Every autumn a path that was fine all summer turns into something you have to think about before you step on it. The usual response is to hire a machine once a year, blast it back to clean, and then watch it go green again by the following February.
That is not a maintenance plan, it is a reset button. The reason it does not hold is worth understanding, because the alternative takes a fraction of the time.
What the slippery bit actually is
It is algae, and it is alive. A green film or a powdery deposit on stone, paving, tarmac or decking is algae; the black version is the same thing further along. Alongside it you get mosses and liverworts in the joints and lichens on the surface, and all of them need the same thing to get going: moisture.
That is the whole mechanism, and it explains everything else. Algae grows faster in shade and where the air does not move. Build-ups happen after wet winters. Anywhere water sits rather than runs, you get growth; anywhere it drains and dries, you mostly do not.
It also explains why the annual blast fails. Cleaning the surface removes what has grown. It does not change a single one of the conditions that grew it.
What the RHS actually recommends, in its own order
The RHS lists non-chemical methods for hard surfaces, and the order they come in is the interesting part. Brushing is first:
Brush with a stiff broom or wire brush to take off established growth
A pointed weeding tool or old knife for moss in the cracks between paving
A pressure washer, to remove growth quickly
Prune back overhanging plants so the surface gets more light
Make sure surfaces slope enough that water does not stand
Improve the drainage around them
Keep drains clear of leaves and debris
Note where the machine sits. It is on the list, and it does what it says — it is fast. But it is one entry among several, and the ones on either side of it are about conditions, not cleaning.
The RHS is also plain about the chemical shelf: non-chemical control works, so there is no need for mosskillers or patio cleaners. Most of those products are non-persistent anyway, which means repeat applications, which means you are buying the same reset button in a bottle.
Why a regular sweep wins
Brushing on a regular basis stops growth taking hold in the first place. That is the sentence that matters, and it is the difference between the two approaches.
A blast-clean deals with a season's growth in one go and leaves the surface bare, damp and shaded — which is exactly the condition algae likes. A sweep every few weeks through autumn takes minutes, removes the leaf litter that holds moisture against the stone, and never lets the film get established enough to need a machine.
It is also the version that keeps working on the surfaces a pressure washer is awkward around: steps, thresholds, the strip along a wall, gravel that just needs raking, the edge where a path meets a lawn.
I do hard-surface sweeping as a standing item on regular visits for exactly this reason — it is a small job done often rather than a big job done annually, and it is the one that actually holds.
The bits people miss
Leaves are the fuel. A path under leaf litter stays wet, and the thin black film develops underneath within a week or two. It then stays slippery long after the leaves have gone, and light paving gets stained into the bargain. Prompt beats thorough on paths — clearing autumn leaves goes into which leaves have to move and which are better left where they fell.
Drains and gullies. A blocked gully makes standing water, and standing water makes algae. Clearing them is on the RHS's own list and takes a minute.
Overhanging growth. A shrub leaning over a path keeps it in shade and drips on it after rain. Cutting it back changes the conditions permanently, which nothing else on this page does.
Lichens are different. The crusty grey and orange patches are lichens, they colonise slowly, and they are not the slip risk — the algal film is. Scrubbing lichen off old stone is mostly cosmetic work.
The fall of the paving. If a slab holds a puddle, no cleaning routine fixes that. That is a laying problem, and it is worth knowing which it is before paying anyone to clean it repeatedly.
What to do this autumn
Get the leaves off the hard surfaces before the first properly wet week rather than after. Brush the paths and steps while they are dry — dry algae comes off far more easily than wet. Clear the gullies. Cut back whatever is leaning over the route you actually walk.
Then keep doing the first two every few weeks until the leaves have finished. That is the entire routine, and it is why the path stays walkable in January.
If you would rather it was just handled — swept properly on a regular visit, with the drains and the overhanging stuff dealt with at the same time — that is a normal thing to book.