these can be difficult jobs and very messy , it would be better if you could use a flat surface cleaner as this wouldnt cause as much mess and get a even finish ,my advice would be use a catladder as the roofs become very slippy when wet also tie the pipe round top of catladder so as not to be struggling with the weight from the floor up, do it in about 6ft sections from top to bottom. on completion gutters windows splashes on walls and floor areas will all need cleaning as you will of made a mess of them. hope this helps and be carefull roofs are lethal.