The first thing to do about roof moss is check the gutters and outlets. Moss rarely needs removing unless it is blocking gutters, outlets or other drainage points. A mossy roof does mean the gutters need clearing more often.
Moss grows because the rough surface of tiles traps dirt from rainwater, and fallen leaves add to it. It holds water after rain stops, so the tiles underneath stay damp for longer. The warning sign is thick moss packed between the tiles, which can block the water channels and let rain reach the layer underneath.
Don't reach for a pressure washer. A high-pressure jet can damage the surface of tiles and slates and shorten their life. It can also strip colour from concrete tiles and drive water up under the overlaps. Walking on a roof can crack tiles, and hairline cracks can go unnoticed and get worse in winter. Moss can be lifted gently with a trowel or stiff brush on a dry day, then the tiles treated to slow regrowth. That is not a permanent fix, so the roof needs regular checks.
Acting early keeps rain off the layer under your tiles and saves a cracked tile from a well-meant clean. Call me before anyone climbs up there. I'm Walter at Clandon Roofing Company Ltd, and I do this work across Guildford. I'll tell you straight whether the moss needs to come off, and here is how I handle general roof repairs.
