Roof cleaning and moss removal

Moss is not only a look, and Johnny explains the mechanism on his own site better than most roofers bother to. It takes the north and the shaded slopes first, because that is where a roof stays damp longest after rain, and it grows in the joint between one tile and the next rather than on the face of them. That is the part that matters. As the clump thickens it lifts the tail of the tile above it a few millimetres clear of the one below, and a tile being held open is a tile that lets wind-driven rain past the lap instead of over it. The clump also works as a sponge, holding water against the roof for days after the weather has cleared, so the battens and the felt underneath stay wet in a way nobody designed them to be. In a cold snap that trapped water freezes and takes the surface off the tile with it. Then the whole lot washes down into the gutters in the first heavy autumn rain and blocks the outlets, which puts the water back onto the roof it was meant to be leaving. One problem becomes three, and none of them announce themselves until something shows on a ceiling.