Okay. So, you must wash your roof after every 12 to 24 months. Initially, you have to spray the roof cleaner often like after 2-3 months. But when the mold and grease start to loosen up then you can ensure that your rood is clean and free from further growth other than, it also depends on your roof i.e., if at any time you feel a mild discolouration, moss, mold, or mildew just spray it and give your roof a good start.