Moss on a roof looks alarming and is usually less serious than it appears, but it is worth managing for two real reasons. Here is what it does, what not to do about it, and how to keep it down.
Does it harm the roof?
Moss does not eat into sound tiles or slates. The genuine problems are indirect: thick growth holds moisture against the surface, which matters in a hard frost when trapped water freezes and flakes the face of a tile, and clumps break off and block gutters and valleys, backing water up under the edges. On a sound roof it is mostly cosmetic; on an old or north-facing one it is worth staying on top of.
Why not to pressure-wash
This is the big one. A pressure washer blasts the protective surface off tiles and slates, drives water under them, and takes years off the roof's life. It does far more damage than the moss ever would. Whatever you read, do not jet-wash a roof to clean moss.
Removing it safely
Brush it off dry with a stiff brush, working downward so you do not lift the tiles, and clear the gutters and valleys while you are there. Do this from a safe platform or leave it to someone who works at height; a wet mossy roof is treacherous. There is no rush, so wait for dry weather and good access.
Keeping it down
Moss returns wherever the roof stays damp and shaded, so reduce that: cut back overhanging branches to let in light and air. Fitting a strip of zinc or copper along the ridge works well, since rain washes a trace of metal down the slope that moss dislikes, slowing regrowth for years. For the general principles, see how to get rid of moss.
Moss as a deliberate green roof, and the weight question
So far this has been about unwanted moss. Moss can also be the point: a living green roof, deliberately grown on a shed, garage, extension or low-pitch deck. Here moss has one big advantage, which is weight.
A conventional green roof is heavy. An extensive sedum or substrate system typically adds somewhere around 60 to 150 kg per square metre once it is fully saturated, and an intensive roof garden a great deal more again. That is real structural load, and many roofs, especially older ones and lightweight timber structures, cannot take it without strengthening and an engineer's sign-off. The weight is the single most common reason a green roof is ruled out.
Moss is one of the lightest living coverings there is. A moss mat over a thin layer of growing medium weighs far less than a sedum-and-substrate build, which sometimes makes a living roof possible on a structure that could never carry the conventional kind. Two cautions, though. First, moss holds a lot of water, so always reckon on the saturated weight plus any snow load, not the dry weight, when you or an engineer check the roof. Second, a light mat catches the wind, so it needs securing and good edge detailing against uplift.
It is not a free pass: you still need sound waterproofing beneath it, a shaded or north-facing aspect and a low pitch suit moss best, and anything you deliberately install should have its loading checked rather than assumed. But where a traditional green roof is simply too heavy, a moss roof is often the version that can actually go ahead.
What it does to rainwater collection
If you harvest rainwater into a butt or tank, a moss or green roof works against you, on both quantity and quality. It is the same water-holding that makes moss useful elsewhere, turned into a drawback here.
Quantity. Moss is a sponge. A moss roof soaks up a large share of the rain that lands on it and evaporates much of it straight back to the air, so light showers can produce almost no run-off until the moss is saturated. Over a year an extensive green roof commonly retains something like half the rainfall, which means your tank sees far less than the same area of plain tile or metal would deliver.
Quality. The water that does run off a living roof carries organic matter, tannins that stain it brown, nutrients, and fine particles from the moss and any growing medium. That is fine for watering the garden, but it discolours stored water, feeds algae in the butt, and clogs fine filters faster, so it is poorly suited to clean or drinking use without good filtration and a first-flush diverter.
The flip side. That same retention is a real benefit for stormwater: it slows and reduces run-off and eases the load on drains in a downpour. So it is a genuine trade-off. If your aim is to fill a tank with as much clean water as possible, keep moss off the collecting roof and run a hard tile or metal surface to the gutter. If slowing run-off and cooling the building matter more to you, the green roof wins, but expect less water in the butt and browner water at that.