TL;DR
Composite decking is low maintenance, not maintenance free -- it grows mildew, it stains, and most manufacturers cap pressure washing somewhere around 1,500 PSI with a fan tip. Warm soapy water and a soft bristle brush handles routine cleaning, and the single most important step is checking your specific brand's care sheet before any product touches the boards.
Composite is not the same material as wood, and it does not fail the same way
Composite boards are wood flour and plastic pressed together, usually with a capped polymer shell on newer products. That shell is what resists staining and fading, and it is also what you can permanently damage.
Older uncapped composite, common on decks built before capping became standard, has exposed wood fiber in the surface. That fiber feeds mildew exactly the way real wood does, which is why homeowners who were promised a no-maintenance deck end up with black speckling in the shaded corners.
Neither type rots the way lumber does, and neither needs staining. But both need washing, and both can be wrecked in an afternoon with the wrong tool.
Source: Composite decking manufacturer care and warranty documentation
Does composite decking really get mildew?
Yes. Mildew grows on the film of pollen, leaf tannin, and organic debris that settles on any outdoor surface, and it does not need the deck itself to be a food source. Shaded, damp, poorly ventilated areas grow it first. Regular washing removes the food layer, which is the real prevention.
Check the warranty before you check anything else
This is the step that saves money. Composite warranties are specific and they do exclude damage from cleaning.
Almost every manufacturer publishes a care sheet naming a maximum pressure, an approved cleaner type, and a list of prohibited products. The common limits look like this, though yours may differ:
- Maximum pressure typically around 1,500 PSI, with a fan tip, held at a stated distance
- Approved cleaners usually soap and water, or a composite-specific deck cleaner
- Commonly prohibited chlorine bleach on some brands, acid-based cleaners, solvents, abrasive pads, and wire brushes
- Never paint stripper, acetone, or oven cleaner on a capped surface
The care sheet is usually a one-page download under the product name. Ten minutes reading it is cheaper than a board replacement, and using a prohibited product is one of the few ways a homeowner can genuinely void the coverage.
The routine wash that handles ninety percent of it
Twice a year is enough for most composite decks -- spring and again after leaf drop.
Clear and sweep. Take furniture, mats, and planters off. Anything sitting on the deck creates a shaded, damp footprint that grows mildew and can leave a shadow in the fading pattern.
Clear the gaps. Debris packed between boards is the main reason composite decks stay wet. Run a putty knife or a joint tool down each seam. This one step prevents more mildew than any cleaner.
Wash with soap and warm water. A bucket of warm water with dish soap, applied with a soft bristle brush, is the manufacturer-approved method on nearly every product. Scrub the full length of each board rather than in circles, because circular scrubbing on an embossed grain pattern shows as swirl marks in low sun.
Rinse thoroughly. Soap residue left on composite attracts dirt and can leave a haze. Rinse each section before moving on so nothing dries with cleaner still on it.
Pressure washing composite: allowed, but on a leash
Many brands permit a pressure washer within limits, and it genuinely speeds up a large deck.
Use a 40-degree or wider fan tip, stay at or below the pressure your care sheet names, hold the wand at least 8 to 12 inches off the surface, and move with the board. Never use a zero-degree or 15-degree tip. On capped composite, a narrow tip can cut through the shell and expose the wood-flour core, and once the cap is breached that spot will stain and mildew permanently.
The failure mode is the same one described in why high pressure ruins a wood deck, just with a different visible result -- instead of furring, you get a dull scoured patch that never matches the boards around it. More of the general errors are collected in deck cleaning mistakes.
Never mix cleaners on a deck
If the first product did not work, rinse it off completely before trying a second one.
Never combine a chlorine bleach product with an acidic cleaner, a rust remover, or a brightener -- that reaction releases chlorine gas. Never combine bleach with an ammonia-based cleaner. This matters more on a deck than indoors, because people work bent over the surface with vapors rising toward them, and "it is outside" is not the protection it feels like.
Mold in the shade is a ventilation problem
A deck section that grows black speckling every year is telling you something about airflow and water, not about your cleaning schedule.
Look at what is holding moisture. Skirting with no vents, a low deck close to damp ground, a downspout discharging under the frame, or shrubs grown tight against the edge all keep the underside wet. Trimming back plantings, redirecting the downspout, and clearing the gaps between boards will do more than doubling your cleaning frequency. Railings collect their own growth on the underside of the handrail, covered in cleaning railings and spindles.
If growth keeps returning across a large area despite good drainage, that is a moisture problem in the structure, and remediation of persistent mold is a separate trade from cleaning. For how composite fits alongside other outdoor materials, see the exterior surface cleaning overview, and what a deck cleaning service covers for the hired-out version.
FAQ
Can you pressure wash composite decking?
Most manufacturers allow it up to roughly 1,500 PSI with a fan tip held 8 to 12 inches off the surface, but the limit varies by brand and the care sheet governs. Never use a narrow tip. On capped composite, a concentrated stream can breach the shell and expose the core, which stains permanently.
What is the best cleaner for composite decking?
Warm water with dish soap and a soft bristle brush is the method nearly every manufacturer approves. For mildew or tannin staining, a composite-specific deck cleaner with oxygen bleach and a 10 to 15 minute dwell works well. Avoid solvents, abrasives, and any product your care sheet prohibits.
Why does my composite deck have mold on it?
Mildew grows on the layer of pollen, leaf tannin, and organic debris that settles on the surface, not on the composite itself. Shaded, damp, poorly ventilated areas grow it first. Clearing debris from between the boards and improving airflow underneath prevents far more of it than cleaning does.
Will bleach damage composite decking?
Some brands permit diluted chlorine bleach and some prohibit it outright, and it can lighten colored boards unevenly. Check your care sheet first. Oxygen bleach is the gentler alternative and is accepted more broadly. Never combine either with an acidic cleaner or an ammonia-based product.
Do I need to seal or stain composite decking?
No. Composite is not meant to be sealed or stained, and a coating on a capped board typically will not adhere properly and will peel. If your deck has faded, that is ultraviolet weathering, which is normal on the first season and then stabilizes. Some brands sell a color-restoring product.
How do I get grease stains off a composite deck?
Blot the grease immediately, then wash with warm soapy water and a soft brush. The sooner you get to it, the better. Grease that has warmed into the surface may lighten but often will not fully clear, which is why a grill mat under a cooking area is worth having.
How often should composite decking be cleaned?
Twice a year suits most decks -- once in spring and again after leaf drop. Decks under trees or in heavy shade often want a third pass. The debris between boards should be cleared whenever it builds up, because that trapped material is what keeps the surface damp.
Where this is one task among many, recurring house cleaning services take the broader load.
Deck size, board age, whether the surface is capped, and how much growth has established all change what the job takes. Every deck is different, so get a quote and we will look at yours and walk you through the options.