This is a very simple solution that Wendyl came up with a few years ago to kill the green algae-like growths that would grow on the roof of her caravan. This spray also works anywhere you may find unwanted algae growing on fences, roofs, carports etc.
All you need to do is mix 1 part soda ash with 4 parts warm water. Dissolve thoroughly then put in a spray bottle with a good nozzle which will really wet the mould – the mould magic bottle and spray nozzle work really well. Wait for a fine day with no chance of rain and spray onto the algae. Leave to dry and within a few days it will all have turned brown and died. It really works, but if you have an older caravan and don’t want the paint to come off under it you might like to wash it off once the lichen is dead.