How to clean a timber deck without wrecking it

Decks are the surface people damage most often with a pressure washer, because timber looks tough and isn't. A deck that's gone grey, green and slippery can absolutely be brought back — but the job is 80% chemistry and 20% water, and turning the pressure up is exactly the wrong instinct.

First, work out what you're dealing with

Problem	What it is	Fix
Silver-grey surface	UV-degraded timber fibre	Deck cleaner / brightener with oxalic acid
Green film, slippery underfoot	Algae and mould	Oxygen bleach cleaner, soft brush
Black spotting	Mould growing in old oil residue	Cleaner plus agitation, sometimes a strip-back
Dark brown streaks	Tannin leaching, common in merbau	Oxalic acid based cleaner
Flaking, patchy finish	Failed oil or stain coat	Strip and recoat — cleaning alone won't fix it
Furry, splintered boards	Previous over-pressure damage	Sand back; it won't wash out

Safe pressure for decking

Timber decking sits at the low end of the scale. As a guide:

• Hardwood (merbau, spotted gum, blackbutt): up to about 1200 PSI, 40° fan, 30 cm back.
• Softwood (treated pine): 500–800 PSI. Pine is much easier to gouge.
• Composite: under 1500 PSI if the manufacturer permits it at all — check the warranty.
• Never: turbo or 0° nozzles, or holding the tip close to "get that stubborn bit".

Move continuously along the length of each board, following the grain. Working across the grain leaves visible bands and lifts the fibres.

Step by step: cleaning a timber deck

1. Clear the deck and sweep the gaps. Get furniture and pots off, and clean the debris out of the board gaps with a paint scraper or a thin blade. Trapped wet leaf litter is what rots boards from below.
2. Check for loose nails and screws. Punch down or re-screw anything proud before you start — a raised fixing catches the brush and your feet.
3. Wet the deck and surrounding plants. Pre-wetting stops the cleaner soaking unevenly into dry timber, and protects nearby garden beds.
4. Apply deck cleaner and let it dwell. Ten to fifteen minutes, kept damp. Don't let it dry on the boards.
5. Agitate with a stiff-bristled deck brush. Along the grain. This is the step people skip, and it's the one that does most of the work.
6. Rinse at low pressure. Fan tip, well back, along the grain, one board at a time so you don't leave lap marks.
7. Apply a brightener if the timber is still grey. Oxalic acid based products neutralise the cleaner and restore the natural colour. Rinse thoroughly afterwards.
8. Let it dry completely. 48–72 hours in mild weather. Longer in a Canberra winter, when decks in shade can stay damp for the best part of a week.

Composite decking is a different job

Composite boards don't go grey and don't need oiling, but they do grow mould in the surface film — particularly in shade, and particularly in the textured grain pattern. Warm water, a mild detergent and a soft brush handle it. If you do use a pressure washer, keep it low, use a fan tip, and check the warranty first, because some manufacturers void it the moment a pressure washer touches the boards.

Re-oiling: timing matters in Canberra

Canberra is hard on decks — high UV in summer, hard frosts in winter, and big daily temperature swings that keep timber moving. Most decks here need re-oiling every 12 months on exposed areas, stretching to 18–24 months for sections under a pergola or verandah.

• Best window: spring or early autumn. Mild, dry, and low overnight moisture.
• Avoid: oiling in direct hot sun (the oil skins over before it penetrates) or in winter, when the boards won't dry out enough beforehand.
• Test first: flick water onto the boards. If it soaks straight in, the deck is thirsty and ready. If it beads, the existing coat is still working.

When it's a pro job

Call someone in if the deck is elevated or over a slope, if a previous coat is flaking and needs stripping, if there's been pressure damage that needs sanding back, or if it's simply large enough that the dwell-time juggling becomes unmanageable. Deck and timber cleaning in Canberra typically runs $149–$400 depending on size and condition — see the full cost guide. If your deck sits alongside pavers or a pool surround, the pavers and sandstone guide covers those at the same time.

FAQs

Can you pressure wash a timber deck?

Yes, but only at low pressure. Stay in the 500–1200 PSI range with a 40° fan tip held around 30 cm back, and always work along the grain. High pressure tears out the soft spring wood between the grain lines and leaves a permanently furry, splintered surface.

What is the best thing to clean decking with?

An oxygen bleach (sodium percarbonate) deck cleaner for general grey and grime, or a dedicated deck cleaner with oxalic acid if you also need to lift tannin and black water stains. Both are far gentler on timber than chlorine bleach, which strips colour and corrodes fixings.

Can I pressure wash composite decking?

Most composite manufacturers allow it under about 1500 PSI with a fan tip, but check your specific warranty first — some void it entirely. Composite usually only needs warm soapy water and a soft brush; the mould that grows on it lives in the surface film, not in the board.

How do I get the grey out of my deck?

The grey is UV-damaged surface fibre. A deck cleaner or brightener containing oxalic acid dissolves it and returns the timber close to its original colour. A gentle low-pressure wash removes the loosened layer, and the deck must dry fully before any oil goes on.

How often should a deck be cleaned?

Once a year for most Canberra decks, ideally in spring. Decks in shade, under trees or near a pool go slippery faster and often need a clean twice a year. Cleaning before it becomes slippery is a safety measure, not just cosmetic.

How long after cleaning can I oil my deck?

Wait until the timber is genuinely dry — typically 48 to 72 hours in mild weather, longer in a Canberra winter. Oiling a damp deck traps moisture and causes the finish to go blotchy and peel within months.