Pressure Washing and Jet Washing in South London

Most of what we are called to is hard surfacing gone green over a winter or two. Some of it is a job in its own right; much of it is half a day added to something we are on site for anyway.

The green film is algae. Single-celled, it needs only moisture and shade, so it lives on north-facing paving, under trees and against fences. It rinses off easily and comes back quickly, because nothing about a wash changes the shade or the damp.

Moss is the one that does structural damage. It colonises the joints, holds water against the pointing all winter, and when that water freezes it levers the joint apart. Once the pointing has gone, water gets under the slabs, and after that it is only frost and time before one lifts or starts to rock. Clearing moss out of the joints is worth doing for that reason alone.

Black spotting on sandstone is lichen, and a different problem entirely. Lichen anchors itself into the surface of the stone, so it cannot be washed off at any pressure the stone will survive. It needs a biocide left on long enough to kill it, then a rinse, and the marks keep fading for a week or two afterwards. Anyone promising to take black spot off in one pass with a lance is promising to take the top of your stone off with it.

Two marks that look like dirt are not. Red-brown staining under a tree is tannin from leaf fall, and it usually lifts. A white bloom on newish block paving is efflorescence — salts drying out of the cement — and washing does not cure it. It weathers off on its own over a season or two, so paying to clean it is money wasted.

What shifts dirt is pressure, water volume, the nozzle and how far that nozzle sits off the surface. Volume matters more than people expect: a machine moving twelve to fifteen litres a minute clears a patio faster than a hire machine at the same pressure and half the flow, with less risk to the stone.

The nozzle is what causes damage. A turbo or dirt-blaster nozzle spins a pencil jet in a tight circle and will cut a visible line into sandstone, old concrete flags or soft brick in about a second. That is where striped patios come from. For open areas we use a rotary flat-surface cleaner — an enclosed head with nozzles on a spinning arm, held at a fixed height so every square metre gets the same treatment — and a fan nozzle by hand at edges and steps.

Some surfaces do not get a lance at all. Lime-mortared Victorian brickwork, which is most of the garden walls and rear elevations around here, has soft joints and a fired skin on the brick face, and pressure removes both. Painted render and external wall insulation come apart the same way. Asbestos cement, still on plenty of old garages and sheds here, must never be pressure washed, because that is how fibres get released. Those surfaces get low pressure and a treatment, or they get left alone.

Roofs are on that list too. We do not jet wash tiles or slates: a lance strips the surface off concrete and clay, drives water under the laps and can void a tile maker's warranty. Roof moss comes off by hand from a roof ladder, and gets treated afterwards — a different method on a different page.

Washing block paving does something that washing a patio does not. It empties the joints. The kiln-dried sand between the blocks is not decoration — it transfers load from one block to the next, which is why the British paving standards, the BS 7533 series, treat jointing as part of the construction rather than as a finish.

Take that sand out and leave it out, and the blocks lose their interlock. They start to rock under a wheel, the edges chip where block meets block, weeds root far more readily in an empty joint than a full one, and the surface creeps out of line. A drive that was jet washed and never re-sanded routinely looks worse after two years than before anybody cleaned it.

So re-sanding is part of this job rather than an extra line on the bill. Kiln-dried sand is brushed in dry, worked down the full depth of the joint and topped up. It has to go in dry: sand brushed into damp joints bridges near the surface and leaves the joint hollow underneath, which looks finished and behaves as though nothing was done.

A wash puts a great deal of water somewhere. In most South London gardens the gully by the back door is a surface-water drain, and surface water here runs to the river rather than to a treatment works. Silt and dead algae are one thing. Detergent, oil lifted off a drive or a strong biocide going down the same gully is another, and it is not meant to happen. Where a job needs chemicals we keep the washings off the gully and soak them away on your own ground.

The wall is the other thing to watch. Pressure throws water and grit sideways, and paving that finishes level with or above the damp-proof course hands that water a route into the brickwork. On older houses whose gardens have been built up over the years this is common, and the honest answer is sometimes that the level is the problem rather than the dirt. We work away from the wall, keep the pressure down along that edge, and say so when what you need is a channel drain rather than a clean.

Cleaning makes a sound surface look new. It does nothing for a surface that has failed. Concrete flags that have begun to spall — the face lifting off in thin plates — are frost-damaged, and washing takes more of the face with it. A patio that has dropped along one edge, or a drive sunk into two wheel ruts, has a base problem underneath; cleaning changes the colour, not the shape.

Where the joints have gone soft and crumbly, a wash and a re-point is the sensible pair of jobs, and it buys years. Where the base has moved, a lift-and-relay of that area is the answer, and we would rather quote for that than sell you a clean that looks good for a fortnight.

On sealing, we will not tell you it is essential, because on most surfaces it is not. Porcelain does not need it. An impregnating sealer on sandstone does slow staining and make the next clean easier, and it has to go onto genuinely dry stone — a separate visit days later, not the same afternoon. A film-forming wet-look sealer over paving still damp underneath traps that moisture and goes cloudy, and there is no cheap way back.