How to clean decking depends mostly on what the boards are made of. Timber decking takes a warm-water scrub with a stiff brush along the grain, plus an occasional vinegar soak for algae. Composite decking needs a gentler touch: soap and water for routine cleaning, and a pressure washer only if the specific product line is rated for it. Mixing up the two approaches is the most common reason a deck ends up scratched, stained or stripped of its finish rather than clean.
Cleaning timber decking
A weekly sweep to clear leaves, dirt and standing water stops timber staying damp long enough for algae to take hold in the first place. For a deeper clean, wet the boards, scrub along the grain with a stiff outdoor brush and a mild detergent, then rinse before the water has a chance to dry on the surface. Decks.com, a building-materials resource used across the decking trade, suggests soaking stubborn green patches with a diluted white vinegar solution for roughly ten to twenty minutes before scrubbing, since the acid loosens algae that plain water and a brush struggle to shift alone.
Once the boards are clean and fully dry, reapplying a decking oil or sealant, following the product's own guidance on how often, sheds water faster afterwards and gives algae less chance to re-establish before the next clean.
Cleaning composite decking
Composite decking is not solid wood, so it needs different handling. Trex, one of the larger composite decking manufacturers, recommends warm soapy water and a soft-bristle brush for routine cleaning, working with the direction of the board's embossed pattern rather than across it. Its higher-performance product lines can generally take a gentle pressure wash, while older composite ranges are better suited to a dedicated composite deck cleaner every six months or so instead of a pressure washer.
If you are not sure which product line your deck is, check the paperwork from when it was installed or fitted, or a receipt if you bought the boards yourself. Composite ranges vary enough between manufacturers, and even between a manufacturer's own older and newer lines, that guessing is not worth the risk on a deck that cost more than a few sheets of timber to lay in the first place.
Trex also warns against homemade or generic DIY deck cleaners, since some can cause damage that does not show up until weeks later. White vinegar diluted in water is fine for hard water marks and mineral deposits, but the boards still need a thorough rinse afterwards. Letting vinegar or hard water dry on the surface is more likely to leave a mark than removing one.
Removing algae and moss from decking
Shaded, damp corners, under furniture, planters or overhanging branches, are where algae and moss return fastest on either material. The Royal Horticultural Society notes that these growths rarely damage most hard surfaces outright, but they make them slippery, and that scrubbing or pressure washing without chemicals is usually enough to clear them. Save a dedicated chemical decking cleaner for a deck that has gone a full season without attention rather than reaching for one on a light green film.
How to clean decking with a pressure washer
A pressure washer speeds the job up considerably, but timber and composite boards both scar easily under a narrow, concentrated jet held too close for too long. Work through this checklist before you start:
- Check whether the manufacturer allows pressure washing at all. Most timber does; some composite ranges specifically advise against it.
- Fit a wide fan nozzle rather than a narrow, concentrated one, and keep it moving rather than holding it over one spot.
- Hold the nozzle well back from the surface and work with the grain or the board pattern, not side to side across it.
- Test a hidden or low-traffic board first before doing the whole deck.
- Wear eye protection and non-slip footwear, and engage the trigger lock whenever you set the wand down.
- Let the deck dry fully before walking on it barefoot or putting furniture back.
The UK Health and Safety Executive's guidance on high-pressure water jetting, written for tradespeople but just as relevant at home, is clear that the safety guard should stay fitted and the trigger lock should never be disabled, and that the jet should never be pointed at yourself, another person, or an electrical fitting.
When to book a professional instead
A large deck, one you cannot confidently identify as timber or a specific composite range, or algae that keeps coming back within weeks of cleaning are all reasonable points to hand the job to someone experienced rather than risk the boards yourself. Steps and raised areas are also worth prioritising if you decide to book help, since a slippery board at the top of a step is a bigger hazard than the same growth on flat, low-traffic decking.
Our guide on when to pressure wash a driveway covers similar timing and safety judgement calls that apply to outdoor pressure washing generally, and how to clean patio slabs without damaging them goes through the same principle of matching method to material for stone and concrete. If decking is just one job on a longer outdoor list, our room-by-room spring cleaning checklist covers what else is usually due around the same time of year, and timing a deck clean alongside those other jobs often saves a second trip outside with the same bucket and brushes.