moss roots slowly destroying the roof tile. The harder and smoother the surface generally means the roof will be cleaner for longer without intervention. Slate tiles and clay tiles perform best at this. Unless they are being covered in a nearby tree sap that will coat them in all the macro nutrients required for growth. There are over 13,000 different types of moss and over 18,000 types of lichen including Common Sheet Moss (Hypnum), Ross Moss Cap (Dicranum), and specialised Granite Mosses (Andreaea). Moss.  Crustose, Foliose, Fruticose and Scamulose are the main types of Lichen we have to mainly deal with in roof cleaning.