I'm not a roof person, but I think the alleged dangers of moss may depend on the roofing material.

I can't imagine hefty concrete tiles succumbing to any of the risks nik enumerates.

Delicate slates, however - well, I can imagine it. But has anyone actually seen it happen?

Robin's roof appears to be covered in traditional ceramic tiles, which would fall somewhere between the two extremes in terms of moss hazard.

Nik and I agree on the matter of blocked gutters. Damned if you do, damned if you don't (strip the moss). Even on sunny roofs, moss can start growing in the well-watered conditions of the gutter itself. There is really no alternative to regularly checking your gutters and clearing them if necessary.

yes, already done, we talked about it a few months ago