What’s that green stuff on my deck? Causes and solutions

Your deck is green – here’s why:

Wooden decks are a bit of a hassle for residents of the South Shore and Boston area to keep looking great.  This is because the floorboards take so much abuse – not only because of foot traffic but also because the sun beats down on them, making them prone to fading and cracking.  Also, snow piles on them during the wintertime, and puddling water wears away at the stain or paint that’s protecting the wood underneath.  As a result, the wood turns green when all of these factors cause mold, mildew, and algae growth.

The green stuff on your deck is mold, mildew, and algae.

Wooden deck floorboards turn green when excess moisture seeps inside the wood’s pores and stays there for a long time.  Mold, mildew, and algae tend to grow in moist conditions, so if your deck holds snow and moisture all winter, it’ll likely develop a slimy green film.

You’ll likely notice the green growth in areas that are shady first.  This is because the thousands of pores in the wood may not have a chance to dry out completely in shadier areas.  In sunnier spots, this green growth is less likely but still not uncommon.

You may also notice more green on your deck underneath areas shaded by tables, chairs, and grills since the sun is not reaching those deck floorboards.

Because the sun hits the deck at different points all day long, green growth can occur all over your deck or just in the shady spots.

Is the green stuff bad for the deck?  Or just ugly?

Water and moisture are the enemies of your deck’s wood.  If water gets into the wood, it will cause swelling and eventually cracking and warping.  When green mildew and algae start to grow on the deck’s wood, they will spread quickly, compounding the moisture and cracking problem.

So, even if you don’t mind the look of the green staining, in order to preserve the integrity of the deck and prevent slips and falls, you need to remove the growth.

How do I get the green stuff off my deck?

Here are a few steps to take to get the green algae, mold, or mildew off of your deck:

• Handwash the deck using a Simple Green brand cleaner.  Follow the manufacturer’s instructions on the bottle.  Simple Green is exactly the stuff we use when you hire us to remove mold, mildew, and algae from your deck before we stain it.
• Cut back trees and bushes so the deck floorboards have more sun exposure throughout the day.  This is a preventative measure you can take after cleaning your deck to slow the green growth from returning.
• Some folks suggest a bleach solution or a vinegar and soap solution, but we recommend using Simple Green because it’s so much easier to use (and comes premixed).

Should I pressure wash the deck?

Only if you are 100% sure can you do it at a light enough setting to prevent damage to the wood. Often, DIYers turn on the washer too high, and the water pressure causes grooves in the wood.

You might also like:

Our deck painting and staining services

The care of Mahogany Decks – a staple of New England homes

Our carpentry services 

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Written by: Paige NeJame – Franchise Owner

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Meredith “Paige” NeJame is a 1988 graduate of Boston College and is still surprised by how much she enjoys running a painting business. “It’s always fun to surprise customers with great service. They don’t expect it from a contractor”