The short answer
What Causes the Green Growth
The green you see is almost always a living organism, not a dye or chemical reaction. Algae, moss, or mildew spores land on the surface and find the conditions they need: moisture, shade, and a little organic debris like pollen or dirt. Composite decking is porous at a microscopic level, so it can hold water against the surface long enough for growth to take hold.
Shaded areas near walls, under trees, or where furniture sits are the most likely spots. If water pools on the deck after rain or from a sprinkler, that trapped moisture speeds up the growth. The green may appear as a thin film, a fuzzy patch, or a slimy layer, depending on the organism and how long it has been there.
How to Clean the Green Off
Start with the gentlest method: a soft-bristle brush and a bucket of warm water with a few drops of mild dish soap. Scrub the green area in the direction of the board grain, then rinse thoroughly with a garden hose. Avoid pressure washers set on high, because they can roughen the composite surface and create more places for growth to return.
For stubborn patches, a solution of one part white vinegar to three parts water can help, but test it on a small hidden area first. Never mix vinegar with bleach or other cleaners, as that creates harmful fumes. After cleaning, let the deck dry completely. If the green returns quickly, the underlying moisture problem may be the real issue.
- Soft-bristle brush and mild soap
- White vinegar diluted with water
- Rinse with a garden hose
- Let the deck dry fully
- Move furniture to let air circulate
Prevent Moisture from Building Up
The key to keeping composite decking green is limiting the moisture that sits on the surface. Trim back overhanging branches so more sunlight reaches the boards. Sweep off leaves and dirt regularly, because they hold water and give spores a food source. If you water plants near the deck, aim the sprinkler away from the boards.
Check for gaps between boards or under the deck where water might collect. If you notice standing water after rain, that is a sign of poor drainage. In some cases, water may be seeping from a nearby source, like a leaking gutter or a sprinkler head. Fixing that source can stop the green from coming back.
When to Look for Water Damage
If the green keeps returning even after thorough cleaning, moisture may be trapped beneath the deck or in the framing. Water that pools under the boards can soak into joists and supports, leading to rot over time. That is a separate issue from surface algae and needs a closer look.
A water damage inspection can help identify hidden moisture sources, like a slow leak from a hose bib or a downspout that empties too close to the deck. The pros use moisture meters and visual checks to find problem areas. If the deck stays damp for days after a rain, consider having a professional look for the source of the moisture.
Does the Green Damage the Deck
Surface algae and moss rarely harm the composite material itself. They are mostly a cosmetic nuisance and a slip hazard when wet. The bigger concern is what the moisture is doing to the structure underneath. Prolonged dampness can lead to mold growth on framing, which may spread to nearby siding or interior walls.
If you see green spreading toward the house or notice a musty smell near the deck, that is a sign of deeper moisture. A mold professional can assess any growth that has moved indoors. For now, regular cleaning and good drainage will keep the surface green in check and protect the deck's appearance.
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Call Now (479) 370-1500Frequently asked questions
Can I use bleach to clean the green off?
Bleach can kill algae and mildew, but it may lighten or damage composite decking over time. It is safer to start with mild soap and water. If you must use bleach, dilute it heavily and rinse immediately. Never mix bleach with vinegar or ammonia.
Will the green come back after I clean it?
It can, especially if the conditions that caused it remain. Shade, moisture, and organic debris will lead to regrowth. Improving sunlight and airflow, and fixing any drainage issues, will reduce the chances. Regular sweeping and occasional washing help keep the surface clean.
Is green on composite decking a sign of a bigger problem?
Usually not, if it is just on the surface. But if the deck stays wet for days or you see green near the house, moisture may be seeping into the structure. A water damage inspection can check for hidden leaks or poor drainage that could cause rot or mold over time.