I jetwashed our roof (Marley concrete tiles, too) when we moved here 20 years ago. It blocked all the downpipes to a depth of about four feet, and, as they enter the gullies under about a foot of concrete, this was an "interesting" problem (to be fair, a few were well blocked already). So if you do have the roof jetwashed, make sure the contractor (temporarily) blocks the tops of the downpipes and cleans out the gutters before removing the bungs!
Did it work or help? Not much. The lichen came back very fast about 1-2 years But it's pretty and thin, so that didn't matter. The moss has followed again, and now it's probably back where it was. Clean moss-free gutters do allow heavy downpours to run off quickly, whereas there's a risk in the first winter storms that the looser moss will clog the downpipes again, but otherwise I'm not sure there's much point.
The moss does damage the Marley tiles over time. Mainly it lifts the sharp sand coating, and that gets removed from the roof when the moss goes (and that too clogs downpipes). But then a jetwash, or even hard frosts, will do the same.
Copper salts are poisononus to plants, so the copper wire thing ought to work - the carbonic acid in the rain will cause weak copper salt deposition on the roof. How much copper wire you would need I couldn't guess.
The other issue, apart from bright green smears on the roof, is increasing the propensity to lightning strikes. This is probably trivial on a bungalow, but here, in a tall house on a hill, with next door having been recently struck, it's a consideration.
You do get corona discharge from thin wire, which is the most popular theory as to how lightning conductors work, so it might help, but only as long as it's really well earthed by a thick wire on the outside of the building (outside is important).
Personally, I think I"d leave well alone, but I would check the gutters and downpipes about now each year.
E.
