Composite decking is a wood-plastic material with a manufactured surface, and pressure marks on it do not weather away the way they do on wood. Clean it with a deck-appropriate cleaner, a soft brush, and low-pressure rinsing. Most of what looks like dirt on composite is a mildew film feeding on organic material sitting on the boards, not staining of the material itself.
Key Takeaways
- Composite resists rot and insects; it does not resist scarring or heat
- Pressure marks and brush scratches on composite are permanent
- Mildew grows on the organic film on top of the boards, not in them
- Capped and uncapped composite behave differently; know which you have
- Check the manufacturer's cleaning guidance, since aggressive cleaning can void a warranty
Why Composite Needs Controlled Cleaning
Composite decking is sold on being low maintenance, and it is: it does not need staining, it does not rot, and insects leave it alone. Low maintenance is not no maintenance, and the maintenance it does need is more particular than wood's.
The material is a blend of wood flour and plastic, usually with a manufactured surface texture and, on most modern boards, a protective polymer cap. That surface is the thing to protect. Unlike wood, which can be sanded or which weathers marks away over a few seasons, a scar in composite is there permanently. A pressure wand held too close leaves a stripe that will be visible for the life of the deck.
The second thing to understand is what is actually growing. Composite does not absorb water the way wood does, so mildew is not growing in it. It is growing on the film of pollen, leaf litter, and organic debris sitting on top of the boards, particularly in the grooves and along the shaded edges. That is good news for the removal: what needs breaking down is the film, not something embedded in the material.
Which means chemistry and a soft brush do the work, and pressure is unnecessary.
Identify the Buildup and the Board Surface
Capped or uncapped. Boards made in roughly the last decade are usually capped, with a polymer shell over the composite core. Older uncapped boards are more porous, stain more easily, and are more prone to mildew getting a grip. Check the board edges or the manufacturer's mark on the underside if you can see one.
What is on the boards:
- Green or black film, especially on shaded boards and in the grooves. Mildew on the organic layer. The most common issue and the most treatable
- A dull gray cast across the whole deck. Usually accumulated pollen and film rather than fading
- Dark ring stains under planters and furniture feet, from trapped moisture and organic material
- Tannin staining from leaves left to break down in place, which appears as brown blotches
- Rust marks below metal furniture feet or fasteners
- Grease and food staining near a grill or table
- White chalky residue, which on older uncapped boards can be efflorescence-like mineral bloom
- Actual fading, which is UV weathering of the surface and does not clean off
The distinction matters because mildew film, organic staining, and UV fading have completely different answers, and only the first responds well to washing.
Test and Choose a Suitable Cleaning Approach
Check the manufacturer's guidance first. Composite decking is usually warranted, and most manufacturers publish specific cleaning instructions including pressure limits. Exceeding them can affect the warranty, which makes this the one deck material where reading the documentation is worth the time.
Use a cleaner formulated for composite decking, or a mild deck cleaner. Products intended for biological growth work well. Avoid anything abrasive, anything acidic, and any solvent-based cleaner, which can craze the polymer cap.
Brush, do not scrub hard. A soft-bristle brush worked along the grain of the board texture. Stiff bristles and any abrasive pad leave scratches that are as permanent as pressure marks, and cross-grain scrubbing shows more than working along the length.
Low pressure only for rinsing. Where a pressure washer is used at all, it should be at the low end with a wide fan tip held well back, and moving continuously. Many manufacturers specify a maximum pressure; some advise against pressure washing entirely.
Test first, in a corner or under a piece of furniture, and let it dry fully. Composite looks different wet, and any lightening, dulling, or texture change only shows once dry.
A Practical Cleaning Sequence
- Clear the deck completely. Furniture, grills, planters, rugs. Outdoor rugs in particular trap moisture and are frequently the cause of the discoloration underneath them
- Sweep, then clear the gaps between boards. Debris packed in the gaps holds water against the board edges and is where mildew starts. A putty knife or a stiff wire works
- Rinse the deck with plain water to remove loose material
- Pre-wet surrounding plantings and the siding at the house wall
- Apply cleaner to a manageable section and let it dwell per the product instructions, without letting it dry
- Brush gently along the board length with a soft brush
- Rinse thoroughly at low pressure, working with the board direction
- Move to the next section before the previous one dries
- Do the railings, stairs, and fascia boards, which are usually dirtier than the field and are more visible
- Rinse the whole deck once more, plus the surrounding plantings
- Let it dry fully before replacing furniture, and put feet or pads under anything that sits directly on the boards
Mistakes That Leave Scarring or Discoloration
- Pressure washing at concrete settings. The classic and permanent one. Leaves visible stripes and can raise the surface texture
- Holding the wand in one place or moving slowly. Even at moderate pressure, dwell time in one spot marks composite
- Stiff-bristle brushes or abrasive pads, which scratch the cap
- Scrubbing across the board rather than along it, which makes any scratching far more visible
- Acidic or solvent-based cleaners, which can craze or dull the polymer surface
- Letting cleaner dry on the boards, leaving a residue that streaks
- Ignoring the gaps between boards, so the mildew source stays in place and returns quickly
- Leaving outdoor rugs and planters directly on the deck, which is the most common cause of persistent discolored patches
- Using a wood deck stripper or brightener, which is formulated for wood chemistry and is not appropriate here
- Expecting faded boards to come back. UV fading is a change in the material surface, not dirt
When to Call for Professional Help
Composite is a reasonable homeowner cleaning job on a routine basis. Bring in help when:
- The deck is large enough that consistent dwell time across it is impractical
- Staining is not mildew: rust, deep tannin, grease, or ring marks that have been there for seasons
- The boards are older uncapped composite, which is more porous and less forgiving
- A previous cleaning left marks you are trying to correct
- The deck is elevated or has railings and stairs that are difficult to work safely
- You want the house wall and siding above the deck cleaned at the same time, which should be done first so runoff does not land on a finished deck
- The manufacturer's warranty conditions are strict and you would rather the work be documented
On the sequencing point: if the siding above the deck also needs washing, do the wall first. Our guide on the right order to clean your home's exterior covers why.
Request an Assessment
WashLabs Exteriors cleans composite decking with deck-appropriate chemistry and low-pressure technique, and sequences deck work with any siding or roof cleaning so the deck is finished last.
Call (610) 665-3995 or request a free assessment. See our patio and walkway washing and house washing services.