Patio Slab Biocidal Treatment and Pressure Washing

The owner reckoned it had been about ten years. That is the kind of thing people say slightly apologetically, as though a patio left alone is a moral failing rather than simply what happens when a garden is used for living in. What it actually meant was that we were not looking at dirt. We were looking at a mature ecological community that had had a decade to sort itself out, which is a different problem and, as it turned out, a more hopeful one.

There is an order to how these things arrive, and a decade is long enough to see the whole sequence. Algae comes first, because it asks for almost nothing — damp and light will do. Moss follows once there is enough trapped organic matter in a joint to root into. Lichen comes last and slowest, because lichens grow at a pace measured in millimetres per year, which is exactly why an established black spot population tells you more about elapsed time than any amount of surface grime does.

The thing people expect after ten years is damage, and that is not what was under it. The terracotta came back. The pigment was intact, the slab faces were sound, and the patio that emerged was recognisably the patio that had been laid — not a worn-out version of it. That surprises almost everyone, and it is worth sitting with, because it changes what neglect actually costs.

It is also a live argument in the literature rather than a settled fact, and we would rather say so. Whether a biofilm damages the stone beneath it or shelters it is genuinely contested — researchers have proposed weighing one against the other directly, and lichen thalli are described as capable of physically shielding a surface, an umbrella effect, as well as binding it. Which way it falls depends on the organism, the substrate and the conditions. On this patio the outcome was clearly benign. That is an observation about this job, not a general licence to leave things.

What it does mean practically is that a decade of neglect did not call for a decade's worth of force. The instinct is that the worse it looks, the harder you must hit it, and on pigmented concrete that instinct will cost you the colour you are trying to recover. So the pressure stayed low at both ends — a light wash first to take the loose surface dirt off, and a light rinse at the end. The chemistry did the lifting in between.

That hour in the middle is the part that gets skipped, and it is the part that actually works. The product is only active while it is wet; let the surface dry and it stops doing anything at all, quietly, without looking any different. So the hour is not waiting. It is an hour of keeping a whole patio uniformly damp, misting anything that starts to go off, and working the worst of the black spot by hand. That is time somebody has to be paid for, which is why it is the first thing to disappear from a cheap quote.

There is a step before the rinse that almost nobody mentions either. The treated surface is washed down with clean water to neutralise the cleaner before the pressure washer comes anywhere near it. Go straight in with the machine and you are atomising active chemical into the air around you, and around the garden. It costs a few minutes and it is the difference between a controlled job and a cloud.

Look at the lawn in the after photographs. It is green, whole, and shows no scorch or dieback anywhere along the edge — and that is not luck. Every border and run of grass was soaked with clean water before a drop of product was mixed, because a plant already full of water takes up very little of anything else, then rinsed again at the end. This stuff will kill vegetation if it is allowed to. The proof that it was handled properly is not a paragraph of reassurance; it is the grass still being there.

Which brings up the thing the search results will not tell you. Ask how to clean a patio like this and you will be handed a cheerful list that includes buying the trade-strength cleaner and doing it yourself. The manufacturer's own datasheet for the product we used says something different: severe skin burns and eye damage, very toxic to aquatic life, contact with acids liberates toxic gas, FFP3 respiratory protection, and in plain capitals, for use by professional operators. None of that is hidden — it is printed on the label. It simply does not survive the journey into a summary written to be encouraging.