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Composite Decking Mushrooms

Composite Decking Mushrooms TIPS

  • Some composite decking materials are made with wood fibers
  • Many deck problems are caused by FAILURE of installer to follow instructions
  • Read updated author's notes at bottom of column
  • Ventilation is very important as is spacing between decking boards
  • CLICK HERE to Get Tim's FREE & FUNNY Newsletter!

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.

Mushrooms At My House Too

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.

IMPORTANT NOTE: The mushroom growth problem was caused by the home builder, not a defect in the Trex decking. I didn't build the house I currently live in.

The builder didn't:

  • provide any spacing between the decking boards
  • it was too close to the ground
  • it was enclosed on THREE SIDES

Because the deck was enclosed on the three sides, it was impossible to get air moving under the decking. You need air moving to evaporate any water that will fuel growth of mold and mushrooms.

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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

Spacing Is Everything

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.

Missing Specification

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.

Old Aggressive Marketing

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.

Restore Color

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.

Wood Is Mushroom Food

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.

Water + Food = Mushrooms & Mold

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.

CLICK HERE to get FREE and FAST BIDS from local deck contractors who know how to follow instructions.

Preventing Problems

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.

Regrade Soil

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 Everything

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 UPDATE:

In the summer of 2016, I replaced all of the generation one Trex decking at my home with Trex Transcend. The Transcend product has the same, or similar, core of wood fibers that are encapsulated by plastic.

But the big different is the top of each plank and the top third of each side is capped with a solid plastic that's embossed with wood grain. The capping material also has some extra coloration to simulate the real look of wood. My wife is a very tough critic of all things that are made to simulate wood. She feels this is the closest she's ever seen of any product to replicate real wood.

The small samples you get to hold in your hand look odd and unrealistic. But when the planks are down on the entire deck and you look out across it from a distance, it looks like real wood.

Here are some photos of my new Trex Transcend deck:

This is a photo of the decks under construction. The long deck with the furniture on them is the original 10 x 60-foot deck. The lower deck is new and is 24 feet wide and extends out 16 feet from the original deck. Copyright 2017 Tim Carter

The deck is complete with the Trex Transcend railing system. It was a pleasure working with this new material. It's engineered for easy installation and has all hidden fasteners for the decking and railing system. Copyright 2017 Tim Carter


You can get a feel of the coloration in the medium-brown colored decking. The deck has a picture frame of darker material - Lava Rock - and the main decking is Tiki Torch. Copyright 2017 Tim Carter

CLICK HERE to get FREE and FAST BIDS from local deck contractors who know how to follow instructions.

Column 903


8 Responses to Composite Decking 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?

  5. What about mold issues under the new Trex capped material. The underside is not capped like Timbertech, which is capped on all 4 sides. Assuming adequate board spacing and adequate air flow under the deck, is Timbertech a better mold-free board to install over Trex?

    • Great question. If you install the Trex as they say in their installation instructions you should NOT get any mushrooms as I show above. Mold is a different issue and just about ANY product can have mold grow on it if there's: food, water and favorable temperatures.

      All three are ALWAYS present under any deck unless it's built in the Atacama Desert.

      Just read and follow the instructions that come with any of the composites to have minimal or no issues.

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