When should you not power wash a deck?
Don't use high pressure on a timber deck at all, and especially not on pine. BRANZ, New Zealand's building research body, advises against water blasting decks because the pressure raises the timber fibres and leaves the boards splintered. The surface turns furry and rough underfoot, and bare feet find the splinters first.
The damage is easy to spot on a deck that has been blasted a few times: raised fuzz on the boards, splinters along the edges and a rough, uneven surface. Sanding can bring it back, but that removes wood each time and decks only have so much to give.
Other times to hold off:
- when the boards are soft or rotten, which a screwdriver pushed into the timber will show; that is a repair job, not a cleaning one
- on a deck high off the ground that is old or loose; BRANZ suggests professional advice for decks more than 1 m up
- right before rain, when a treatment would wash off the boards before it has worked
The BRANZ guide to timber deck problems is worth reading if your deck has more going on than dirt.
How are timber decks cleaned?
Timber decks are cleaned with a deck treatment to kill mould and algae, a stiff brush worked along the grain and into the grooves, and a gentle rinse. Pine (usually treated radiata) gets the softest approach because its surface damages easily. Hardwoods such as kwila are tougher but bleed dark tannin when wet, which can stain concrete and paving below.
The crew sweeps the deck, wets the surrounding plants and protects the ground below, then applies the treatment and gives it time. Scrubbing works the cleaner into the grooves and the gaps between boards, where most of the mould lives. The rinse uses low pressure and a wide fan, or a soft-wash setup, held well back from the timber.
A note on cleaners. BRANZ recommends oxygen bleach over chlorine bleach for decks, because chlorine bleach is toxic to plants, harmful to the environment and corrosive to metal fixings. Oxygen bleach (sodium percarbonate) breaks down into soda ash and hydrogen peroxide in water, and it is the base of many timber deck washes.
- Pine: low pressure only, scrub along the grain, and keep the cleaner off nearby cladding.
- Kwila and other hardwoods: expect red-brown run-off, especially from a newer deck and from the board ends. The crew rinses paving and concrete underneath straight away so the tannin doesn't set. Tannin removers reduce bleed but rarely stop it entirely.
- Old oiled or stained decks: cleaning lifts dirt and growth, not the old finish. Flaking coatings stay flaking until they are sanded or stripped, which is outside this service.
How are composite and PVC decks cleaned?
Composite and PVC decks are cleaned with warm soapy water, a soft brush and a thorough rinse. They don't splinter like timber, but the coloured outer layer can scuff, and bleach or acid cleaners can lighten it. Every maker publishes its own care guide, and the crew follows the one for your boards. Mould on composite feeds on the dirt and pollen sitting on top, not the board itself.
One major composite maker's published care guide lays out the approach: soap, a gentle scrub with a soft brush, then a rinse, with a pressure washer allowed only with a fan tip held at least about 20 cm from the boards and within the maker's pressure limit. The same guide warns that dirty water left to dry on the boards leaves a film, so the final rinse matters.
If you know the brand of your decking, tell us when you ask for a quote. If you don't, the crew tests the cleaner on a spot under the barbecue or a pot plant first.
Why is my deck slippery?
A slippery deck almost always has a film of algae or mould on it. It grows where boards stay damp: under trees, on the shaded side of the house, near downpipes and between tight boards that can't drain. The film turns slick as soon as it rains. Cleaning removes it, and a treatment slows it coming back, but shade and poor drainage bring it back over time.
BRANZ lists moss and mildew as one of the most common deck problems and points out that most trouble happens where timber can't dry out between wettings. Boards laid with no gap, or with gaps packed with leaves and dirt, hold water for days after rain.
What helps between cleans:
- sweep leaves off regularly and clear the gaps between boards
- trim back plants that shade the deck or rub against it
- move pot plants and outdoor mats now and then so the boards underneath dry
- check that downpipes and gutters aren't spilling onto the deck
The exterior cleaning industry's code of practice suggests cleaning timber decks once a year, and most shaded North Shore decks need at least that, ideally in spring before the outdoor season.
What needs to happen before you oil or stain a deck?
The deck needs to be clean, free of mould and fully dry before any oil or stain goes on, or the coating won't soak in evenly and growth can carry on underneath. That is where our job ends. The crew cleans the deck and leaves it ready for oil or stain. How long to wait before coating depends on the timber, the weather and the product, so check the label.
Timber needs to dry right through, not just on the surface. After a clean on a warm, breezy day that might be a couple of days. In a damp North Shore autumn it can be much longer. New hardwood decks are a special case: coating makers generally recommend letting new kwila weather for some weeks before it is oiled, so the finish can penetrate.
We don't oil or stain decks. If you plan to do it yourself, our guide on how to clean a deck covers the cleaning steps, and the coating maker's instructions cover the rest.
What deck cleaning costs
Washing a small deck of up to about 20 m² typically costs $150 to $250, a medium deck $200 to $350 and a large one of up to around 70 m² $300 to $400, going by published Auckland and NZ figures. Heavy mould, steps, balustrades and tight access add time. We stop at clean, so the deck is ready for you or someone else to oil or stain.
| Job | Typical range |
|---|---|
| Small deck, up to about 20 m² | $150–$250 |
| Medium deck, about 20–40 m² | $200–$350 |
| Large deck, about 40–70 m² | $300–$400 |
A guide only, built from figures Auckland and New Zealand operators and cost guides publish (checked October 2026). Heavy mould, steps, balustrades and tight access add time. We confirm your price after seeing the deck.
How a deck wash runs
- Look-over: the crew checks the timber type, the finish, any soft or loose boards and what sits underneath the deck.
- Prepare: furniture and pots come off, plants around the edge are wetted or covered, and paving below is protected.
- Treat: a deck cleaner goes on to kill mould and algae and is left to work.
- Scrub: boards are brushed along the grain, with the grooves and gaps worked clean.
- Rinse: a low-pressure rinse carries the dirt off, steered away from stormwater drains.
- Dry and hand back: the deck is left clean to dry, ready for oil or stain if you plan to coat it.
Questions people ask about deck washing
What is the best way to clean a deck?
Sweep it, apply a deck cleaner to kill mould and algae, let it work, scrub along the boards with a stiff brush and rinse at low pressure. That cleans timber without raising the grain. Composite boards get warm soapy water and a soft brush instead. Do it on a dry, mild day with no rain forecast so the boards can dry afterwards.
What should you not use to clean decking?
Avoid high-pressure water blasting on timber, because it lifts the fibres and leaves splinters. Avoid chlorine bleach on timber decks too: BRANZ notes it harms plants and corrodes metal fixings, and an oxygen-based cleaner is the safer choice. On composite boards, skip bleach, acids, solvents and stiff wire brushes unless the maker's care guide says otherwise.
How long before I can oil the deck after cleaning?
Wait until the timber is dry right through, which can take a couple of days in warm, breezy weather and longer in damp months. The oil or stain label gives the maker's own drying time and moisture check, and that is the one to follow. New hardwood decks usually need several weeks of weathering before their first coat.
Can you clean the deck and the house on the same day?
Yes, and it is a common pairing. The house is usually washed first so any run-off from the walls doesn't land on a freshly cleaned deck, and the deck is done last. Booking both together also means one visit and one setup. Ask about our house washing service when you request a deck quote.