I use a garden hose to rinse off the lichen. It is time consuming. But $400 for 6 hours of work is still worth it in my eyes. However, it does still take of some granules but, these are the granules that the lichen are rooted to and I'm not sure but, I think they'll end up coming off no matter what because the lichen is rooted to it. I could be wrong. I did a roof once that had lichen and let it sit for about a month and a half and went back (after many rainy days) and it was still there. A lot did come off but, there was a noticable amount left. So I'd assume that it would take 3 or so months to fall off and this is only if you've definitly killed it down to the roots. Otherwise it will just grow back. The trick is before you start rinsing it off, make sure that it is pure white because if it hasn't died completely it is tough to get off. An extra 10 minutes spent applying more chemical will save you an hour of rinsing. Or you can just try to convince the customer that it will damage the shingles if you rinse it off and this is the safest way to do it and still try to get the $400 (or whatever) out of them. Some will go for it. If anyone has ever left it up there dead and gone back to check please comment because if I'm damaging the shingles I'd like to know it. And let me know how long it took to come off and if you could see little black spots where the lichen used to be. It's gonna be a tough sell to tell the customer that i can kill the lichen, it's gonna be more noticeable (pure white) for 3 months and they still need to pay me. But if thats the only way to do it then I can argue that