Decking: Jet Wash, Soft Wash, or Leave It Alone?

I do not clean decking. Driveways and patios are the whole list, so nothing here is a pitch, and the last section is how to judge whoever you do get in.

Decking is the surface where a pressure washer does visible damage fastest, and where the damage is least reversible.

What pressure does to timber

Wood is not uniform. It grows in annual rings, and each ring has soft early wood laid down in spring and denser late wood laid down later in the season. That alternation is what gives timber its grain.

A jet washer erodes the soft early wood faster than the dense late wood.

What you get is a surface where the hard grain stands proud and the soft grain has been cut away. It feels furry. Under bare feet it is splintery. And because the surface area has increased enormously, it holds dirt and moisture better than it did before, so it goes green faster afterwards.

This is not a subtle effect. One careless afternoon with a lance can add years of apparent age to a softwood deck.

The only remedy is sanding the boards back flat, which on any reasonable sized deck is a bigger and more expensive job than the cleaning was.

The pressure that is actually safe

If you are going to use a pressure washer on softwood decking:

Wide fan, 40 degrees or more, never a turbo nozzle.

Long standoff, 300mm minimum.

Work along the grain, following the length of each board, never across it.

Keep moving. No dwelling on a stubborn mark, because that is exactly where the furring will show.

Moderate pressure. If your machine has a regulator, turn it well down.

Even done correctly this removes some material. On a deck that is already weathered and soft, it is worth asking whether it should happen at all.

Soft washing instead

For most tired timber decks, this is the better answer.

Apply a proper deck cleaning solution. Give it real dwell time, ten or fifteen minutes. Agitate with a stiff brush along the grain. Rinse at low pressure, which can be a garden hose.

It is slower, and it does not have the theatre of a jet cutting a clean stripe through green boards. It also leaves you with a smooth deck rather than a rough one, which after two or three cleanings over a decade is a very large difference.

Chemistry removes the growth. Pressure removes the growth and some of the timber.

The gaps between boards matter more than the boards

The thing most often skipped, and the thing that actually shortens a deck’s life.

Leaf mulch, silt and moss pack into the gaps between boards. That material holds water against the board edges and against the joists underneath, and joists are where rot starts, because they are the parts that never dry and never get looked at.

Clearing the gaps out is worth more to the structure than cleaning the surface is. A putty knife or a purpose-made deck gap tool run down each gap takes an hour on a typical deck and is the single most useful maintenance job on it.

Composite decking

Composite cleans easily, and the pressure limit still applies for a different reason.

Composite boards have a moulded surface texture, often a brushed or embossed grain. Excess pressure scours that texture flat, leaving shiny patches that catch the light differently and never match the rest.

Most composite needs a wash with detergent and a brush plus a mould treatment, and nothing more. It does not need oiling and it does not need sealing.

If yours is still under warranty, check the manufacturer’s guidance. Several specify a maximum pressure and will point at it if you make a claim.

When to leave it alone

Soft boards. If you can push a screwdriver into a board, that is rot. Cleaning a rotten deck makes it look considerably better without making it any safer, which is arguably worse than leaving it green.

Badly cupped boards. Where each board has curved so water sits in a channel along it. Usually the boards were laid the wrong way up, or there is no ventilation underneath. Cleaning will not change the shape.

A deck you are replacing this year. Obviously.

Anything with loose or protruding fixings. Get those dealt with first. A raised screw head is the thing on a deck most likely to hurt somebody, and it is easiest to spot on your knees at close range before any water goes near it.

Oiling afterwards

Oiling is a decorating job rather than a cleaning one, and doing it properly needs several consecutive dry days, which is why most cleaning outfits will not touch it.

What matters is the order, and people get it wrong. Clean first. Let the timber dry properly, which is at least two or three dry days and considerably longer in autumn. Then oil.

Putting oil onto damp or dirty timber seals the dirt and the moisture in underneath, and the finish lifts within a season.

The green film and why it returns

Algae on decking is the same organism you get everywhere else and it responds to the same things.

A north-facing deck, a deck in shade, or a deck sitting directly on soil with no airflow underneath will green over again within a year regardless of how well it was cleaned.

A mould and algae treatment after cleaning is £1.50 a square metre and buys a meaningful extension. Improving the ventilation under the deck, if that is possible, is free and does more.

My rates

£5.50 a square metre for decking, softwood, hardwood or composite. Treatment £1.50 on top. Minimum job £150, which on decking binds quite often because most domestic decks are between 15 and 30 square metres.

Send a close photograph of the board surface with your enquiry. It tells me the timber type and its condition, which is most of what decides the right method.

Telling softwood from hardwood from composite

It changes the method, and a lot of people genuinely do not know what they have.

Softwood. Usually pressure-treated pine or spruce, greenish when new, fading to silver-grey. Light. You can dent it with a thumbnail. Boards are typically grooved on one face. By far the most common and the most easily damaged.

Hardwood. Balau, ipe, oak or similar. Noticeably heavier, denser, darker, often with visible plug-covered fixings rather than exposed screws. A thumbnail will not mark it. Takes more pressure safely, though it still fibrillates if you push it.

Composite. Uniform colour through the board, no knots, no grain variation between boards, often with a slightly plastic feel and a repeated embossed texture. Ends may show a hollow chambered profile.

If you are unsure, look at a cut end or an offcut under the deck. Composite has a uniform speckled core with no grain running through it; timber does not.

Slipperiness, which is the real safety issue

Wet algae on smooth timber is one of the more slippery surfaces you will encounter in a garden, and decking is usually raised, which makes a fall worse.

Grooved decking is often assumed to be the anti-slip option. In practice the grooves fill with debris and hold water, and a smooth board that drains and dries can be safer than a grooved one that stays damp. This is contested in the trade and I am telling you my own observation rather than a rule.

What genuinely helps: keeping the gaps clear so the deck drains, treating with a mould and algae product after cleaning, and on a deck used through winter, anti-slip strips or a grit-additive finish on the steps at least.

If somebody in your household is unsteady, do the steps as a priority.

What a clean will and will not achieve

Will. Remove the green film and the surface dirt. Take out the black mould spotting on most softwood. Clear the gaps. Make the deck safe to walk on. Reveal the actual condition of the boards.

Will not. Restore silvered timber to its original colour, which needs a brightener or an oil. Remove deep black staining where water has sat around a fixing for years. Flatten cupped boards. Fix rot. Remove tannin bleed from an oak deck, which is a stain in the timber.

Silvering is worth a word because it is the thing people most often expect cleaning to reverse. Ultraviolet breaks down lignin at the timber surface and the grey is what is left. A clean removes the dirt sitting on top of the grey, and the deck comes up looking lighter and cleaner and still grey. Getting the colour back needs an oxalic acid deck brightener and then an oil, which is a decorating job.

Anybody worth hiring will tell you that beforehand rather than let you look at a clean silver deck and feel short-changed.

How to judge whoever you get in

Since it will not be me, three things to listen for.

What pressure, and how do they know? The right answer is a number under 1000 psi held well back, or a soft wash with a cleaner and a brush and almost no pressure at all. “We’ll be careful” is not a number.

What are they doing about the algae? Blasting the green off puts it straight back. A mould and algae treatment after the wash is what buys you the season.

What will it look like? If they promise the grey will come off, they are describing a brightener and an oil, which is a different job at a different price. Ask which one you are buying.

The patio it sits next to, if there is one, I can do.