Can You Jet Wash Composite Decking to Remove Algae and Dirt?

Jet washing is one of the most satisfying forms of garden maintenance. Watching years of grime, algae, and green slime disappear in seconds is genuinely rewarding - and composite decking is no exception. But there's a catch.

The question isn't just can you jet wash composite decking - it's should you, and if so, how? Done correctly, jet washing is one of the most effective ways to clean composite boards and restore their appearance. Done incorrectly - with too much pressure, at the wrong angle, or with the wrong nozzle - it can strip the board surface, raise the grain, or push water into the board structure in ways that cause problems down the line.

At Paving and Slabs Ltd, we want your composite decking to look its best for decades. Here's the definitive guide to jet washing it safely and effectively.

Yes, You Can Jet Wash Composite Decking - With Conditions

The short answer is yes. Most quality composite decking products are suitable for jet washing, and regular washing is actually recommended by most manufacturers as part of routine maintenance.

The conditions matter, however:

1. Pressure must be kept within safe limits - typically no more than 1,400–1,500 PSI (100 bar)
2. Nozzle angle and distance must be correct - never directly perpendicular at close range
3. The right nozzle type must be used
4. Jet washing is appropriate for cleaning, not for high-pressure stripping

Exceeding these parameters - particularly with a powerful domestic or trade jet washer - can damage even quality composite boards by eroding the cap layer or raising fibres on uncapped products. Understanding your product is important. See our guide on what composite decking is made of for a clear explanation of capped vs uncapped construction and why it matters for cleaning.

The Right Equipment for Jet Washing Composite Decking

Jet Washer Pressure

• Maximum recommended: 1,400–1,500 PSI (approximately 100 bar)
• Most domestic jet washers operate at 100–140 bar; stay at the lower end of your machine's range
• Trade jet washers can reach 200+ bar - do not use at full power on composite decking
• Higher pressure does not clean more effectively; it simply damages the surface

Nozzle Type

• Use a fan/flat nozzle (25-degree or 40-degree tip) - these distribute the water pressure across a wider area
• Never use a zero-degree (pencil jet/red tip) nozzle - this concentrates all the pressure into a single point and will damage composite boards immediately
• A rotary/turbo nozzle is too aggressive for composite boards and should be avoided
• If your machine has a soap/detergent nozzle setting, this is ideal for applying pre-treatment cleaners

Nozzle Distance from Surface

• Maintain a minimum distance of 300mm (30cm) between the nozzle and the board surface
• Working closer than this increases effective pressure dramatically - at 150mm, the same nozzle can exert double the surface pressure it does at 300mm
• Keep the distance consistent throughout to ensure even cleaning

Step-by-Step Guide to Jet Washing Composite Decking

Step 1: Pre-Sweep

Before applying any water, sweep the deck surface thoroughly with a stiff broom to remove loose debris - leaves, soil, grit, and any other dry material. This prevents debris from being pushed into board joints by the jet washer.

Pay particular attention to clearing between the board gaps - accumulated debris in joints holds moisture and promotes algae growth. A gap tool or stiff brush works well for this.

Step 2: Pre-Treat Heavy Algae or Staining (Optional but Recommended)

If your deck has significant algae, moss, or surface staining, a pre-treatment cleaner will dramatically improve the jet washing result:

• Apply a composite-safe decking cleaner or a diluted white vinegar solution (50/50 with water) to the affected areas
• Allow to dwell for 10–15 minutes - this loosens the algae and biological growth before you apply water pressure
• For rust staining specifically, see our detailed guide on removing rust from composite decking before proceeding

Step 3: Set Up the Jet Washer Correctly

• Attach a 25-degree or 40-degree fan nozzle
• Set the machine to a moderate pressure setting (not maximum)
• Stand back and test on a less visible area of the deck first - a section under furniture or near the back of the garden

Step 4: Wash in the Direction of the Board Grain

This is critical. Always move the jet washer nozzle in the same direction as the board grain - never across it.

• Moving across the grain forces water sideways into the board material
• Moving along the grain allows water and loosened material to travel to the board ends where it can drain away
• Use smooth, continuous strokes - don't stop the stream in one place
• Overlap each stroke slightly to ensure complete coverage

Step 5: Maintain Consistent Distance

Keep the nozzle at a steady 300–400mm from the surface throughout. Irregular distance creates uneven cleaning that leaves tide marks and inconsistent appearance.

Step 6: Clear the Joints

After washing the board surfaces, direct the water along the board joints to clear debris that was displaced during washing. A lower pressure rinse is more appropriate here than the full jet - you want to flush the joints, not pressure-blast the board edges.

Step 7: Final Rinse

Do a final low-pressure rinse of the entire deck surface, moving away from the house to direct dirty water off the deck rather than back towards the building or into adjacent planted areas.

Step 8: Allow to Dry Before Use

Composite decking can be slippery when wet. Allow the surface to dry fully before walking on it, particularly if elderly or young children will be using the deck. For ongoing slip prevention strategies, see our guide on how to stop composite decking feeling slippery.

Dealing with Stubborn Algae and Green Slime

Green algae is the most common biological growth on UK composite decking and typically responds well to jet washing combined with pre-treatment. For heavy algae that has established over several seasons:

1. Apply a specialist algae remover or composite-safe biocide solution and allow to dwell for the full recommended period (sometimes up to 30 minutes)
2. The algae should visibly die and lose its green colour - this means the treatment is working
3. Jet wash using the technique above - loosened dead algae rinses away much more easily than live growth
4. For any remaining patches, repeat the pre-treatment and washing process

Prevention is better than cure: Algae thrives in shaded, damp conditions. If your deck is in a persistently shaded area, consider whether adjacent shrubs or structures can be managed to increase airflow and light. Regular brushing to remove debris also dramatically reduces algae establishment.

How Often Should You Jet Wash Composite Decking?

Annual jet washing is sufficient for most composite decks that are regularly swept and maintained. A thorough jet wash in spring (to address winter grime and any algae established over autumn and winter) keeps the deck looking fresh for the season ahead.

Decks in particularly shaded locations, or those adjacent to overhanging trees, may benefit from twice-yearly jet washing.

Between jet washes:

• Brush weekly or as needed to remove debris
• Mop up spills and food debris promptly
• Address rust staining immediately - see our rust removal guide for the right technique

Can You Jet Wash All Types of Composite Decking?

Most modern composite decking products from reputable manufacturers are jet wash compatible. However:

• Very old or degraded composite boards may have weakened surfaces that cannot withstand jet washing pressure - inspect carefully first and start with the lowest pressure setting if in doubt
• Grooved deck boards need particular care - direct the jet along the grooves, never across them
• Some manufacturers specify maximum pressures in their care guides - always check your product documentation

For composite decking products supplied by Paving and Slabs Ltd, our team can advise on the manufacturer's recommended cleaning guidance. Browse our composite decking range and contact us with any product-specific cleaning questions.

Jet Washing vs Other Cleaning Methods: When to Use Each

Cleaning Challenge	Best Method
General dirt and grime	Jet wash (annual)
Algae and green slime	Pre-treat then jet wash
Rust staining	Oxalic acid or vinegar treatment - NOT jet washing alone
Surface scratches	Heat gun or touch-up kit - see our scratch removal guide
Food and oil stains	Warm soapy water + brush; jet wash to finish
Leaf tannin staining	Oxalic acid or proprietary cleaner; then jet wash

Summary

Yes - you absolutely can jet wash composite decking, and it's one of the most effective maintenance tools available. The key requirements are: staying within the 1,400–1,500 PSI pressure limit, using a fan nozzle at a consistent 300–400mm distance, moving always with the board grain, and pre-treating heavy algae or staining before applying water pressure.

Done correctly and annually, jet washing keeps composite decking looking as good as the day it was laid. Combined with our guidance on stopping composite decking from fading, removing rust stains, and addressing surface scratches, you have everything you need for a deck that looks outstanding year after year.

Explore our full composite decking collection or contact our team for expert advice on the right product for your outdoor space.