Most moss on a building is harmless, and some of it is worth keeping; a gutter is where that stops being true. A channel packed with moss and rotted leaves cannot carry water away, so the water it should be shifting runs down the wall instead. The growth itself is slow and undramatic. The damage it sets up, over a wet winter or two, is neither.
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Why a gutter fills with moss
A gutter gathers everything the roof sheds and holds it in a shaded, permanently damp trough, which is close to a moss's idea of paradise. Grit and mineral granules wash down off the tiles, leaves and seeds drop in from above, and the mixture settles into a moist compost lying in the bottom of the channel. Spores and windborne algae root in that silt within a season. North-facing runs, and any length kept in shadow by a tree or a taller wall, stay wet longest and green over first. Once a cushion takes hold it behaves like a sponge, pressing water against the metal or plastic and catching yet more debris, so a little moss becomes a lot faster than you would expect.
What a blocked gutter costs
The moss is not really the trouble; the dam it builds is. Water backing up behind a plug of moss and leaf litter has to go somewhere, and it goes over the front lip or over the back. Over the front you get the tell-tale curtain of drips down the face of the house, staining render and keeping the wall wet enough that moss then starts creeping up the brickwork itself, as the page on moss on walls describes. Over the back is worse, because water spilling behind the gutter runs onto the fascia board and soffit, and softwood there rots unseen until the paint blisters or the board begins to sag. In a hard freeze a full gutter turns to a block of ice, heavy enough to drag the brackets off the fascia or lift the tiles at the eaves. A downpipe choked at its outlet is the most frequent single fault, since everything the channel carries is funnelled to that one narrow point.
Where the moss is coming from
Clearing the gutter without looking uphill only ever treats the symptom. Most of what fills a gutter has come off the roof, and a tiled roof already thick with moss sheds a steady fall of loosened cushions and gritty granules every time it wets and dries. If you find yourself scooping the same green sludge out twice a year, the roof above is usually the culprit, and keeping that growth in check is the subject of moss on roofs. Overhanging trees are the other dependable supplier, dropping leaves each autumn and a fine drizzle of twigs and seeds the rest of the year.
Clearing it without coming to harm
This is ladder work, and the ladder deserves more thought than the moss. Foot it on firm level ground, tie it off or have someone hold it, and move it along the wall rather than stretching sideways from the top rungs. Choose a dry spell, when the debris has firmed up and lifts in handfuls. Scoop the bulk out by hand or with a plastic trowel into a bucket hung from the ladder, working away from each downpipe so you are not ramming material down into the outlet. With the channel clear, flush it through with a hose from the far end and watch the downpipe run; a sluggish drain means a blockage lower down that a length of hose or a drain rod usually shifts. Leave the pressure washer in the shed, since the force flings wet muck everywhere and can spring the joints on old plastic guttering. Where the run is above a single storey or the pitch is steep, this is a job to pay someone for rather than to risk.
Keeping them clear
Timing does most of the work here. I clear mine once in late autumn when the leaves are down, then glance along them again in spring, and that plain rhythm keeps well ahead of any real build-up. Cutting back a branch that hangs over the roofline reduces the leaf fall at source and lets the gutter dry out between showers. Mesh guards and brush inserts earn their keep on a run beneath a tree, though they want lifting and rinsing now and then, or the clog simply moves to the top of the mesh. The lasting answer, where a gutter greens over year after year, lies in dealing with the moss on the roof that feeds it; the general approach to that sits in removing moss.
When it points to a bigger problem
A gutter that fills unusually fast is often telling you something about the roof. Old concrete tiles shed their granules heavily as they age, and a sudden thickening of gritty sediment can mean the tiles are nearing the end of their working life. Damp that lingers on the wall below, or a fascia still soft weeks after clearing, is worth having looked at properly rather than repeatedly scrubbing green off the brick. In those cases the moss is only the messenger, and the message is about the fabric behind it.