How to Clean Composite Decking Without Damaging It

Composite decking is one of the smartest investments you can make in your outdoor space — but one of the most common misconceptions new owners have is that it never needs cleaning. In reality, composite decking is low-maintenance, not no-maintenance. And if you clean it the wrong way, using the wrong products or the wrong tools, you can do more harm than good.

This guide explains how to clean composite decking safely, what products to avoid, and how to keep your boards looking their best year after year — whether you're the owner of a brand-new Millboard deck or a Trex installation that has been down for a few seasons.

Why Composite Decking Still Needs Cleaning

Composite boards are engineered to resist moisture, rot, and fading — but they're not immune to the outdoors. In the UK climate, where damp, shaded gardens are common, organic material like algae, moss, and pollen settles onto the surface and into the grooves between boards. Left untreated, this creates a thin, slippery biofilm that reduces grip and dulls the appearance of the boards.

The boards themselves aren't the problem. It's what grows on top of them. This is why a simple, regular cleaning routine makes all the difference — and it's far less effort than sanding, staining, or treating a timber deck each year.

What You'll Need Before You Start

You don't need specialist equipment. For a standard clean, gather:

• A stiff-bristled brush (not wire) or a soft deck brush

• A bucket of warm water with washing-up liquid, or a composite-specific deck cleaner

• A garden hose with a decent flow rate

• A pressure washer (optional — see below for how to use it safely)

Avoid anything abrasive or metal-bristled. Even on textured boards, aggressive scrubbing can wear down the protective cap layer over time.

How to Clean Composite Decking Step by Step

Step 1 — Clear the Deck

Remove all furniture, planters, and objects from the surface. Sweep away loose debris — leaves, dirt, grit — with a soft brush or broom. Pay particular attention to the gaps between boards, where debris accumulates and traps moisture.

Step 2 — Wet the Surface

Use a garden hose to wet the deck thoroughly. This softens any dried-on dirt and makes the cleaning stage more effective. Work from one end of the deck to the other so you're always pushing dirty water away from yourself.

Step 3 — Scrub With Soapy Water

Apply your cleaning solution and scrub the boards with your brush, always working with the grain — in the same direction as the board's texture. Never scrub across the grain, as this can leave visible scratch marks, particularly on lighter board colours. Work in sections of two or three boards at a time.

For stubborn patches of mould or algae, let the soapy water sit on the surface for two to three minutes before scrubbing. A diluted white vinegar solution (one part vinegar to three parts water) can help with organic growth without harming the board surface.

Step 4 — Rinse Thoroughly

Rinse every section with clean water, again working with the grain. Make sure no soap residue remains — dried detergent can leave a white haze on darker boards. Check that water is flowing freely through the gaps between boards; if it isn't, use the jet from your hose to clear any blockages.

Can You Use a Pressure Washer on Composite Decking?

Yes — with the right settings. The key rules are:

• Maximum 1,500 PSI

• Fan-tip nozzle only (never a pinpoint jet)

• Hold the nozzle at least 20–30 cm from the surface

• Always work with the grain of the boards

Avoid pressure washing uncapped composite boards — these are softer and more porous, and high pressure can open up the surface. Capped composite boards such as those from Millboard or Trex are designed to withstand controlled pressure washing. If you're unsure whether your boards are capped or uncapped, check with your installer before reaching for the pressure washer.

What NOT to Use on Composite Decking

This is where many homeowners make costly mistakes. Avoid:

• Bleach: causes discolouration and degrades the cap layer over time. Trex specifically warns against bleach use on their boards.

• Wire brushes or metal scrapers: these will scratch and scour the surface permanently.

• Oil-based products or deck stains: composite boards don't absorb them — they sit on the surface, attract dirt, and become slippery.

• High-pressure pinpoint jets: even on capped boards, a concentrated jet can force water into the board's core.

Stick to mild, pH-neutral cleaners designed for composite surfaces, or plain soapy water for routine cleans.

How to Deal With Specific Stains on Composite Decking

Algae and Mould

The most common issue in UK gardens. A warm soapy scrub usually shifts light growth. For heavier build-up, use a composite-specific cleaner containing a mild surfactant. For persistent algae, NewTechWood recommends a dedicated composite deck wash rather than household remedies.

Oil and Grease

Act quickly — composite boards are easier to clean when the stain is fresh. Blot (don't spread) the oil with an absorbent cloth, then apply washing-up liquid directly and scrub. Rinse thoroughly. Dried oil that has been sitting for days may require a dedicated degreaser.

Food, Drinks, and Other Organic Matter

These clean up easily with warm soapy water and a brush. Red wine, juice, and condiments shouldn't stain capped composite boards if you attend to them the same day. Rinse promptly and the cap layer will do most of the work.

How Often Should You Clean Your Composite Decking?

A thorough clean twice a year — once in spring and once in autumn — keeps most decks in good shape. Spring cleaning removes the damp residue from winter; autumn cleaning clears fallen leaves before they have a chance to stain the boards or block drainage gaps over the colder months.

In between, a quick sweep and a rinse with the hose takes five minutes and prevents the build-up that makes the big cleans harder.

If your deck is shaded by trees or a building and stays damp for long periods, you may need to clean more frequently — particularly to stay ahead of algae. Take a look at our completed installations to see how we've designed low-maintenance decks across Merseyside and Cheshire.

How to Clean Composite Decking — The Short Version

Composite decking is designed to make your life easier, not harder. A mild clean with soapy water and a brush, twice a year, is enough to keep most boards looking sharp and safe underfoot. The key is to stay away from bleach, metal tools, and high-pressure jets — and to act promptly when something spills.

If you're unsure whether your boards are still performing as they should, or if you're thinking about a new composite deck for this summer, we're happy to talk it through. Visit our Composite Decking page for an overview of what we offer, or get in touch to arrange a consultation with our team in Merseyside and Cheshire.