How to Clean a Deck

1. What Is Actually On the Deck

Deck discolouration comes in four varieties, and they respond to different chemistry. Identifying yours takes a minute and saves buying the wrong product.

That last row is the one that catches people out. No cleaner removes a failed film finish; it needs a stripper and a different afternoon, which our guide to stripping deck stain covers. If patches of your deck are peeling rather than worn, read that before buying a cleaner.

The tannin streaks in the fourth row are also worth calling out, because they are the one problem a cleaner makes worse rather than better. Alkaline cleaners darken tannin-rich woods like cedar and redwood; only the acidic brightener step brings them back.

2. Choosing the Cleaner

Percarbonate (Oxygen Bleach) — the Default

Sodium percarbonate dissolves into hydrogen peroxide and soda ash in water, lifting grey, dirt, algae and mildew without attacking the wood's lignin. It is what most commercial deck cleaners are built on, it does not kill nearby planting, and it will not corrode fasteners. For nine decks out of ten this is the correct and complete answer.

Sodium Hypochlorite (Chlorine Bleach) — Spot Use Only

It kills mould faster than anything else, which is why professional cleaners still carry it. It also degrades lignin, leaves the surface furry, corrodes hardware over years and burns planting. Used diluted on a stubborn black patch and rinsed hard, it is a reasonable tool. Used across a whole cedar deck, it is why some decks look fuzzy and pale for years afterwards.

Oxalic Acid Brightener — Not Optional Before Staining

Not a cleaner at all. It neutralises the alkalinity left behind by the cleaning step, removes iron and tannin darkening, and restores the wood's natural colour. Skipping it is the most common reason a properly cleaned deck still takes stain unevenly.

Dish Soap and a Brush — for Routine Cleaning

For a mid-season wash of a deck with a sound finish on it, warm water with a little mild detergent and a stiff-bristle brush is entirely adequate and will not touch the coating. Save the percarbonate for the annual clean and for prep before staining.

Whichever you buy, check the label for how much area a container covers and how long it needs to dwell. Those two numbers determine whether you need one jug or three, and our tested deck cleaner picks compare them directly.

3. The Cleaning, Step by Step

Clear and Sweep

Furniture, planters and grills off the deck entirely. Sweep, then run a putty knife or a purpose-made gap tool down the joints between boards — the packed debris in there holds water against the wood and is where mildew starts and where rot eventually follows. This ten-minute job does more for a deck's lifespan than the cleaning itself.

Wet Everything, Including the Plants

Soak the deck boards with plain water first: dry wood absorbs cleaner unevenly and blotches. Then thoroughly wet every shrub, bed and lawn area around and below the deck, and keep wetting them through the job. Diluted runoff on wet foliage is largely harmless; concentrated cleaner on dry leaves is not.

Apply and Let It Dwell

Apply with a pump sprayer or a roller, working in sections you can manage in ten minutes — usually 100 to 150 square feet. Let it dwell for the time on the label, typically 10 to 15 minutes, and mist it with water if the surface starts to dry. A cleaner that dries on the deck stops working and leaves streaks.

Scrub With the Grain

A stiff nylon brush on a pole, worked along the boards rather than across them. This is where the grey actually comes off, and it is the step most people skip in favour of more pressure. Pay attention to the shaded edges, the bottom of the stair treads and the area under where the table sits.

Rinse Thoroughly, Then Rinse Again

Every trace of cleaner has to come off, including what has run down into the joints and onto the framing below. Residue left behind interferes with the brightener and, later, with the stain. Rinse the surrounding planting down as well while you are there.

4. Using a Pressure Washer Without Ruining the Boards

A pressure washer is a rinsing tool on a deck, not a cleaning tool. Used that way it saves an hour; used as the primary cleaner it permanently damages softwood, and the damage only becomes obvious once the stain goes on and the furred areas drink twice as much.

The tell-tale sign of over-pressuring is a wavy, striped pattern in the boards that follows the arc of the wand. It cannot be washed out; it has to be sanded out, which is a different weekend entirely. If you see it starting, drop the pressure or switch to a wider tip immediately.

You do not need a pressure washer at all. A garden hose with a firm nozzle, a pump sprayer for the cleaner and a stiff pole brush will clean any deck perfectly well. It takes perhaps an hour longer and carries none of the risk, which for a first-time deck cleaner is a good trade. If you are renting one anyway, our guide to pressure washer nozzle colours covers which tip is which.

5. The Brightener Step Nobody Does

After cleaning, the wood surface is alkaline. Cedar and redwood in particular come out of the process looking darker and slightly muddy, and people often assume the cleaner did not work. It did — the colour is a pH effect, and an acidic brightener reverses it in about fifteen minutes.

Apply oxalic acid brightener to the still-damp deck, let it work for the labelled time, and rinse. Three things happen at once: the pH returns to neutral, iron and tannin darkening around fasteners lifts, and the grain opens up so the wood accepts stain more readily and more evenly.

That last effect is the reason this step matters even to people who do not care about colour. A cleaned-but-unbrightened deck accepts less stain, and accepts it inconsistently between the sun-bleached and sheltered areas. If a previous staining job came out patchy for no obvious reason, a skipped brightener is the most likely culprit.

6. Cleaning a Deck Before Staining

The cleaning is the same; what changes is that the drying and the final surface now genuinely matter. Three additional rules apply:

• Brighten, without exception. The stain's evenness depends on it more than on anything else you do that weekend.
• Allow 24 to 48 hours of drying, and up to four days in humid or shaded conditions. The deck feeling dry underfoot is not the same as the wood being dry.
• Test with water, not with your eyes. Sprinkle water on several boards. Soaking in and darkening means ready; beading means there is still moisture in the surface and stain will sit on top of it.

Whether to sand afterwards depends on how the boards feel. Cleaning raises grain slightly and pressure washing raises it a lot, so run a hand across a few boards once dry. Smooth means stain away; rough or splintery means a light pass with 60 to 80 grit first, which our guide to sanding a deck before staining covers in detail.

From there, the rest of the prep — masking, checking the forecast, setting popped nails — is covered in how to prep a deck for staining, and the staining itself in how to stain a deck.

7. Composite and PVC Decks

Composite cleans easily and damages easily, which is an awkward combination. Use a cleaner labelled composite-safe, a soft or medium brush, and plenty of rinsing. Percarbonate cleaners are generally fine; anything abrasive or strongly acidic is not.

Keep pressure well down — most manufacturers cap it around 1,500 psi with a wide fan held back — because erosion marks on composite are permanent. There is no grain to sand back and no realistic repair short of replacing the board.

Black spotting on composite is nearly always surface mould growing on organic debris trapped in the embossed grain rather than anything happening to the board itself. Clean the debris out of the grain and the spotting stops returning. And if the reason you are cleaning is that the colour looks tired, read whether composite decking can be stained before reaching for a coating — the cleaning is often the whole fix.

8. Specific Marks and Spills

• Grease and barbecue splatter. A degreasing detergent while the mark is fresh. Old grease that has soaked into bare wood usually needs sanding.
• Rust runs from fasteners and furniture. Oxalic acid brightener, applied directly to the stain rather than the whole board.
• Leaf and tannin marks. Also oxalic acid. Wet leaves left over winter leave a surprisingly dark print that no amount of scrubbing removes.
• Planter rings. A mix of trapped moisture, tannin and mildew. Clean, brighten, and put feet under the pots afterwards.
• Tree sap. Mineral spirits on a rag for the bulk of it, then wash the area normally. Do not scrape it off a finished board.
• Old paint or stain drips. Chemistry, not force. A stripper on the spot, or careful sanding — a pressure washer will only fur the wood around them.