A north-facing wall that greens over every winter, a rendered gable freckled with dark moss, brickwork furred along its damp base: sights like these set off a familiar unease that the house itself is being quietly eaten away. In the great run of cases the moss is marking where water lingers, and works no harm of its own. Learning to read that difference tells you whether to fetch a brush or leave the wall in peace.
Why a wall turns green
Moss and its frequent partner, a filmy green alga, take hold on masonry wherever the face stays wet long enough between showers. Shade comes first, which is why the north and east sides green while the sunny south wall stays clean. Anything that keeps the brick damp adds to it: a gutter leaking a streak down the wall, splashback off a path or bed banked against the base, a blocked downpipe, a shrub leaning close enough to stop the surface drying. Rough, thirsty materials hold water far longer than smooth ones, so weathered brick, soft lime render and textured pebbledash green sooner than dense engineering brick or a fresh coat of paint. In effect the moss maps the wall's moisture for you, picking out the slow-drying, shaded zones a damp meter would find.
Does it harm sound masonry?
On sound, well-pointed brick or stone the answer is reassuringly dull. The moss sits on the outer skin and draws nothing from it; its rhizoids, the fine threads that stand in for roots, grip only the surface grain and reach no deeper, so there is no acid eating into the brick and nothing prising the courses apart. Small cushions of wall screw-moss and its relatives, profiled in wall screw-moss, have colonised mortar joints for as long as walls have stood, and firm Victorian brickwork carries them without the least ill effect. The climber that genuinely wrecks masonry is ivy, whose adhesive rootlets work into cracks and can lever a weak joint open; a flat film of moss behaves nothing like it.
Where it does flag trouble
The picture shifts once the mortar or render is already going. Moss will root cheerfully into a soft, sandy joint or a hairline split in render, and once in it holds moisture against the gap, hurrying along the winter freeze-and-thaw crumbling that any wet joint suffers. Here the moss marks a weakness that predates it: a joint mossy enough to pick loose with a fingernail wants repointing whether or not anything green is growing there. Soft lime renders and painted walls are the other places to watch, since a permanently damp, mossed film of paint blisters and peels sooner than a dry one. On any wall, a thick green band low down often signals splashing off the ground or a damp course bridged by heaped soil, a trail worth following back to its start.
The damp-indoors question
Many people worry that a mossy outer wall is drawing wet through to the rooms behind, so it helps to keep the two apart. Moss on the outside does not, by itself, soak the inside of a solid wall; damp indoors has its own origins, chiefly rain driving through failed pointing or condensation forming within a cold room. What the moss reliably tells you is that the outer face stays wet. Where you also meet damp indoors on that same wall, read the moss as a helpful clue and go over the pointing, the guttering and the ground levels, rather than treating the green itself.
Cleaning a wall without harming it
If you want the wall clear for looks or ahead of painting, wait for a dry spell and brush the moss off with a stiff bristle brush, never a wire one, then rinse the last of it down with plain water. Keep a pressure washer well away from soft brick, lime render and pebbledash, since the jet drives water deep into the fabric and can blast the fired face off old bricks, doing lasting harm where the moss did none. A fungicidal wash made for masonry will hold a cleaned wall clear for a season or two, though nothing keeps while the wet remains. The lasting remedy is to help the wall dry: clear the gutters and downpipes so rain stops sheeting down the face, and pull back any soil, mulch or overhanging shrub that keeps it in permanent shade. The same thinking, applied to paths, patios, roofs and decking, is gathered in how to get rid of moss.