How to prevent mushroom growth on decking

October 14, 2011

I had to replace a damaged composite 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 board! I thought that composite decking materials were problem-free. I thought the reason to buy 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, Wash.

I’m not really surprised by your question, as I’ve had firsthand experience with this problem. Recently, I had to repair a poorly designed front porch that was covered with composite decking from a well-known manufacturer. It was important to salvage the decking material, and as I carefully removed each board, I discovered large patches of white fungus and different species of mushrooms growing on the underside of the decking.

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 ventilation.

The written instructions show that there should be a quarter-inch gap between the boards to provide for both expansion and air circulation. I was surprised, however, to discover there was no real minimum clearance distance listed to tell you how close the bottom of the decking can be to moist and dark soil. (Dampness and darkness are the conditions that mushrooms, mold and fungus seem to thrive in.)

Clever marketing has for years has set up the expectation — unrealistic in my opinion — that composite decks require no maintenance.

I was shocked to discover that 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 tells me that they know their 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 composite decking. The literature from the manufacturer says the decking does absorb moisture. It would be reasonable to surmise that the wood fibers used in the decking are not treated to prevent wood rot or decay.

If the wood fibers are not treated, then it is no mystery why you, I and perhaps many others have discovered hidden mushroom growth. The wood fibers inside the decking are getting wet, rotting and feeding the growth of mushrooms. That’s what happens to logs that fall in the forest and get wet.

To prevent the growth of mushrooms, fungus and mold on the underside of your decking, you might want to consider doing what I am. 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 also creating a minimum of a 1 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 poorly draining soil, I recommend that you take whatever measures are necessary to prevent the ponding of water under the decking.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site at www.askthebuilder.com .

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