Q&A / 

Trex Decking Problems and Mushrooms

DEAR TIM: I had to replace a damaged Trex decking board on my ground-level deck and was shocked at what I discovered. On the underside of the decking, mushrooms and fungus were growing out of the decking! I thought that composite decking materials were problem free. I thought the reason to purchase these products was so that you’d never have to replace them. What’s going on? What can I do to stop the growth of the mushrooms? Patti L., Woodinville, WA

DEAR PATTI: Oh my goodness! I’m not really surprised by this, especially since I’ve had firsthand experience with this myself. Recently, I had to repair a poorly designed front porch that was covered with Trex decking. It was important to salvage the decking material, and as I carefully removed each board, I was stunned to discover large patches of white fungus and different species of mushrooms growing on the underside of the decking.

Author's Note: Would you like to see three High-Resolution photos of the mushrooms that were growing out of the Trex Decking? If so, click here.

Mushrooms growing out of the bottom of Trex composite decking can’t possibly be a good thing. Decaying wood inside the Trex is fueling the mushroom growth. PHOTO CREDIT:  Tim Carter

Mushrooms growing out of the bottom of Trex composite decking can’t possibly be a good thing. Decaying wood inside the Trex is fueling the mushroom growth. PHOTO CREDIT: Tim Carter

 The first thing I did once I got inside and cleaned up was to go to the latest written installation instructions published by the manufacturer. I immediately discovered that part of the problem in my case was the boards had not been spaced properly to provide some ventilation.

The current written instructions show that there should be a 1/4-inch gap between the boards to provide for both expansion and air circulation. However, I was shocked to discover there was no real minimum clearance distance referenced with respect to how close the bottom of the decking should be to moist and dark soil. These are the conditions that mushrooms, mold and fungus seem to thrive in.

Clever marketing has for years set up an unrealistic expectation, in my opinion, that composite decks are maintenance free. I can remember years ago that this absolutely was the case with Trex. After it was in the marketplace for several years and problems started to happen, it became clear you had to maintain the decks.

I was shocked to discover in the installation and maintenance instructions the manufacturer even talks about how to stain the deck to restore the decking to its original color. That communicates to me they know the decking is going to fade and will need to be cared for if you want it to look good for years to come. So much for maintenance free.

The mushrooms are growing because they are feeding on the wood fibers inside the Trex decking. The literature from the manufacturer says the decking does absorb moisture. I’m reasonably confident the wood fibers that are used in the decking are not treated to prevent wood rot or decay. I can’t find any information in the Trex literature that says they do treat the wood to prevent rot.

If this is the case, that the wood fibers are not treated, then it makes perfect sense why you, I and perhaps tens of thousands of others are experiencing hidden mushroom growth. The wood fibers inside the decking are getting wet, rotting and fueling the growth of mushrooms. That’s what happens to logs that fall in the forest and get wet. It makes sense rot could happen with composite decking that uses wood fibers or wood flour that’s not treated to prevent rot.

To prevent the growth of mushrooms, fungus and mold on the underside of your decking, you may want to consider implementing what I’m doing. The front porch I’m rebuilding is going to have new and vastly improved ventilation.

Not only will I be spacing the decking boards so that there is a minimum of 1/4 inch between the boards, I’m creating a minimum of a 1 and 1/2-inch gap around three sides of the decking where it used to connect directly to the house.

This air gap should be plenty to help keep the relative humidity on the underside of the decking to a point where the growth of the organisms will be stopped or significantly slowed.

I’m also going to try to regrade the soil so there’s at least 16 inches of air space between the soil and the underside of the decking. It helps that I have well-drained soil under the porch I’m working on. If you have poor-draining soil, I suggest you take whatever measures are necessary to prevent the ponding of water under the decking.

Air flow is perhaps the best way to ensure there’s no or minimal growth of mushrooms and fungus. The goal is to keep the moisture content of the wood fibers inside the Trex as dry as possible. Air movement absolutely accelerates drying.

Since the manufacturer admits the decking does absorb moisture, anyone with Trex decking needs it to dry as soon as possible when it gets wet. This does become problematic in areas that experience lots of rain over months of time. In these locations, it’s probably best to avoid the use of composite decking materials that contain wood fibers that have not been treated with chemicals that prevent wood rot.

Author's Note: Would you like to see three High-Resolution photos of the mushrooms that were growing out of the Trex Decking? If so, click here.

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6 Responses to Trex Decking Problems and Mushrooms

  1. This issue was written up in a scientific journal 14 years ago. Morris and Cooper 1998. Recycled plastic/wood composite lumber attacked by fungi. Forest Products Journal Vol 46 No. 1 pages 86 to 88
    The name of the product was carefuly not mentioned but the description was so accurate that the manufacturer became aware of the problem. They claimed it was surface mold but it was definitely wood-rotting fungi.

  2. Lots of luck locating your deck further above the ground for more ventilation. Our deck is 8 ft high and has fungus all over the bottom of the trex boards and on top as well.

  3. So how can I clean the mushrooms in the grooves of my composite deck without damaging the deck further, or ruining the color? Any thoughts? Is there a fungicide out there that is safe for the composite deck?


  4. We put in a Trex deck in 2000. We live in a dry climate, yet there is tons of fungus growing on the underside of the deck. When the builder installed the deck, he placed the boards tightly together so there is no drainage between them. Will it damage our deck if we drill tiny holes between the boards at periodic intervals to provide drainage?

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