Composite decking is sold as maintenance-free, grows mould anyway, and can absolutely be damaged by a pressure washer — a combination that surprises nearly everyone who owns it. Safe deck cleaning on Trex and its cousins is a matter of following the manufacturer’s numbers.
Why ‘maintenance-free’ grows mould
Composite boards are wood fibre in a polymer matrix, usually with a harder cap layer. The embossed grain texture that makes them look like timber also gives organic material somewhere to sit: pollen, leaf tannin and airborne spores settle into the pattern, and in shade the surface grows mould exactly like wood does.
Two things are happening at once, and it helps to separate them. The first is simple deposition — a film of pollen, dust, pet hair and broken-down leaf litter that settles into the embossing and gets pressed in by foot traffic. The second is colonisation. Mould and algae do not need to eat the board; they need moisture, shade and something organic to start on, and the film already sitting in the grain pattern supplies all three. That is why the green never appears in the middle of a sunny deck and always appears along the shaded edge, under the table, and in the two feet nearest the house where nothing dries.
It cleans off readily — the material is genuinely easier to clean than timber — but it does need cleaning, which is the part the marketing skips. The good news is that the biology is only ever on the surface. There is no oxidised layer to strip and no grain to raise, so the job is chemistry, dwell and rinse, with none of the pressure anxiety that comes with washing a timber deck without furring the grain.
Capped or uncapped: know which you own
This is the single most useful thing you can establish before anyone quotes the job.
Early composite decking was uncapped — one homogeneous material all the way through, wood flour and polymer mixed and extruded. The wood fibre goes right to the surface, which means the surface absorbs. Spills soak in, tannins soak in, and mould has something at the surface it can genuinely feed on. These boards also fade and chalk with UV exposure in a way later products do not.
Later generations are capped: the same core, wrapped in a thin polymer shell on three or four sides with no exposed wood fibre at all. That shell is what makes modern composite stain-resistant and easy to clean. It is also what you damage if you get the pressure wrong, and it cannot be repaired or refinished once marked.
Telling them apart is usually straightforward. Look at a cut end or the underside of a board: a capped board shows a visibly different skin over a coarser core, while an uncapped board is the same speckled material through and through. Uncapped boards also tend to look chalky and lightened on the sun side after a decade. If you have the paperwork or a brand and range name, better still.
The warranty numbers
Most composite manufacturers publish care guidance with a maximum pressure and a minimum nozzle distance, and several warranties are conditional on following it. Exceed the numbers and the cap layer can be permanently marked — often as a lighter, roughened patch that no product will restore.
The numbers differ by brand and by product line, which is why guessing is not good enough. What they have in common is the shape of the instruction: a modest pressure ceiling, a fan tip rather than a concentrated one, a stated standoff distance, and movement kept parallel with the board. Nothing published by any manufacturer permits a turbo or rotary nozzle on composite. What that tip does to a cap layer is scour it dull in a spiral pattern that shows up in every raking light for the rest of the deck’s life.
The safe approach is the one we use anyway: an appropriate cleaner, soft brush agitation in the grain direction, and a low-pressure, wide-fan rinse from a respectful distance. The chemistry does the work; the water rinses. It is the same principle that separates soft washing from pressure washing everywhere else on a property — force is a liability on any surface designed to shed water rather than absorb it.
The two composite-specific problems
Early-generation uncapped boards stain deeper and may retain some tannin shadowing after cleaning — honest expectations apply. And spot-cleaning one bad area often leaves a visibly cleaner patch, which is why the right unit of work is the whole deck, not the stain.
Tannin shadowing
Tannins are the natural pigments that leach out of oak leaves, acorn husks and pine needles when they sit wet on a surface. On an uncapped board those pigments migrate into the porous wood fibre at the surface rather than resting on top of it, and what remains after cleaning is a soft-edged grey-brown ghost in the shape of whatever sat there. An oxalic-based treatment lifts a good deal of it. Some of it stays. Anyone promising a total removal on uncapped composite has not looked closely at the board.
The patch problem
Composite dulls and shades gradually with UV and weathering, so the whole surface has quietly drifted from where it started. Clean one square metre of it and you have not made that square metre dirty-free; you have made the surrounding ten look filthy by comparison. Cleaning edge to edge is the only way to end up with a deck rather than a repair.
Know your board brand if you can. It changes the chemistry we choose and what we promise about the result.
Composite decks in Southeast Missouri
Our conditions are hard on composite in a specific way: the material resists rot beautifully and does nothing at all about biology.
Mature oak and pine cover means a heavy spring pollen load, then a summer of leaf drop from storms, then autumn proper. Every one of those events lands organic debris in the embossing. The humid summers keep it damp, and the river-valley fog around Cape Girardeau and Perryville means a shaded composite deck can stay wet into the afternoon for weeks. That combination produces the classic Southeast Missouri composite complaint: a board that looks fine in the open and green-black along the tree side.
The gap between the boards matters more here than the boards do. Needles and leaf fragments pack the gaps, hold water against the board edges and feed the growth line you can see running along every joint. Clearing the gaps is part of the job, not an extra. Under mature trees around Marble Hill and Fredericktown it is often worth timing the deck for the same visit as the roof and gutters, since the debris source is the same and the clean-up sequence runs top-down anyway.
One winter note: ice-melt products tracked out of the house are a common source of white residue and, on some formulations, staining on composite. Sand or grit is the safer choice on a composite surface, and any residue is better rinsed off in spring than left to sit.
Common questions
Will cleaning restore the colour my composite deck has faded from?
No. Cleaning removes what is on the board — film, mould, tannin deposits — and reveals the board’s current colour. UV fading is a change in the material itself, and there is no product that reverses it. What surprises people is how much of what they had assumed was fading turns out to be film, so the deck usually comes back further than expected. But the faded part stays faded.
Can I use household bleach on composite decking?
Some manufacturers permit a dilute solution and some explicitly do not, and a few will treat it as a warranty issue. Where it is allowed it needs proper dilution, a controlled dwell and a thorough rinse, because residue left in the embossing keeps working after you have stopped watching. Check your brand’s care sheet before anything comes out of the cupboard.
What about the mould growing under the deck and on the framing?
The framing under a composite deck is almost always timber, and it is the wettest, shadiest wood on the property. It is worth a look whenever the surface is cleaned. If the joists or ledger feel soft to a screwdriver, that is a structural conversation rather than a cleaning one, and it is the same test used to decide whether a deck is worth restoring at all.
How often does a composite deck actually need cleaning?
Once a year suits most decks here, usually in late spring once the pollen has finished. Heavily shaded decks under mature trees do better with a second pass in autumn after the drop. Decks in full sun and open ground can often stretch longer.
Can the deck be done at the same time as the house?
Yes, and it is the sensible order — the house comes first so that rinse and runoff land on the deck before the deck is finished rather than after. If the siding is due anyway, pairing it with house washing saves a second mobilisation and gets both surfaces finished on the same weather window.
Tell us what the deck is made of when you send photos — composite and timber are quoted as different jobs because they are. Read more about deck cleaning →