If the roof covering is of slate either Caithness or Welsh/Spanish/Chinese/Canadian...there should be no problem when cleaning as long as the cleaning action is from the top down thus preventing moss/soil etc from being pushed up between the slate gaps (as this is for roof drainage)... if they are of a clay tile type there may be damage beneath any moss as they can get a little flakey...again in a clean down action... Concrete tiles loose their resin coating after few weathering years which allows a key for moss growth...the moss roots can work their way through the porous concrete and start to drip in wet weather onto the felt often found underneath... if there is no under-felt this can cause damp patches in the building beneath it...algae also loves concrete tiles creating a further key for moss... the more the roof is sheltered from the sun the more the moss will grow... most roof covering in the last 50 years tend to have a felt based underlay...this is a bitumen bonded pressed paper fabric with strands, often nylon running through it to add strength when laying... Modern under-slaters felt tends to be of a breathable material which is lighter and covers faster... if either of these are under the roof covering there is more protection from the elements as well as any pressure washing you may be considering... Although cleaning and painting the roof is a modern idea, it has problems as it often seals any gaps left for water drainage as well as adding the potential for the roof to sweat causing timber decay in the roofs structure...
A copper wire/strand/strip is a slow method of moss removal as it needs the weather to make it oxidise. It is this oxidisation, when washed down the roof that poisons the moss via its roots...Hope this helps...Angel...
I'm not perfect, which is a shame really, as everyone else is!
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