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Deck Turning Green? Clean Off the Algae Before It Stains

Green film and dark spots are algae and mildew on damp, shaded boards. Scrub them off safely, then use a deck cleaner to slow the return.

By Jamilur Rahman Updated November 5, 2026 · 6 min read

Time
2 hours
Cost
$20-$50
Difficulty
Easy
Wooden deck boards covered with a green algae film and dark spots, with a stiff deck brush resting on them
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. Wood or composite? Check before you clean
  9. What to skip
  10. FAQ
  11. Related problems

Quick answer

Why is my deck turning green? Algae and mildew grow on boards that stay damp and shaded, and the film turns slippery fast. Sweep, wet the deck and scrub with a stiff brush and a deck cleaner made for algae and mildew, then rinse well. A cleaner that kills the growth, not just the surface film, keeps the boards clear for longer.

Free fix vs. permanent fix

Free fix · 1 to 2 hours

Soap, water and a stiff brush

  • Sweep and clear debris from between the boards
  • Scrub with dish soap and water, with the grain
  • Rinse well and let the boards dry for a day or two
  • Wear non-slip shoes, since wet algae is slick

How long it lasts: Weeks to a season. Where boards stay damp, the film grows back.

Most readers choose Permanent fix
Deck cleaner for algae and mildew (illustration)
Illustration

A deck cleaner for algae and mildew

Soap lifts the surface film but can leave spores in the grain. A deck cleaner made for algae and mildew, often oxygen-bleach based, soaks into the wood and breaks down the growth, so you scrub less and the green takes longer to return.

What to get: Deck cleaner for algae and mildew

How long it lasts: Often a season or more, depending on shade and rain. It cleans deeper than soap.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Garden hose with nozzle

    For wetting and rinsing the boards

    You have it

  • Putty knife

    For scraping debris from the gaps between boards

    You have it

  • Gloves and safety glasses

    Cleaner splashes can sting

    You have it
  • Our pick
    Deck cleaner for algae and mildew (illustration)
    Illustration

    Deck cleaner for algae and mildew

    Choose one labeled for your decking type

    Check price
  • Long-handled stiff deck brush (illustration)
    Illustration

    Long-handled stiff deck brush

    Saves your knees and scrubs evenly

    Check price

Why it happens

1

Shade and damp

Algae need moisture. Boards under trees, beside a wall or on the north side stay wet longer and turn green first.

2

Spores in the grain

Wood is porous, so algae and mildew spores sit in tiny cracks. Pollen, leaves and dirt feed them.

3

Debris between boards

Leaves and dirt in the gaps hold water against the wood and block airflow, which feeds mildew.

4

A worn finish

When the sealer wears off, boards soak up water. Composite boards do not rot, but they also grow mildew where dirt collects.

Try this first (free)

Do these in order, starting with the simplest.

  1. Clear and sweep

    Tool used: Putty knife

    Move furniture and planters. Sweep the deck and scrape out leaves and dirt from the gaps.

  2. Wet the boards and protect plants

    Tool used: Garden hose with nozzle

    Soak plants beside the deck with the hose and cover small ones. Wet the deck so the cleaner does not soak in unevenly.

  3. Apply the cleaner and wait

    Tool used: Gloves and safety glasses

    Mix the cleaner as the label says, using warm water. Oxygen bleach powder is a common choice. Work in sections of a few boards. Let it sit 10 to 15 minutes and keep the boards wet.

  4. Scrub with the grain

    Tool used: Long-handled stiff deck brush

    Use a stiff nylon-bristle deck brush and scrub along the boards. Skip wire brushes, which gouge the wood.

    Long-handled stiff deck brush (illustration)
    Illustration

    Tool for this step

    Long-handled stiff deck brush · Saves your knees and scrubs evenly

    Check price on Amazon

  5. Rinse and let it dry

    Tool used: Garden hose with nozzle

    Rinse until the water runs clear. If you use a pressure washer, stay under about 1,500 psi on wood, use a wide fan tip and keep it a foot away. Let the deck dry for a day or two.

  6. Cut the damp

    Trim branches that shade the deck, sweep leaves weekly and keep planters on feet so water cannot sit underneath. A water-bead test shows if the sealer still works. If water soaks in, it is time to reseal.

When free fixes aren't enough

If soap and scrubbing clear the surface but the green returns within weeks, spores are staying in the grain. A deck cleaner that kills growth does more than a bucket of soap. Skip chlorine bleach on wood, which can lighten and weaken it and harm plants and metal fasteners. If dark patches stay after cleaning, press a screwdriver into them. If it sinks into soft wood, the board is rotting and needs replacing. Wet algae is also slippery, so a deck that stays green is a fall risk, especially on stairs.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
Deck cleaner for algae and mildew (illustration)
Illustration
Deck cleaner for algae and mildew
algae and mildew that soap leaves behind in the grain Anyone whose deck turns green every season and who would rather scrub less. Check price
Budget pick
Long-handled stiff deck brush (illustration)
Illustration
Long-handled stiff deck brush
scrubbing time and uneven cleaning Anyone with a deck that only needs a seasonal scrub. Check price
Best pick
Deck cleaner for algae and mildew (illustration)
Illustration

Deck cleaner for algae and mildew

Fixes: algae and mildew that soap leaves behind in the grain

Look for: Labeled for algae and mildew, and for your decking type; Oxygen-bleach or other deck-safe formula, per the label; Clear dilution and dwell times for the size of your deck; Safe around plants and pets once rinsed, per the label.

Who it's for
Anyone whose deck turns green every season and who would rather scrub less.
Honest downside
Your decking maker bans certain cleaners. Check the care guide for composite boards first.
Check price on Amazon
Budget pick
Long-handled stiff deck brush (illustration)
Illustration

Long-handled stiff deck brush

Fixes: scrubbing time and uneven cleaning

Look for: Stiff nylon bristles, not wire; A long handle that lets you scrub standing up; A head wide enough to cover a board.

Who it's for
Anyone with a deck that only needs a seasonal scrub.
Honest downside
You already own a good stiff broom. A brush alone will not kill spores in the grain.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on deck care guides from decking makers and extension services, cleaner labels, and the problems and fixes people report in reviews. We do not test products by hand.

Wood or composite? Check before you clean

Composite decking makers often name the cleaners and tools they accept, and some warn against stiff brushes or high pressure. Look up your brand's care guide first. Pressure-treated and cedar boards are softer than they look, so keep pressure low and scrub with the grain.

What to skip

Wire brushes and a pressure washer on full blast

Both gouge soft wood and leave fuzzy, splintery boards that hold even more moisture. If you use a washer, keep the pressure low and the tip wide.

Frequently asked questions

Why does my deck turn green, and why is my deck green in some spots only?
Algae grow where wood stays damp and shaded. The shady spots under trees or beside walls dry slowest, so they turn green first.
Why is my deck turning black?
Dark patches are usually mildew. Weathered wood, tannin stains in cedar and rust stains near nails also look dark. If a scrub with cleaner lifts it, it is mildew. If it stays in the grain, it is a stain.
Why does my deck have black spots?
Small black spots are usually mildew colonies, often where dirt or leaves sit between boards. A deck cleaner and a scrub clear them, and better airflow keeps them from returning.
Why does my deck have white spots?
White spots can be mineral residue from hard water or sprinklers, or a light mildew film. If a scrub with water and a soft brush lifts them, it is minerals or dirt.
Can I use bleach on my deck?
Chlorine bleach can lighten wood fibers and harm plants and metal fasteners. Oxygen-bleach deck cleaners are gentler on most wood decks, but follow your decking maker's care guide.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Deck cleaner for algae and mildew (illustration)
Illustration

Our pick: Deck cleaner for algae and mildew

Check price