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If your wood deck is turning green, algae and mold may be the culprits! Read on to find out why this may be happening along with some tips on how to keep it from happening again.

Shakir Malik

Shakir Malik

Shakir Malik, Home Expert at Life Hype.

Caused By Fungi Like Mildew and Algae

A deck gets its color from wood stain. Over time, wood stains can fade, and if the deck isn’t cleaned or maintained, the deck can end up with faded green spots. Green on wood decking is usually caused by fungi like mildew and algae.

The most common is dry rot, which rots the boards of the deck and makes them brittle, causing them to break if you step on them. Another type of fungus is “wet rot,” which grows under the surface of the wood. You can remove dry rot by sanding off the surface and replacing any rotted boards.

Wet rot will need to be removed with a power washer and a deep penetrating wood stain. It’s a good idea to install plastic or other waterproofing material around the deck to prevent moisture from getting trapped. Second, seal the wood with a water repellent.

Next use a clear wood preservative. This will prevent the wood from rotting. To prevent the decking from turning green, treat it every six months with a penetrating fungicide. Addressing this issue now will save you time, money, and headaches in the future.

Algae and Mold From Moisture

When wood decks turn green, the culprit is moisture in the wood allowing algae and mold to grow. To prevent this, wood decks need to receive plenty of sun exposure, as well as be kept clear of accumulated organic debris like fallen leaves.

Andre Kazimierski

Andre Kazimierski

Andre Kazimierski, CEO, Improovy.

James Walton

James Walton

James Walton from Wood Router Guru.

Three Reasons Why Wood Decks Turn Green

Deck wood can turn green due to a variety of reasons, but the most common cause is algae growth. Algae can grow rapidly on moist surfaces, such as wood decks. The sunlight and moisture will help the algae grow and turn the surface of the deck green.

Another common cause of deck greening is fungal growth. Fungi prosper in moist environments, so if there is water damage or rot on the lumber, the fungus will grow and turn the deck green.

One more common cause is water collecting on the deck.

What are the symptoms of wood decks turning green?

If you’re seeing greenwood on your deck, it’s time to take action. Here are the most common symptoms of a wood deck turning green:

1. Spots or patches of discoloration will appear on the surface of your deck.
2. If water isn’t allowed to soak into the wood, it will start to mold and rot.
3. You may also see insects or other pests moving around on the surface of your deck – this is a sign that the wood is infested with insects and decay.
4. The fibers in the wood will start to break down, leaving behind a greenish-black film on the surface of your deck.
5. Finally, over time, the rot will spread through the deck and cause it to collapse in areas where it was weakened by decay or insect damage.

How can you prevent your wood deck from turning green?

There are ways to protect your deck from turning green, and understanding the options is key.

There are many things that homeowners can do to protect their deck from turning green. One easy solution is to keep the deck clean and free of debris. This will help prevent moisture from accumulating, leading to cracking and rotting. Additionally, homeowners can seal any cracks or holes in the deck with a sealant. This will help protect the wood from water damage and decay.

The wood should be protected from the elements and pests, so it lasts for years to come. There are a few things you can do to keep your deck looking its best and protect it from damage:

Clean it Regularly: If it’s not clean, bugs and bacteria will build up, which will cause decay and rot.

Seal it: Decking exposed to the elements can become wet and freeze, causing buckling or warping. A sealer will help protect the wood against water damage and weathering.

Use a Coating: A durable coating can help keep moisture away from the wood while preventing pests, such as termites, from gaining access.

Chemicals Used In Treatment Process

Wood decks begin to turn green as a result of the chemicals used in the treatment process breaking down. The lumber used to construct a deck is treated to protect the wood from the elements however this treatment needs to be maintained. If the wood is not resealed every few years, the wood will take on moisture and begin to decay. Part of this process is algae growth on the surface of the boards giving them a green appearance and also causing them to be slippery when wet.

To avoid the green appearance and associated decay, [you should] reseal the deck surface every three years or so. The best test to know if it is time to reseal your deck is to place a few drops of water on the wood surface. If the water beads up and rolls away, the treatment of the wood is still doing its job, however, if the water soaks in, it is time to reseal the deck.

Customers can often get frustrated applying deck paints to their deck as they are prone to chipping. We recommend a product like Thompson water seal, as these stain-type products are rather inexpensive, easy to apply, and are not prone to chipping as they soak into the wood rather than adhering to its surface.

Curt Visser

Curt Visser

Curt Visser, Owner of Property Revolution, LLC.

Michael Rubino

Michael Rubino

Michael Rubino, President, and CEO of All American Restoration.

High Moisture in the Structure

Oftentimes, when a deck turns green, it means that mold has settled in due to high moisture in the structure. This isn’t ideal because as the mold grows, it reproduces by releasing microscopic spores into the air. Some species of mold also produce microscopic toxins called mycotoxins when threatened. A colony near a door or window that leads into the home means a higher number of particles will be making their way inside, decreasing the indoor air quality, contaminating any surfaces, and increasing the likelihood of a colony developing in the home.

Typically, mold spores only need two major components to transition into a living colony: a food source and a moisture source. Wood, glue, or other organic particles on the deck itself offer perfect edible options. Most woods are porous or semi-porous, which means that water can get trapped and build up within these small pockets.

If these two conditions are met for 24–48 hours, mold can begin growing on the deck. With over 100,000 species of mold identified so far, this growth can appear in a variety of colors (green, black, brown, white, yellow, etc.) and textures (powdery, slimy, velvety, etc.). Similar to a plant, mold grows roots called hyphae, so once a colony is established, these tendrils can reach into the semi-porous and porous surfaces, making total removal difficult.

Steps to remediate mold from wood include abrasively removing the roots using processes such as sanding or wire brushing. From there, the procedure should include HEPA vacuuming, applying hydrogen peroxide, allowing this to dry, sanding the surface, HEPA vacuuming again, applying Benefect Decon 30, wiping the surface with a microfiber towel, and allowing it to dry. This helps remove roots, spores, and other contaminants like mycotoxins and bacteria. Afterward, seal the surface for further protection.

To prevent mold from developing on a deck:

    ● Seal any wood used for the structure
    ● Clean often to remove edible particles as well as spores
    ● Actively work to reduce as much moisture as possible from the surface
    ● Ensure that drainage goes away from the deck and doesn’t build up below

This is a crowdsourced article. Contributors’ statements do not necessarily reflect the opinion of this website, other people, businesses, or other contributors.