Every March the phone does the same thing. A deck has come through winter looking tired, a homeowner has watched a video, and a machine has been dragged out from under a bag of compost where it sat since 2019. By the time I turn up, the boards are usually worse than they were in February. The grain stands up like the bristles on a yard brush. Pale fibrous stripes run down the length of the run where somebody held a 15-degree tip six inches off the timber and walked backwards in a straight line, the way you would with a patio.
Decking is not a patio. It is a soft, layered, thirsty material that happens to be flat, and that flatness is what fools people.
I wash decks most weeks between April and September, all over Greater London, and I’d say pressure has wrecked more timber in this city than rot has. Nearly all of it comes down to two things you set before the trigger is ever pulled: what’s on the end of the lance, and how far that end is from the wood.
What actually happens to timber when the water hits it too hard?
Softwood decking – and almost everything laid in a London garden is softwood, whatever the merchant called it – is built in rings. Each ring has a soft, open, fast-grown band (earlywood) and a dense, tight band (latewood). Those two bands have wildly different hardness. Water at 150 bar does not know the difference between dirt and earlywood. It removes both, at roughly the same speed, and it leaves the latewood standing proud.
That’s the whole mechanism, and it behaves nothing like sanding. You are selectively excavating half the timber and leaving the other half as a ridge, which is why a badly washed deck develops that washboard feel underfoot within a season – ribs you can catch a sock on.
The other failure is more brutal and quicker: fibre lift. Push hard enough and the jet gets under the surface fibres and peels them upward, like flicking through the pages of a paperback with your thumb. They stand up, they dry stiff, and now the board is a splinter farm. For my money, that’s the damage people find hardest to accept, because it happens under a stream of clean water, in front of them, and it looks like it’s working while it’s doing it.
Why the damage often looks like progress on the day
A ruined deck comes up beautifully. That’s the cruel part. Blasted earlywood is fresh, raw, pale timber, so the contrast against the untouched section makes the operator think they’ve found the setting. They speed up. They get closer. The deck gets brighter and rougher in the same movement, and nobody notices the roughness until it dries three days later and the whole thing has gone the colour and texture of a wolf’s back.
Which nozzle should be on the lance when you’re standing over decking?
The nozzle decides more than the machine does. Colour coding is near enough universal: red is 0 degrees, yellow 15, green 25, white 40, black is the low-pressure soap tip.
Red should never come out of the bag on a garden job. It’s a pin of water carrying the machine’s full energy through a spot the size of a match head, and it will cut a groove in seasoned pine on a single slow pass. I’ve seen a homeowner in Barnes write his own initials into a board that way without meaning to. Yellow is barely better on timber – fine for lifting moss out of block paving joints, wrong here.
Green (25) is where most decking work should sit, for my money, and white (40) for anything soft, silvered, or already suspect. A lot of contractors will tell you 40 is too weak to clean. What they mean is that it’s too slow, which is a complaint about wanting to be off site by two.
Then there’s the turbo nozzle – the dirt blaster, the one that spins a zero-degree jet in a cone. It shifts algae off concrete like nothing else. On a deck it is a router bit made of water. I won’t have one in the van on a decking day. Not stored, not tucked in the box for “just in case the steps are bad.” The temptation on a stubborn patch is real and the tool wins the argument.
The flat-surface cleaner question
The rotary flat-surface cleaner – the round hovercraft-looking thing – splits opinion. It runs two spinning 15-degree jets under a skirt, so the dwell time on any point is tiny and the standoff distance is fixed by the housing itself. On hardwood, on a well-laid balau deck, it can give an even finish that no hand pass will match. On soft, weathered pine it still eats earlywood, only evenly, so you get a deck that’s uniformly 1mm thinner and furred rather than stripey. Consistency is not the same as safety.
How much pressure is too much for a garden deck?
The numbers people quote at me are always machine numbers. “It’s a 150-bar machine.” That tells me almost nothing, because pressure at the pump is not pressure at the timber, and the distance from the tip is doing more work than the dial ever will.
Rough working figures I hold in my head: hardwood tolerates around 100 bar (roughly 1,500 psi) at a sensible standoff. Softwood, which is your deck, wants half that or less – 40 to 70 bar, near enough. Old, silvered, south-facing softwood that’s been baked for six summers might not take 40 without lifting.
Most domestic machines from a shed start around 110 bar and can’t be turned down, because the manufacturer sells pressure as the feature. That’s the trap in the whole DIY decking clean: the cheapest machine on the shelf is more dangerous to timber than my petrol unit, because mine has an unloader valve I can wind down to 50 bar and hold there. Theirs has an on switch.
Distance does the regulating
If you can’t adjust pressure, you adjust geometry. Every inch you back off drops the force on the wood sharply – the jet spreads and slows. A 25-degree tip at 300mm is a different tool from a 25-degree tip at 80mm. Keep the lance moving with the grain, keep the angle shallow rather than pointing straight down, and never stop. A pause is a gouge.
Test on the worst-placed board you can find – under the barbecue, behind the pot – never in the middle where you’ll be looking at it every summer for the next decade.
Why does the damage only show up months later?
The deck dries and it looks alright. Everyone is happy. The invoice clears.
Then autumn arrives and the trouble surfaces, because that furred, opened surface is a superb sponge. Water goes in three or four times faster than it did. It sits in the lifted grain, the boards cup, the fixings start to work loose, and the black spot fungus that was living on the surface now has a habitat with depth to it.
Coatings tell the same story. An oil that should soak and level now disappears into the timber – two coats gone where one used to sit – and it dries patchy because the raw earlywood drinks and the latewood ridges don’t. I re-oiled a deck off Fairfoot Road in Bow last September that had been washed by somebody the previous spring with a turbo nozzle. The client thought the oil was faulty. The oil was fine. The wood was a colander.
The one that goes straight to matchsticks
Thin composite-era softwood, 22mm, laid over joists at 600mm centres – if that’s been furred hard and then held moisture through two winters, you’re not oiling it. You’re pricing a replacement. Honest answer on the day beats a nice-looking job that fails by Christmas.
What should you do instead if the deck is already grey and green?
Chemistry first, pressure last. Almost everything on a London deck is biological – algae and that black lichen film – and it dies to a sodium hypochlorite solution with a surfactant, dwelling for ten or fifteen minutes. Kill it, wait, rinse it away.
My sequence is dull and it works: sweep the deck and clear the gaps between the boards with a jointing knife, wet the surrounding planting and keep wetting it, apply the mix at low pressure through a black tip, let it dwell, agitate with a stiff-bristle brush on a pole where the growth is thick, then rinse at 40 degrees and 50-odd bar. The pressure washer’s real job on a decking clean is rinsing. Nothing else.
What the machine is actually for
Rinsing. That’s it. The pressure washer on a decking job is the last ten minutes, not the first two hours, and if you’ve bought a petrol unit to save yourself the brush work you’ve bought the wrong thing. The brush is where the job is. A stiff-bristle deck brush on a pole, worked along the grain with the chemical still wet, will lift more black growth in five minutes than a 25-degree tip lifts in twenty, and it takes nothing off the board. Cold water and patience beat hot water and haste on softwood every time. I know how that sounds coming from someone who owns four pressure washers.
Grey is different from green. Grey is UV-damaged surface lignin, and for my money no chemical or jet takes it off without taking timber with it. That’s a job for an oxalic acid brightener, or for accepting the silver and oiling over it. A roof terrace off Rivington Street I look after has been silver for four years by choice, and it’s the soundest deck on my books.
If your deck is furred already, don’t wash it again this year. Sand it at 60 grit, brush the dust out of the gaps, oil it, and leave the machine in the van.
