After being suggested by Geneva Point I gave the product a try. We have some quarter size spots on lichen starting on the roof and many small spots of moss. About 10 days ago I sprayed 200 square feet of the roof and now all the lichen and moss in that area is dead.
I had picked-up a half gallon locally for $30, rather expensive even given that you dilute it 5:1 but it works. Today I ordered a full gallon at Amazon.com, $28 plus shipping.
http://www.amazon.com/Wet-Forget-105...8080959&sr=1-1
Once all the lichen and moss is dead, I'll install zinc strips at the peak.
http://www.zincshield.com/home.html