My jury is still out on roofs? I need to do more research. I see guys cleaning with heavy duty amounts of chlorine and I wonder how good that can be, and if any pressure is used the rock gets washed off? Need to talk to more people about it. We have a lot of roofs around here that need it and the general public doesn't understand whats going on with the discoloration. I have been looking at the zinc strip installation as well as we know what we see on the roofs under the vents caps etc. Interesting. Also wonder what insurance premiums are like for companies who have guys walking on roofs with all that slippery chem on it. Like i said much more to research.