An algicide or abicide sprayed on from the top downwards will kill off whats there now......watch for the run off though.....however I have investigated this and come up with a "revolutionary new way" to treat this problem...
I`m not a scientist and whether it works is yet to be seen but apparrently if you fix a half inch wide length of copper wire all along the ridge tiles, the resultant oxidation of the copper causes a mild solution of copper oxide which I`m told will wash down the roof when it rains and will keep on top of the spores as they start to grow. Apparently moss / algae & lichen hate copper solutions and will not grow.
Now don`t ask me where I found that one out `cos I can`t remember....I`ll try and hunt it down again though. :
That would be a system similar to Copper Ridge?Popular in Scotland?
Great up to 10 ft from the actual strip,not so good further away as dilution of oxides is much to sparse to be of any use.Also when homeowners see the strip turn green from oxidisation can be funny about the effect on the appearance of their home!(one likened it to turning her house into a freight train)
For a simple test on the effect of oxides on these type of problems look no further than lead flashings under windows and the lack of algae growing beneath.
Algae adapts to living on roof tiles as the original fired glaze degrades over time leaving a rougher surface for root growth.Most of the time algae has migrated from nearby trees.
Zinc Sulphate Monohydrate is soluable in water and although costly is effective as an algaecide.So to are Copper Sulphates but you must beware of staining existing brickwork with these minerals.
If in doubt,or you have any further questions shoot me an e-mail
Kevin
Aquatechnique