By late autumn a shaded deck often wears a soft green bloom, and the first frosty morning turns it into something you cross carefully. That slipperiness is the whole of the problem worth taking seriously; the moss itself does very little to the timber. Deal with the grip and you have dealt with decking moss, and there are gentler ways to do it than blasting the boards.
Why a deck greens over
A deck offers moss almost everything it wants. The boards lie flat and hold a film of rainwater long after a wall or a fence has drained, and the anti-slip grooves milled into so much softwood decking act as little gutters that keep grit and moisture sitting in the timber. Add the shade of a house wall, an overhanging tree or a north or east aspect, and the surface rarely dries between showers. Spores and algae drift in on the wind, settle in the damp channels, and within a couple of wet seasons a new deck has a green skin. Composite boards green over too, since the moss is only using the surface film of grime and needs nothing from the material beneath it.
Green film, and why it is so slippery
Much of what makes a deck lethal in the wet is not moss at all but a thin algal slime, the same slick coating that greens a shaded paving slab; the cushiony moss tends to gather in the grooves and along the edges where feet fall less often. Both hold water against the boards and both turn glassy underfoot once wet, which on a raised deck with a drop or a step is a genuine hazard rather than a cosmetic quibble. If you want to know which growth you are looking at, the leafy green tufts are moss and the smooth paint-like stain is algae, a distinction drawn out in telling moss from algae. For safety it hardly matters, since the cure for both is the same.
Clearing it without wrecking the wood
Start with the mildest thing that works: a stiff broom or a scrubbing brush and a bucket of warm water, worked along the length of each board so the bristles reach into the grooves. Most of a winter's growth lifts with that alone, especially while it is soft and damp. Where you want a cleaner finish, a proprietary deck cleaner does the job without the harm that neat bleach or a path-and-patio biocide can do to timber and to the plants at the deck's edge. Rinse afterwards and let the boards dry. A pressure washer is where most decks come to grief: held close or on a needle jet it tears up the soft summer grain, leaving the surface rougher and more water-holding than before, so the moss returns faster. If you must use one, drop to a low setting, fit a wide fan nozzle, keep it well back and follow the grain. Then let everything dry hard before anyone walks on it.
Slowing the return
Once the boards are clear, the useful work is drying them out. Cutting back a branch or lifting the skirt of a nearby shrub lets in the light and moving air that a deck in permanent shade never gets, and both do more good than any coating. Keep the surface swept, since fallen leaves and pine needles rot down into exactly the damp grime moss roots into, and check that the gaps between boards are clear so water drains away rather than pooling. A fresh coat of decking oil or stain sheds water for a season or two and buys time, though on a shaded deck nothing keeps green growth off for good. Autumn, before the first hard frosts, is the sensible time to clear and re-treat.
When a little green is fine
On a quiet, low deck with no drop and no steps, a light patina in the shaded corners does no harm and saves you the bother; sweep the walked-on middle and leave the fringes be. This is the opposite of the approach on a shaded path or terrace, where a soft moss seam is often something to nurture rather than scrub, as moss in paving and patios describes. A deck asks for the reverse only because a wet board underfoot behaves so differently from a wet stone at ground level. For clearing moss off harder surfaces and drives, removing moss covers the rest of the garden.