How to clean composite decking and get rid of mould

A bucket of warm water, a squirt of dishwashing liquid and a soft deck brush clean most composite decks. Scrub along the length of the boards, hose it off, done. Where it goes wrong is mould on a shaded deck, barbecue fat left for a month, and a pressure washer held too close. Each of those has a fix, and each maker writes its own limits into the care guide, so the first job is to find the guide for the boards you have.

How to clean composite decking: the basic wash

Sweep first. ModWood says a regular sweep stops dirt and leaves building up into marks and mould, and that the gaps between the boards need clearing too. Then wet the deck with the hose, mix warm water with dishwashing liquid, and scrub with a soft to medium bristle brush along the grain of the boards. Rinse well, because Trex warns that dirty water left to dry leaves a film on the surface.

That routine is what the makers ask for almost word for word. Ekodeck's care guide for its capped Designer Series says to scrub with hot water and dishwashing liquid using a soft-to-medium brush before rinsing, and it wants that general clean done at least once a year¹. Its FAQ gives the same yearly wash with warm soapy water and a garden hose rinse for the rest of the range. NewTechWood suggests warm soapy water on a rag, mop or soft broom, and recommends a monthly clean⁹. Trex says warm soapy water and a soft bristle brush, and adds that the brush is what lifts dirt out of the embossed grain pattern⁶. ModWood names sugar soap mixed with water and worked in with a yard broom⁴.

One Ekodeck Designer Series owner on ProductReview said they just "soap and hose them down with water"¹⁵. A Whirlpool member wrote in August 2024 that their Trex deck, in full northern sun, had had nothing done to it, "not even a sweep or pressure wash", and was still in the same condition as new¹⁶. It had been down for five years. I wouldn't copy the second one, but it shows that the wash is about looks and mould rather than keeping the board alive.

Sometimes the wash doesn't work. A Trex owner in Melbourne wrote that within a week of installation the deck looked "caked in mud" from the gravel paths beside it, and that neither a pressure washer nor the soapy-broom method shifted it¹². The installer tried vinegar and then a stronger cleaner the maker had suggested, and the owner says the maker later called that cleaner a non-recommended product. If the normal wash fails, I'd get the maker's advice in writing before trying anything stronger.

The pressure washer question, maker by maker

This is where decks get wrecked, and the makers disagree with each other on where the line is, so read yours.

ModWood is the strictest: do not use high pressure cleaners on its boards at all⁴. NewTechWood says a high-pressure cleaner is not recommended because it may damage the boards⁹. Ekodeck allows one on its capped Designer Series with conditions: a fan spray nozzle only, and it warns that a needle or point jet can do damage that cannot be repaired. Its stated limits are a maximum of 2000 psi and a nozzle kept at least 30 cm off the boards¹. Trex allows a fan tip on its current capped lines but not on its old uncapped ones, and says going past its limit could damage the boards and void the warranty. Its figures are 3100 psi and a tip no closer than 203 mm to the surface⁶.

Each limit is a pressure and a distance together, so check both against your machine before you start, because the makers hold owners to them. When one NewTechWood owner complained on ProductReview that the boards shed a furry layer when cleaned, the maker replied that its warranty assessment had found a high-pressure cleaner had been used, and that damage from inappropriate cleaning is not covered¹³. The owner's answer was that the pressure cleaner came out because the mould wouldn't shift.

On Whirlpool a member noted in 2024 that Ekodeck advises against pressure washing but reckoned an ordinary domestic unit at a sensible distance would be fine¹⁶. Another, back in 2012, said if the deck gets mould "pressure clean it off" and it comes up like new¹⁸. My advice sits between them. If your maker allows a washer at all, use the fan nozzle and stay inside its pressure and distance. Keep the wand moving along the board. A pencil jet held still to chase one stain is exactly what the Ekodeck warning is about.

Composite decking mould: why the shaded end goes black

Mould shows up first where the deck stays damp, such as the shaded end or the boards under a tree. On most boards it grows on the film of dust, pollen and leaf tannin that sits on the surface, rather than in the board itself. Ekodeck puts the cause as constant moisture and shade and says a dirty deck promotes the spread. Trex says the same: pollen and dirt left on the surface form a biofilm that the mould feeds on, and regular cleaning removes the food. NewTechWood says surface mould can occur on its boards but never penetrates them and washes off⁹.

Owners see it after wet years. One ModWood owner on ProductReview said that after the heavy rains of 2022 the deck "began to exhibit a lot of black mould"¹⁰. Another on the same listing said mould had become an issue over a couple of rainy years and that they removed it with "sugar soap and a stiff broom"¹¹. A third ModWood owner says the deck "turns green and extremely slippery every winter", even though he keeps it clean¹⁰. A Whirlpool member wrote in December 2016 that a ModWood deck they built in 2012 had split and gone mouldy, while the two Millboard decks they laid after it still looked new¹⁷. A reviewer on the NewTechWood listing said mould formed on five decks he built with it, and titled the review "mould forms on capping"¹³.

The common thread in those is damp, and capped boards are in the list as well as uncapped ones. ModWood says the brushed face of its uncapped board picks up dirt and mould more easily because the wood fibre is exposed, while the smoother, more plastic face stays cleaner⁴. So a cap helps, but it doesn't stop mould on a damp, dirty deck. For a shaded deck I'd still choose a capped board and plan the cleaning as well. The capped vs uncapped guide covers the difference, and the ground-level deck guide covers airflow under a low deck.

To get mould off, use the product your maker names. Ekodeck names 30 Seconds Mould Off for its Designer Series and says to clean more often if the mould keeps coming back¹. ModWood names Wet and Forget as a slow, long-term option, and a two-part composite cleaner with an anti-fungal treatment as the quick fix⁵. In a reply to one owner on ProductReview it adds that the anti-fungal treatment breaks down with sun and moisture, so keeping mould away is ongoing work¹⁰. Trex's care page says warm soapy water and a soft brush removes both the mould and its food source on its capped boards. Its mould bulletin goes further for its composite and PVC decking in general, naming a commercial deck wash with detergent and bleach applied to a dry deck, and it warns that the bleach will lighten the surface⁷. That is a Trex instruction for Trex boards. Don't read it as permission to bleach a different brand without that maker's say-so.

One more thing on mould and the warranty. Ekodeck's Classic warranty is one example of how it is handled. Mould claims are limited to the first two years, and staining from mould, mildew, dirt and grease is excluded³. Read your own maker's terms for the same clauses, and treat mould as a cleaning job rather than a claim.

Tannin marks, leaf stains and pot-plant rings

A new deck, especially an uncapped one, throws brown water marks in its first weeks. That is tannin coming out of the wood fibre at the surface. ModWood says these marks appear in the early life of the deck and generally disappear with a good soaking of rain and ordinary weathering, or sooner with a diluted composite cleaner⁵. Trex gives two answers. On its older uncapped boards you allow at least twelve weeks of weathering or use a deck brightener, and on its capped boards you use the brightener once the deck is dry⁶.

Leaf stains work the same way. One Ekodeck Classic owner with trees all around the house noticed "'wet' marks around where the leaves had been" after a week of rain, and said they dried up and the marks went¹⁴. Pot plants are worse, because the ring never dries. A Whirlpool member who trialled a small area of ModWood while choosing decking for a house built in 2010 said it was badly marked by dirt and water from pot plants and the marks "never came out properly"¹⁸. ModWood's own advice is to raise pots off the boards and move them around now and then to avoid shadow marks.

Barbecue fat and oil

Grease is the stain that does not weather out. Ekodeck's answer is an aerosol car or engine degreaser, sprayed on and lightly scrubbed, followed by the normal wash to take the degreaser off again¹. ModWood names a degreaser for oil-based stains too, and warns that it will lighten the colour of the boards, so follow the directions on the pack exactly⁵. Trex ties its stain warranty to speed. Food spills must be cleaned off within seven days⁶, which is not long when the barbecue is used every weekend.

The ones that beat people have soaked in. One Ekodeck Classic owner said cooking-oil patches were "impossible to remove even scrubbing"¹⁴. That board is uncapped, with the same colour all the way through, and ModWood says much the same of its own uncapped boards: oil-based stains can be difficult to remove, and the exposed wood fibre is prone to fatty stains⁴. ModWood suggests a splat mat or a drop board under the barbecue. Check the mat against your maker's care advice first, because Trex warns that rubber-backed and polypropylene rugs can draw colour out of its boards⁸.

Composite decking cleaning products: what voids a warranty

Keep it to soap, the maker's named cleaner, and the maker's named degreaser. These are the things that put a warranty at risk.

• Sanding a capped board, which Ekodeck says causes damage to its Designer Series that cannot be repaired¹. Trex says sanding voids its warranty for anything the sanding causes⁶, and NewTechWood says plainly that composite cannot be sanded back like timber⁹. The exceptions are uncapped boards whose makers allow it. ModWood says heavy scratches on the brushed face of its uncapped boards can be sanded or wire-brushed out⁴, and Ekodeck says you can sand out scratches or marks on its uncapped Classic board².
• Pressure beyond the stated limit, or a point jet, on any board that allows a washer at all.
• Bleach or solvent on a board whose maker has not named it. Trex warns that bleach or acid will lighten its boards⁶, and NewTechWood warns against spilling undiluted chlorine or pool salt on its boards⁹. Test anything new on an offcut, not on the deck.
• Steel wool and wire brushes on a capped board. A scratch through the cap can't be sanded back out, and the fading and scratches guide covers what you can do instead.

Static is the odd one. Some decks build up a shock in dry weather, and Ekodeck and NewTechWood both say an anti-static spray helps and that static is not covered by warranty¹⁹. Ekodeck calls noticeable static rare.

A cleaning routine by season

Autumn: sweep weekly while leaves fall and clear the gaps between boards with a plastic scraper or the edge of a trowel. Winter: check the shaded end after rain and wash it as soon as it darkens. ModWood's point is that the longer a deck stays dirty, the more work it takes to clean. Spring: the full wash of every board along the grain, with the maker's cleaner if there is any black left. Summer: the barbecue mat, and a hose-down after parties. Once a year, get under the deck if you can and clear the leaf litter, because the boards need air underneath to dry out after rain.

Before you buy a cleaner

I'd start by finding out whether your boards are capped or uncapped, then find the care guide for that exact range and keep it with the receipt. If the deck is already mouldy, buy the product named in that guide rather than whatever is on special. If the mould comes back every winter, the problem is shade and damp, and no cleaner will stop it coming back. The guide on problems and disadvantages owners report covers the shaded-deck issue, and the myths guide deals with the idea that a composite deck doesn't need any upkeep. If you are choosing boards now, the composite decking hub lists the ranges sold in Australia and lets you filter them by cap.