A green roof need not mean sedum. On a shaded shed, a north-facing garage or a bin store tucked under trees, the very damp and gloom that starve a sedum mat are what moss likes best, so a moss roof can clothe a structure that would otherwise stay bare felt. It weighs next to nothing, asks for no mowing or feeding, and once it has settled it runs itself.
Why moss suits a roof
Most off-the-shelf green roofs are planted with sedum, the low succulents that ride out the heat and glare of an exposed roof in full sun. Set that same mat in shade, beneath overhanging branches or on a slope turned away from the south, and it thins, weakens and lets weeds march in. Moss runs the opposite way. It seeks out the cool, the damp and the soft light that defeat sedum, and it carries barely any weight, which counts for a great deal on a thin shed roof that could never bear a deep planted system. Soaked right through, a skin of moss weighs a fraction of a soil-based roof and wants none of the irrigation those systems depend on.
What goes under the moss
A moss roof is still a roof, so the layers below earn their keep. Begin with a sound, fully waterproof deck; over boarding, a butyl or EPDM pond liner is the usual home-build answer, carried up under the edges so no water can creep beneath. On top of that a thin drainage course, even a sheet of dimpled plastic or a spread of coarse grit, keeps the moss from standing in trapped water, which rots it from below. Because moss roots into almost nothing, a shallow base is plenty: a centimetre or two of a lean, gritty mix, or one of the water-holding mats sold for green roofs, does far better than deep rich soil, which only feeds weeds. Finish the perimeter with a batten or upstand to stop everything sliding off, leaving gaps or a gravel margin so rain can drain away at the edge.
Getting the moss to take
Three approaches will establish it. The fastest is to lay sheets or cushions of moss lifted from elsewhere, pressing them firmly onto the damp base so they make full contact and butting the pieces tight together. For a larger or more awkward roof the slurry works better: blend moss with rainwater or buttermilk to a thin paint, spread it evenly and mist it while the fragments regenerate, much as described in spraying moss slurry at scale. You can also simply let it arrive, since a shaded, grit-topped roof in a mossy district often greens over within a couple of years if you keep it weeded and damp. Whichever you pick, moisture in the first few weeks decides the outcome, while the rhizoids are still gripping; water with rainwater through any dry spell until the moss has knitted down.
Slope, aspect and the right moss
Aspect rules everything here. A roof facing north or east, or one shaded by trees or a taller building, holds the damp and gives moss its best chance, whereas a baking south pitch in full sun will defeat it as surely as it rewards sedum. A gentle slope sheds heavy rain without letting the surface dry too fast; a flat roof wants reliable drainage so it never ponds. For the covering itself, lean on the creeping carpet mosses that knit into continuous cover and cling on a slope, the plait and feather mosses rather than the loose, tumbling cushions. The plait moss that mats over a shaded woodland boulder will happily mat over a shaded roof, and there is more on it in plait moss (Hypnum cupressiforme).
Looking after it
As roof coverings go this is about as undemanding as they come, though it is not quite a case of laying it and forgetting it. Sweep off fallen leaves in autumn before they smother and rot the moss, particularly under trees, and tug out any seedlings of grass, willowherb or birch that try to gain a foothold, since these are what eventually break a moss roof apart. Through a long summer drought the moss will brown and go dormant, which unsettles people new to it; almost always it is merely thirsty and will green up with the next steady rain, so leave it be rather than stripping it off. Keep gutters and outlets clear so water keeps moving. This is a different matter from the unwanted moss that gathers on a house's tiled roof, a nuisance dealt with in moss on roofs. Done with a little care, a moss roof gives you a soft green covering that cools the structure beneath and soaks up rainfall, with a patch of habitat thrown in, for very little weight and almost no work.