Cleaning Composite Decking (Trex): Pressure Settings and Soaps

Composite decking (Trex, TimberTech, Fiberon, and similar capped or uncapped boards) promised an end to staining and sealing. What it did not promise was an end to cleaning. The embossed wood-grain texture that looks so good also traps pollen, leaf tannins, sunscreen, and — in shaded, damp areas — mold and mildew that root into the grain pattern.

The catch: composite manufacturers publish strict pressure limits, and exceeding them can void your warranty. The good news is that composite responds beautifully to gentle washing with the right soap. This guide covers the pressure limits for major brands, which chemicals are safe (and which are banned), the step-by-step gentle wash process, and how to handle mold lodged in the embossing — using a standard rental pressure washer set up correctly.

Why Composite Is Different from Wood

Composite boards are a blend of wood fiber and plastic (polyethylene or PVC), often with a protective polymer cap. That construction changes the cleaning rules:

• The cap is thin. Capped composites have a protective outer layer measured in thousandths of an inch. Aggressive pressure or abrasive scrubbing wears through it, exposing the core — which stains permanently.
• Heat sensitivity. Dark composite boards get hot in sun; harsh solvents and high heat can discolor or warp them.
• Warranty limits are real. Trex, for example, specifies maximum 3100 PSI with a fan tip at 8+ inches — but also warns that damage from pressure washing is not covered. Staying well under the maximum is the smart move.
• Mold roots into texture. The embossed grain gives mold something to grip. Surface blasting tears the visible mold off but leaves roots in the grooves — chemistry must do the killing.

For natural wood decks, the rules are entirely different — see our wooden deck cleaning guide.

Manufacturer Pressure Limits and Safe Settings

Always check your specific product's care guide — but these are the widely published limits and our recommended working settings:

Brand / Type	Published Max PSI	Our Recommended Setting	Nozzle
Trex (capped composite)	3100 PSI	1500 PSI or less	25°–40° fan, 8–12 in
TimberTech (capped)	3100 PSI	1500 PSI or less	25°–40° fan, 8–12 in
Fiberon (capped)	3000 PSI	1500 PSI or less	25°–40° fan, 8–12 in
Uncapped composite	Varies	1200 PSI or less	40° fan, 12+ in
PVC decking (Azek etc.)	3100 PSI	1500 PSI or less	25°–40° fan, 8–12 in

Notice our recommendation sits at roughly half the published maximum. The maximum is a "do not exceed or else" number, not a target. At 1500 PSI with proper soap and dwell time, composite cleans completely — the extra pressure buys nothing but risk.

Approved Soaps — and Chemicals to Never Use

Safe and recommended:

• Dish soap and water (mild solution) for general grime — the manufacturer default.
• Oxygen-bleach-based composite deck cleaners for mold, mildew, and tannin stains.
• Specialized composite deck cleaners (brand-specific formulas exist for Trex and TimberTech).
• White vinegar solution for light mineral deposits.

Banned or risky — do not use:

• Chlorine bleach (sodium hypochlorite) at strong concentrations can lighten and damage composite, and most manufacturers warn against it. Oxygen bleach is the safer alternative.
• Solvents (acetone, MEK, paint thinner) attack the plastic component.
• Abrasive cleaners and wire brushes scratch the cap layer permanently.
• High-alkaline strippers meant for wood decks — wrong chemistry for composite.

When in doubt, test your cleaner on a scrap piece or a hidden board end first.

Step-by-Step Gentle Wash

1. Sweep thoroughly. Remove all loose debris — grit underfoot during washing acts like sandpaper.
2. Pre-rinse gently. Wet the deck with a garden hose or the washer's 40-degree tip at low pressure.
3. Apply composite-safe cleaner. Use a pump sprayer or soap tip, working in sections. For mold, use an oxygen-bleach deck cleaner and allow the full label dwell time (usually 10–15 minutes).
4. Soft-bristle agitation. Scrub with a soft-bristle deck brush along the grain pattern. This is the key step for mold in the embossing — the brush reaches where water sheets over.
5. Rinse at 1500 PSI or less. Using a 25- or 40-degree fan tip at 8–12 inches, rinse with the grain in overlapping passes. Let the water do the carrying, not the blasting.
6. Detail the gaps. Flush debris from between boards with a gentle fan — trapped organic matter in gaps is a prime mold nursery.
7. Final rinse. Rinse the entire deck once more, plus any siding or windows that caught overspray.

Fixing Mold Lodged in the Grain Pattern

The most stubborn composite problem is mold that has rooted into the embossed texture. If a standard wash leaves gray-green shadowing in the grain:

• Extend the dwell. Re-apply oxygen-bleach cleaner and give it 20–30 minutes (keeping it damp), then agitate again with the soft brush.
• Brush twice, pressure once. Two rounds of chemical + brushing beats one round of harder pressure every time — and it is warranty-safe.
• Address the cause. Mold returns to the same spots because of moisture and shade. Improve drainage, trim vegetation for airflow and sun, and keep the gaps between boards clear.
• Know when it is permanent. On uncapped composite, deep-set tannin or mold staining that survives two proper treatments may be permanent. A soft wash approach with extended dwell is the last gentle option before accepting it.

Frequently Asked Questions

What PSI is safe for Trex composite decking?

Trex allows up to 3100 PSI, but we recommend 1500 or less with a 25- or 40-degree fan at 8–12 inches. The maximum is a do-not-exceed limit, not a target.

Can I use bleach on composite decking?

Strong chlorine bleach is risky and most manufacturers warn against it. Use oxygen-bleach-based composite cleaners instead — effective on mold without the damage risk.

Will pressure washing void my composite deck warranty?

Damage from pressure washing is typically excluded from warranties. Staying well under the PSI limit with fan tips and proper distance protects deck and coverage.

How do I get mold out of the grain pattern on composite?

Apply oxygen-bleach cleaner, dwell 20–30 minutes kept damp, and agitate with a soft-bristle brush along the grain. Repeat the cycle rather than raising pressure.

Can I use a wire brush on composite decking?

No. Wire brushes and abrasive pads scratch through the cap layer, exposing the core to permanent staining. Soft-bristle brushes only.

How often should composite decking be cleaned?

Thoroughly twice a year — spring and fall — plus prompt cleanup of food, grease, and leaf piles. Shaded decks may need an extra mid-summer wash.