Low maintenance is not no maintenance
Composite boards do not need staining or sealing like wood, but they still get dirty. Pollen, dust, leaf litter and food spills settle on the surface and in the grooves, and that layer is what feeds mold and mildew. The boards themselves are not rotting. The film on top is what turns dark.
For most decks, the routine is to sweep, clear the gaps between boards, apply a cleaner made for composite, scrub with a soft brush along the grain, and rinse. A pressure washer is optional and only helps if it is used gently.
Check your manufacturer's care guide first
Every composite brand publishes care instructions, and they differ. Some allow pressure washing within limits on pressure and nozzle distance, some recommend only a garden hose and brush, and some warn that pressure washing can affect the warranty. Older uncapped boards and newer capped boards also behave differently.
Look up your brand and product line before you start. If you are unsure which product you have, check closing paperwork, ask your builder, or look for stamps on the underside of a board where it is visible.
Safe pressure washing technique
If your manufacturer allows it, use a wide fan tip, never a zero-degree or narrow tip and never a turbo nozzle. Start the spray off the deck and bring it in, keep the wand moving along the length of the boards, and never stop in one spot. Holding the tip close or pausing can scar the surface, leaving lighter fuzzy streaks that do not wash out.
Work with the board direction, rinse the gaps so debris flows out, and finish with an even full rinse. A lot of the work should come from the cleaner and the brush, not the water pressure.
Mold, mildew and dark spots
Shaded decks along the coast, where the marine layer keeps things damp, and decks under trees grow mildew fastest. A cleaner formulated for composite decks, given proper dwell time, usually lifts it. Avoid strong chlorine mixes unless your care guide says they are fine, since some composites can discolor.
Mold that keeps coming back usually means the gaps are packed with debris and the deck stays wet underneath. Clearing the gaps and trimming back plants for airflow matters as much as the cleaning.
Outdoor rugs and planters sitting on the deck also hold moisture against the boards. Move them a few times a year, and use planter feet so water can drain and air can circulate underneath.
Grease, rust and leaf stains
Grill grease is the most common stubborn stain. Wipe fresh drips right away and use a degreaser approved for composite on older spots. Rust marks from metal furniture feet and leaf tannin stains often need specific stain removers sold for composite decking. Spot test first in a corner.
Expect some older stains to lighten rather than vanish. Composite can absorb oils into its surface, and deep scuffs or burn marks are damage, not dirt.
Mistakes to avoid
Do not scrub with wire brushes or metal scrapers, which scratch the surface. Do not use acetone or harsh solvents. Do not leave cleaner to dry in direct sun on a hot Valley afternoon, since it can leave residue. Clean in the morning or on a cooler day instead.
Also watch what is below the deck. Wash water can drip onto plants, a patio or a lower walkway. Rinse those areas at the end.
Finally, do not use a surface cleaner made for concrete unless your deck maker approves it. The spinning bar can catch board edges and screws, and the fixed distance does not suit every board profile. Hand work with a fan tip and a brush is slower but safer on most decks.
Getting it done
A once-a-year deep clean plus regular sweeping keeps most composite decks looking good. Deck cleaning is part of our fence and deck service, and the instant quote calculator on this page is the easiest way to get a number for your size. At Say Wash It, we follow your board maker's guidance on pressure and cleaners, and the flat price is confirmed before work begins. Every visit has a $195 minimum as of October 2026, so pairing the deck with a patio or fence on the same visit is often the better value.
Key points
Low maintenance is not no maintenance
Composite boards do not need staining or sealing like wood, but they still get dirty. Pollen, dust, leaf litter and food spills settle on the surface and in the grooves, and that layer is what feeds mold and mildew. The boards themselves are not rotting. The film on top is what turns dark.
Safe pressure washing technique
If your manufacturer allows it, use a wide fan tip, never a zero-degree or narrow tip and never a turbo nozzle. Start the spray off the deck and bring it in, keep the wand moving along the length of the boards, and never stop in one spot. Holding the tip close or pausing can scar the surface, leaving lighter fuzzy streaks that do not wash out.