Moss is an unsightly and damaging problem to concrete tiled roofs, especially if your tiles are over 20 years old. The problem is that moss retains rainwater like a sponge, drawing dampness through already porous roof tiles, not only causing wet rot, but additional weight stress to your roof timbers.
Moss roots grow into concrete tiles taking in water which freezes in low temperatures - ice expands and the tile crumbles leaving a bullet-like hole for rainwater to pour through.
In my experience, people have their roof power-washed and leave it at that - this is a mistake as it still leaves the roof wide open to the elements.
After a power-wash, damaged tiles should be replaced and cement pointing re-done and then, most important of all, sprayed with Algon anti fungal solution to protect tiles.