Composite boards do not rot, but they still grow mildew in Lowcountry humidity. Here is what actually cleans them, what stains them, and why the pressure setting matters more than the cleaner.
Short answer: soap, warm water, and a soft-bristle brush clean composite decking better than pressure does. The mildew you see is growing in the film of pollen, oak litter and salt residue sitting on top of the board, not in the board itself - so the job is removing that film without abrading the surface. In Charleston that means two cleanings a year, and it means checking your board's own care sheet before anyone points a pressure wand at it.
Homeowners are often told composite is maintenance-free, then find black speckling across the boards by their second Lowcountry summer. Both things are true. Composite decking is wood flour bound in plastic, so it will not rot, splinter or need staining the way a pressure-treated pine deck does. But mildew does not need the deck to be wood - it needs moisture and something organic to feed on, and a Charleston deck supplies both constantly.
What it feeds on is the layer you cannot easily see: spring pollen, live oak leaf litter and catkins, pine straw, and the fine salt-and-dust film that coastal air deposits on every horizontal surface. Add tidal humidity that barely drops for months and shade from mature oaks that keeps boards from ever fully drying, and you have the same growth conditions as any other exterior surface in the county. The mildew is on the deck, not in it, which is exactly why it comes off with far less force than people expect. The same pattern behind the streaking on siding is covered in our guide to Lowcountry humidity, pollen and mildew.
This is the single most useful thing to know about your deck, and it changes the method.
If you do not know which you have, look at a cut end or an under-board edge: a capped board usually shows a visibly different outer layer over the core. Otherwise, find the manufacturer and product line and pull the care sheet. That document is the only real authority on your specific boards.
Warm water and dish soap handles routine pollen and dirt film. For actual mildew, an oxygen-based cleaner (sodium percarbonate, sold as oxygenated deck cleaner) is the usual recommendation, because it lifts growth without the risks chlorine carries on these boards.
What to be careful with:
Whatever you use, test it on an out-of-sight board - under a step or behind a planter - and let it dry fully before doing the whole deck.
Sometimes, at low pressure, and only if your manufacturer permits it. This is where composite decks get ruined, and the damage is not always immediate.
Manufacturers genuinely disagree here: some permit a fan tip at a stated low pressure with a cleaning solution, others advise against pressure washing entirely and treat it as grounds for a warranty claim being denied. The rules also change between product generations from the same brand, so a number someone gives you for their deck may not apply to yours. Your board's care sheet is the authority - check it before, not after.
If pressure is permitted, the failure mode to understand is surface erosion. Too much pressure, or a narrow tip held too close, scours plastic out from between the wood fibers. The board looks acceptable while wet, then dries fuzzy, faded or streaked in the pattern the wand traced - and unlike wood, you cannot sand a composite board back. Keep a wide fan tip, keep it well back from the surface, keep it moving parallel to the boards, and never stop in one spot.
The honest version: on a residential composite deck, pressure is rarely what makes the difference. Dwell time and a brush do. Pressure mostly saves labor on large flatwork, which is why it belongs on driveways rather than deck boards. The broader distinction is covered in our guide to pressure washing versus power washing.
Twice a year suits most Lowcountry properties, and the timing matters more than the count: once after pollen season finishes in late spring, and once after the fall leaf and acorn drop. Decks under mature oaks, on north-facing elevations, or shaded most of the day often want a third pass in late summer, because those boards effectively never dry out. A rinse and a sweep between cleanings does more good than one heavy annual scrub.
Will cleaning restore faded composite to its original color? No. Cleaning removes growth and film, which often makes a deck look dramatically better. But UV fading is a change in the board itself, and no cleaner reverses it. Boards installed at different times will still not match after cleaning.
Do I need to seal composite decking afterward? No. Composite is not sealed or stained like wood, and most manufacturers do not want a coating on it. If a contractor offers to seal your composite deck, ask what product and check it against your care sheet first.
The mildew came back within a couple of months. Why? Usually gaps still packed with debris, a planter or rug trapping moisture, or a shaded section that never dries. Clear the gaps, lift anything sitting flat, and improve airflow before assuming the cleaning failed.
Is a composite deck cheaper to have cleaned than a wood deck? Often, because there is no stripping, sanding or resealing stage - it is a wash rather than a refinish. What drives the number is square footage, growth severity and access. We break the pricing down in our Charleston deck cleaning cost guide.
Should I just hire it out? A small, lightly soiled deck is a reasonable Saturday. A large deck, heavy embedded growth, an uncapped older deck, or anything where you are unsure what pressure the boards tolerate is worth handing to someone who cleans them regularly - the cost of replacing scoured boards is far above the cost of the wash. That is what our Charleston deck and patio cleaning service covers, and it is commonly booked alongside a house wash while the crew is already on site.
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