How do I clean mildew off a composite deck in Vancouver?

Composite decking in Metro Vancouver develops mildew and algae buildup due to our persistent humidity and rainfall, but it cleans off easily with the right approach and won't damage the deck surface like it would on wood.

The most effective method is using oxygen bleach (sodium percarbonate) rather than chlorine bleach. Mix one cup of oxygen bleach powder with one gallon of warm water, or use a commercial composite deck cleaner like Trex Deck Cleaner or Simple Green Oxy Solve. Apply the solution generously to the affected areas and let it sit for 10-15 minutes to break down the mildew. Scrub with a stiff-bristled brush (avoid wire brushes which can scratch the composite surface), then rinse thoroughly with your garden hose or pressure washer on a low setting.

For stubborn mildew spots, make a paste with oxygen bleach and a small amount of water, apply it directly to the stains, and let it work for 20-30 minutes before scrubbing. North-facing decks and areas under overhangs that stay damp longer will need more frequent cleaning — typically every 3-4 months during Vancouver's wet season from October through March.

Avoid chlorine bleach on composite decking as it can discolor the surface and harm surrounding plants when rinsed off. Also skip pressure washing at high settings (over 1,500 PSI) as it can damage the protective cap layer on composite boards. A garden sprayer or pump sprayer works perfectly for applying cleaning solutions evenly.

Prevention is key in Vancouver's climate. Sweep debris regularly so leaves and organic matter don't trap moisture against the deck surface. Trim back vegetation that blocks airflow over the deck — better air circulation reduces the damp conditions where mildew thrives. Consider installing fans under covered deck areas to improve ventilation.

Most composite manufacturers recommend cleaning 2-3 times per year in climates like ours, but decks with heavy tree cover or limited sun exposure may need monthly attention during the wet months. The good news is that unlike cedar decking, composite won't be damaged by the mildew — it's purely a cosmetic issue that cleans off without affecting the structural integrity or warranty of your deck.

This is definitely a DIY-friendly maintenance task that takes 1-2 hours for a typical 300 square foot deck and costs under $20 in cleaning supplies.