I can't tell you the chemistry of why it works, but using Joy, the dishwashing detergent, full strength or very slightly diluted has killed moss and lichen for me on several roofs. I picked up the tip from a woman who was a chemist. Don't rinse it off. just leave it on and the growth will disappear in about 10 days.
I use a piece of old rubber hose slit down the middle to cover the saw's teeth. A heavy duty rubber band holds it on.
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