How to Clean Green Algae Off a Deck

Green algae spreads fast on decks in Ottawa. Humid summers. Heavy rain. Long winters. Shade from trees. All of it creates the perfect surface for slippery green buildup. It looks bad. It feels worse underfoot. If left alone, it weakens wood fibres and shortens the life of your deck.

Here is exactly how to remove it and keep it from coming back.

Why Green Algae Grows on Decks in Ottawa

Algae thrives in damp, shaded areas. North-facing decks see less sun. Boards near railings stay wet longer. Leaves trap moisture between planks. Snow piles sit for months.

Pressure-treated lumber absorbs water. Cedar and pine can hold organic material on the surface. That creates a feeding ground. Once algae takes hold, it spreads across boards and into grooves.

Stop it early. Act fast.

Tools and Materials You Need

Gather everything before you start.

Stiff-bristle deck brush
Garden hose with spray nozzle
Pump sprayer or bucket.
Oxygen bleach or white vinegar
Mild dish soap
Protective gloves
Safety glasses
Pressure washer (optional, low setting only)

Avoid chlorine bleach. It strips colour and weakens wood fibres. It harms nearby plants.

Step 1: Clear the Deck Surface

Remove patio furniture. Lift planters. Sweep away debris. Pay attention to corners and gaps between boards.

Rinse the entire surface with a garden hose. Soak the boards. This prevents the cleaning solution from absorbing too quickly.

Step 2: Apply a Safe Cleaning Solution

Mix one of the following:
Option A: Oxygen Bleach Solution
Mix according to label directions in warm water. This breaks down algae without damaging wood.
Option B: Vinegar and Soap Mix
Combine one part white vinegar with one part water. Add a few drops of dish soap.

Pour into a pump sprayer or bucket. Apply evenly across affected areas. Let it sit for 10–15 minutes. Do not let it dry.

Step 3: Scrub With Firm Pressure

Use a stiff-bristle brush. You have to scrub along the grain of the wood. Focus on green patches. Apply steady pressure. Avoid metal brushes. They scar the surface.

Work in sections. Rinse each section before moving on. Keep the surface wet.

You will see the green film lift. Continue until the wood looks clean and even.

Step 4: Rinse Thoroughly

Rinse the entire deck with clean water. Remove all residue. Leftover cleaner can dry into streaks.

If you use a pressure washer, keep the pressure below 1,500 PSI. Use a wide fan tip. Hold the wand at least 12 inches away. Move steadily. Never hold in one spot.

Too much pressure gouges wood. It raises fibres. It causes splintering.

Treating Composite Decking

Composite boards need less force. Use mild soap and warm water. Scrub gently with a soft brush. Rinse well.

Avoid harsh chemicals. Avoid high pressure. Composite surfaces scratch easily.

Drying and Inspection

Let the deck dry fully. This takes 24 to 48 hours in Ottawa’s summer climate. Inspect the surface in full daylight.
Look for:

Dark patches
Soft spots
Raised grain
Loose fasteners

Repair damage before sealing.

Prevent Green Algae From Returning

Prevention saves time.

Trim back trees and shrubs. Increase sunlight exposure.
Sweep weekly. Remove leaves and debris.
Improve drainage. Ensure water flows away from the deck.
Seal the wood. Apply a high-quality water-repellent stain every 2–3 years.

Always choose a stain for Canadian weather. Ottawa winters freeze and thaw repeatedly. Wood expands and contracts. Proper sealing reduces moisture absorption.

When to Call a Professional

Some decks need deeper restoration. Severe staining. Years of buildup. Structural damage. In these cases, professional cleaning and refinishing deliver better results.

A professional team can sand, brighten, and reseal the surface evenly. That extends the life of your deck and improves safety.

Final Result: A Clean, Safe, Long-Lasting Deck

Green algae makes decks slippery and unsightly. Quick action prevents permanent damage. Use safe cleaners. Always scrub along the grain. Rinse thoroughly. Seal the surface.

Maintain it seasonally. Stay consistent. Your deck will look clean, feel solid, and withstand Ottawa’s harsh climate year after year.

A well-maintained deck adds home value. It creates a safe outdoor space for family and friends. Clean it properly. Protect it fully. Keep it green-free.