Always make sure you use a sturdy ladder to climb up safely to the roof top. Use harness because the roof and shingles would become slippery when wet. Put on boots and non-skid soles. Gather all necessary tools needed for the cleaning purpose. Use garden hose to spray the chemicals prepared for cleaning. Last time I had my roof cleaned I sought expert advise from light weight roofing solutions roof lines east inc for expert advise (http://www.rooflines.com/blog/category/our-products/
). They would definitely help you with safe tips for cleaning.