That's an interesting question. I did a demo today for an apartment complex. When I looked at it the other day I didn't see the large patch of lichen on the top corner that was shaded by a tree. I used 4% ch + GW. using three coats ( the tan shingles were almost completely black). After letting the third coat dwell for about 15 minutes I sprayed the lichen with the hose sprayer set to jet. Eighty percent of it washed right off. I have three other jobs pending the results of this one demo so I'm going to go back in a couple days to give the rest of the lichens time to give up their last breath and rinse them off. Other than the lichen, the twenty year old shingles look like they were just nailed down. I'm confident that the property managment company will agree that they don't need to spend $10k or more for a new roof as they orginally thought. I'm curious to hear others views on the hot water rinsing. Personally the water hose worked well for me.