White Lichen Spots on Concrete Tile roofs

[Karmic]

Hello,

Any ideas in what to add to mixture to remove lichen spots?

 

I typically mix 5-6% SH w/ surfactant, lichen still on roof after few months.

[Apple Roof Cleaning (813) 655-8777]

Try adding some sodium hydroxide, maybe a few gallons to every 30 gallons of mix