Blame it on the customer for not having gutters! LOL!
Gotta love the houses in NC, almost all of them have gutters!
"Mr Customer, I can clean you roof with pressure, or w/o.
IF I use pressure, there will be no effect on plants, but your roof will be damaged.
If I clean your roof w/o using pressure, as your shingle manufacturer recommends, there is a slight chance of an effect on the plants/grass.
We cover, water, and do everything in our power to prevent this. "
This is, IMHO, a good way to sell roof cleaning.
They will pick the non pressure way everytime.
IF you run into some yo yo "guaranteeing" not to damage the plants, they are either pressure cleaning, using sodium hydroxide, or desperate.
When I am asked if my chemical kills plants, I proudly answer yes!
If it can't kill plants, how can it kill the plant growing on your roof ?