Warm water, a mild soap, and a soft-bristle brush worked along the board grain - not a pressure washer held close. A Cape Fear guide to salt film, pollen, oak tannin, and the cleaners that permanently dull a composite deck.
Short answer: clean a composite deck with warm water, a mild soap, and a soft-bristle brush, working along the grain of the board and rinsing before anything dries. A pressure washer is optional and risky - if you use one at all, use a wide fan tip, keep it well back, and check your decking manufacturer's own care sheet first, because most of them publish a maximum pressure and a minimum distance, and exceeding either is one of the few ways to void a deck warranty in an afternoon. On the Cape Fear coast the job is less about dirt than about what the humidity grows in the board texture between washes.
Homeowners often buy composite expecting it to be maintenance-free, then find it looks worse after three summers than the pressure-treated deck it replaced. Both parts of that are half true. Composite will not rot, cup, splinter, or need a stain coat, which is most of the maintenance wood demands. What it does have is a manufactured grain pattern pressed into the surface, and that texture holds a thin layer of pollen, dust, and organic film the way a smooth board does not.
That film is the whole problem. It is food. Once it settles into the grain, the mold and algae that are already in the air here have somewhere to live, and they do not need the board itself to be organic to colonize it. This is why people are surprised: the deck is not rotting, it is hosting. And it is also why cleaning composite is mostly about lifting a surface film rather than stripping anything out of the material.
This is the single distinction that decides what you are allowed to put on the boards, and most people do not know which one they own.
Uncapped composite is the older style - wood flour and plastic mixed all the way through, with the same material at the surface as in the core. It is more porous, it stains more readily, and it is more sensitive to harsh cleaners.
Capped composite, which is most of what has been installed in the last decade or so, has a bonded polymer shell over that core. The shell is what resists staining and fading, and it is also what you can damage with abrasives or the wrong chemistry. Once a cap is scratched or chemically dulled, it does not recover, and there is no sanding-and-refinishing path the way there is with wood.
Not sure which you have? The quickest check is an offcut, or the underside edge of a stair tread, where a shell reads as a distinct outer skin wrapped around a differently-textured core. Failing that, the installer paperwork or the product line stamped on the board underside will lead you to the care sheet, and that document is the only one that governs your specific boards. Where it stays genuinely unclear, work as though the deck is capped: gentle methods cost you nothing on an uncapped board, whereas an aggressive cleaner on a capped one is not undoable.
Three different things show up on composite around Wilmington, and they do not respond to the same treatment.
The first is the flat green film on the shaded boards, usually along the north side of the house, under a pergola, or wherever the deck runs beneath live oak canopy. That is algae, and it is the easiest of the three. Soap, agitation, and a rinse take it off.
The second is mildew - darker, spottier, concentrated where water sits longest. The bottom of the boards near the house wall, the shaded strip behind a grill, and anywhere a planter or a deck box has been sitting all season are the usual spots. Mildew needs a cleaner with some dwell time, not more scrubbing pressure.
The third is not biological. It is a grey or chalky dullness across the whole surface, and on an older uncapped deck that is usually oxidation and embedded grime rather than something growing. A wash brightens it but will not fully return the original color, and no honest cleaning will. If someone tells you a wash will make a fifteen-year-old uncapped deck look new, that is a sales claim, not a technical one.
Clear the deck completely - furniture, planters, grill, rugs, the lot. Anything left in place leaves a clean-edged outline you will see for the rest of the year.
Sweep out the gaps between boards. The debris packed into board gaps is holding moisture against the deck and is a large part of why the shaded end stays green, so a putty knife or a gap tool along the seams does more good than another pass of soap.
Wet the whole surface first, then work in sections small enough that the soap never dries on the board. Apply with a soft-bristle brush - not a wire brush, not a stiff deck broom - and scrub along the length of the boards rather than across them. Composite grain runs with the board, and cross-scrubbing leaves visible marks in the texture.
Rinse each section thoroughly before moving on, and rinse the plantings and any siding below the deck while you are at it. Then let it dry fully before anything goes back, especially rubber-backed mats, which trap moisture and will grow their own outline within a season.
You can, and it is where most of the permanent damage to composite decks comes from.
The failure mode is not dramatic. Held too close, or used with a narrow tip, a pressure washer lifts and furs the surface of the board - on capped composite it scores the cap, and on uncapped it opens the surface so it takes on dirt faster afterwards. The deck looks fine when it is wet, and the striping shows up a week later once it has dried. At that point there is nothing to do about it.
If you are going to use one, the rules are consistent across manufacturers even where the specific numbers differ: widest fan tip you have, keep the wand moving, stay well back, never stop in one spot, and look up your own decking brand's care guide for the pressure ceiling it publishes. If the deck is under warranty, that document is also the thing that decides whether damage is covered.
The honest version is that a brush and soap do this job about as well on composite and cannot hurt it. The pressure washer is a time-saver that carries a risk the brush does not, which is a different trade than it is on concrete.
Late March and April coat everything in this area in fine yellow pine pollen, and a composite deck holds it in the grain better than almost any other outdoor surface. Washing at the front of that window is largely wasted effort - it comes straight back.
The more useful sequence is to rinse the deck periodically through the heaviest weeks to keep the layer from building and bonding, then do the real wash once the drop has finished. Pollen that sits all spring becomes the organic film the summer's algae feeds on, so clearing it is not a cosmetic step.
On the barrier-island and waterfront properties around Wrightsville Beach, Carolina Beach, and the sound side, there is a salt film on every horizontal surface that inland decks never deal with. It is not visible on most composite colors, and it is mildly hygroscopic - it holds moisture at the surface, which extends how long the board stays damp after a rain and gives mildew more hours to work.
Practically that means waterfront decks want a plain fresh-water rinse more often than an inland one needs a full wash, and it means the railing hardware deserves a look while you are out there. The fasteners and any steel connectors take the salt harder than the decking does.
Under the mature live oaks in the older Wilmington neighborhoods, wet leaf litter left on composite leaves brown tannin shadows in the shape of the leaves. The leaf is not staining the plastic - it is depositing tannin into the surface film and, on uncapped board, into the pores.
Caught within a season, these usually come out with the ordinary soap-and-brush wash. Left through a full winter under a mat of wet leaves, they can be permanent on uncapped composite. Keeping leaf litter cleared off the deck through fall is genuinely more effective than any product that promises to remove the result.
Two stains do not respond to general deck cleaning. Grill grease needs a degreaser and needs it before it has weathered in; the longer it sits in summer heat, the more of it migrates into the board surface. A grill mat is the cheap version of solving this permanently.
Rust - the orange staining that shows up under planters, around fasteners, or in a spray pattern near an irrigation head - is iron, not dirt, and scrubbing will not touch it. It needs an oxalic-acid-based rust remover, applied to a wet board and rinsed thoroughly. If the source is well water going through a sprinkler, the staining will return every season until the water is treated, which is worth knowing before repeating the removal every spring.
Avoid full-strength chlorine bleach as a general cleaner, particularly on uncapped composite - it can lighten the color unevenly and it is hard on the plantings and any metal it runs across.
Avoid anything abrasive: wire brushes, scouring pads, powdered cleansers, and stiff-bristle deck brooms all leave marks in the surface that the deck keeps.
Avoid deck stains, sealers, and "restorer" coatings sold for wood. Composite is not meant to take a film-forming coating, it will not bond properly, and removing a failed coating from composite is a far worse job than the dullness it was applied to fix.
And avoid letting any cleaner dry on the board. Most of the streaking blamed on a product is a dwell-time problem - the product worked, it just was not rinsed before it dried.
For most Wilmington-area composite decks, a proper wash twice a year fits the climate: once after pollen drop has finished in late spring, and once in the fall after the leaves are down. Decks in heavy shade, under oak canopy, or on the water often want the shaded sections touched more often than that, and a deck in full sun with good air movement can sometimes go a year.
The signal to watch is not the calendar but the shaded end. When the green film returns to the north side or the strip against the house, the surface has enough organic material on it to be worth clearing, whatever the date says.
The honest answer is that most composite decks are a homeowner job. Soap, a brush, and an afternoon does it, and the equipment that would make it faster is the equipment most likely to damage the boards.
It is worth handing off when the deck is second-story or over a slope where working safely means more than a brush; when the mildew has gone well past a surface film and needs a proper soft-wash approach; when the deck cleaning is part of a larger job and it makes sense to do the siding and the deck and patio in one visit; or when there is rust or grease staining you have already tried once and not shifted.
If you are weighing that up, our page on what deck cleaning costs in Wilmington lays out what actually drives the number, and the difference between soft washing and pressure washing explains why the low-pressure method is the right one for a surface like this. If you are still working out whether what you are looking at is mold or mildew, that comparison is here. And if a screened porch or pool surround is part of the same project, that is a related job worth quoting together.
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