Chapter 1: Why does moss keep returning to the tiles of our canalside home in Kintbury?
What we find
Moss needs moisture, shade and a surface to grip, and a roof close to the Kennet and Avon Canal and under mature trees offers all three. The north-facing slope, which gets the least sun, usually goes first. Spores settle in the rough surface of a concrete or clay tile, hold water like a sponge, and by the following winter the green layer is thick enough to lift the edges of the tiles and fill the joints between them.
It matters beyond appearance. A mat of moss keeps the tile damp, and on a frosty night that moisture freezes and flakes the face of older clay. Loose moss washes into the gutters and blocks the outlets, so water then backs up under the bottom course. From the garden, look for dark green or black patches, lichen crusts and a roof that looks shaggy along the eaves.
What we fix
We clear moss by hand with brushes and scrapers, then treat the surface with a biocide that goes on killing the growth for months afterwards. We do not pressure wash tiles by default, because a jet can strip the protective surface from concrete and drive water under the laps. We empty the gutters before leaving and check the roof for cracked or slipped tiles.
Cost guides report hand removal plus biocide at about £748 for a small roof, up to £978 for a large one, with £863 in between; your fixed quote follows the survey. Where overhanging branches are the cause, we suggest cutting back, because treatment alone buys a few years in deep shade. Our roof cleaning page explains the methods.