How to clean a deck

Updated 2026-10-06

To clean a deck, sweep it, kill the mould and algae with a deck cleaner, scrub along the boards with a stiff brush and rinse gently with a hose. That works on pine, hardwood and, with a softer brush, composite. Skip the water blaster. If you would rather not spend a Saturday on your knees, we arrange deck washing across the North Shore.

What is the best thing to clean decking with?

For timber, an oxygen-based deck cleaner (sodium percarbonate) or a dedicated timber deck wash, mixed as the label says, plus a stiff-bristled brush on a long handle. For composite, warm water with a mild detergent and a soft brush. Both need a garden hose for rinsing. A simple scrub with hot water and detergent also works on a deck that is dirty rather than green.

BRANZ, the building research body, suggests scrubbing timber decks with detergent and hot water first, making sure to get into the grooves and the gaps between boards, then using a bleach solution only if the deck is badly stained. When bleach is needed, it recommends oxygen bleach rather than chlorine bleach. Its full guide to timber deck problems and repairs also covers rot and popped nails.

What you need:

• a broom and a scraper or old putty knife for the gaps
• a deck cleaner suited to your boards, or detergent and hot water
• a stiff brush for timber, or a soft one for composite
• a garden hose with a spray nozzle
• rubber gloves and safety glasses

What should you avoid when cleaning a deck?

Avoid high-pressure water blasting on timber, chlorine bleach on timber, and harsh chemicals or wire brushes on composite. A water blaster raises the fibres of the wood and leaves it splintered. Chlorine bleach harms garden plants, is bad for waterways and corrodes metal fixings. Composite boards can be lightened by bleach and acid products and scratched by stiff bristles.

Some other things that cause trouble:

• Cleaning in hot sun: the cleaner dries on the boards before it has worked and can leave patches. Pick a mild, overcast day.
• Cleaning before rain: the treatment washes off before it does its job, and the run-off goes where you didn't plan.
• Letting run-off into the drains: Auckland Council says no wash water or detergent, even a biodegradable one, may enter the stormwater system. Rinse toward lawn or garden.
• Mixing products: never mix different cleaners, and never mix bleach with anything acidic or with ammonia.
• Kneeling on wet boards for hours: use a long-handled brush, and leave raised decks with loose or soft boards to someone who can check them first.

How do you clean a timber deck step by step?

Clear the deck, sweep it, scrape out the gaps, wet the boards and nearby plants, apply the cleaner, let it sit, scrub along the grain, then rinse thoroughly with a hose. Work in sections of a few boards at a time so the cleaner doesn't dry. Leave the deck to dry fully afterwards. On most decks the job takes a morning to a full day.

1. Clear everything off. Furniture, pots, mats and the barbecue. Check under them for dark rings, which need extra attention.
2. Sweep and scrape. Get leaves out of the corners and dirt out of the gaps between boards so water can drain.
3. Wet the area. Hose the boards, and soak garden beds next to the deck so they absorb less cleaner.
4. Apply the cleaner. Mix it as the label says and spread it with a brush, sprayer or watering can, a few boards at a time.
5. Let it work. Give it the time the label says, usually some minutes, and keep it damp.
6. Scrub. Brush along the length of the boards, never across them, working into the grooves.
7. Rinse. Rinse well with the hose until no foam or dirty water runs off, then rinse any paving the run-off touched.
8. Dry. Leave the deck clear until it is completely dry.

Hardwood decks such as kwila often shed red-brown tannin when they are washed, particularly when fairly new. Rinse concrete, tiles and pavers below the deck straight away, because the colour stains light surfaces if it is left to dry.

How do you stop a deck being slippery?

Remove the green film that causes it, then make the deck harder for growth to return. The slipperiness comes from algae and mould that grow on boards that stay damp. A deck cleaner and a good scrub take it off. Keeping the gaps clear, cutting back shade and moving pots around lets the boards dry between showers, which slows regrowth.

Check where the deck is greenest. Under trees, along the shady south side, under a leaking gutter or beside a downpipe are the usual spots. Clearing a blocked gutter or redirecting a spilling downpipe can do more for a slippery corner than any cleaner.

For steps and ramps that stay slippery after cleaning, grip strips or anti-slip treatments are worth a look, and a quick sweep once a week through autumn and winter keeps leaves from holding moisture against the boards.

How do you clean a composite deck?

Sweep it, wash it with warm soapy water, scrub gently with a soft brush and rinse thoroughly so no dirty water dries on the surface. Composite won't splinter, but its outer layer can scuff and fade. Check your decking maker's care guide before using any stronger cleaner or a pressure washer, because each brand sets its own limits.

Mould on composite usually grows on dirt, pollen and food spills sitting on the surface rather than in the board. Cleaning off that layer regularly removes what it feeds on. Grease from the barbecue should be cleaned off quickly, because some makers set time limits on stain cover.

If a care guide allows a pressure washer, it will usually specify a wide fan tip held well back from the boards. One leading maker's guide, for example, says no closer than about 20 cm. When in doubt, the hose and brush are enough.

How do you get a deck ready for oil or stain?

Clean it as above, make sure every trace of mould, dirt and cleaner is rinsed away, and let the timber dry right through before any coating goes on. Coating over damp, dirty or mouldy boards leads to patchy colour and early failure. The oil or stain label gives the drying time and preparation steps, and those are the ones to follow.

A few checks before you open the tin:

• the boards look evenly pale and dry, with no dark damp patches near gaps or joists
• no rain has fallen in the drying time on the label, and none is forecast while you coat
• any flaking old coating has been dealt with, since cleaning removes dirt and growth, not old finish
• new hardwood decks generally need several weeks of weathering before the first coat, so check the coating maker's advice

Our part stops at a clean deck. The coating itself is up to you or whoever you choose for it.