Q&A / 

Black Spots on Deck Boards

broadwalk next lake

This is Jim's boardwalk. That's a lot of scrubbing. Good news! No more scrubbing! Keep reading. © Copyright 2017 Tim Carter

Black Spots on Deck Boards TIPS

DEAR TIM: I’ve got 1,500 square feet of lakefront boardwalk made from composite decking. The brand is Trex Accents. It started developing black spots within two years of the install.

I’m mad as heck about this as I thought I’d have a maintenance-free deck for life. I’m not able to get any help from the manufacturer nor the the big box retailer that I bought it from.

It’s going to cost me $20,000 to replace it. Can you help me at all? Jim C., Eaton, OH

DEAR JIM: I’ve got great news for you. You’re not going to have to replace that composite decking. I’ve also got a small dose of tough love too.

opens in a new windowCLICK HERE to get FREE & FAST BIDS from local deck cleaning companies.

Half-Truth Claims

Let’s start with a story. I remember years ago when two products were introduced, both had to do with decks. I can clearly remember seeing a counter placard at Hyde Park Lumber Company in Cincinnati, Ohio that said, “Rot-Proof and Maintenance-FREE Deck Lumber”. It was advertising the new copper-chromate arsenic (CCA) lumber for deck framing and deck boards.

We all know how that turned out. You did have to seal the decks every two years and the lumber did rot.

I also remember similar early claims by the manufacturers of composite decking material. As a member of the working press, I was inundated for years by the public relations companies spreading the word about these miracle products.

My Black Spots

My own home here in New Hampshire, built by someone else not me, had this generation one composite decking that did fade and did develop deep black spots. It also allowed algae to grow on it. Millions of other homeowners did the slow burn like you’re doing as they discovered you do have to maintain the decking.

I permanently solved my problem last summer by ripping up all my old generation one Trex and replaced it with Trex Transcends decking.

The Transcends line has a solid plastic cap so no wood fibers are exposed on the visible portions of the boards.

Maintenance-Free Myth

There’s no such thing as maintenance-free when it comes to any outdoor product. The reasons are many. For starters, the sun’s ultraviolet (UV) rays are so powerful they break apart metal atomic bonds. So simple paint, plastic or wood molecules and fibers are child’s play for the sun.

opens in a new windowcloseup tex black spots

Here's a closeup of Jim's Trex Accents. You can see the dark spots. CLICK THE IMAGE NOW to BUY THE PRODUCT YOU NEED TO STOP THE SPOTS!

Spores Everywhere

Mother Nature is also quite adept at showering everything outdoors with mold spores, algae, dirt, diesel soot, dust, etc. Add water to this slew of ingredients and you’ve got perfect conditions for mildew, mold and algae growth on any surface.

Deck Cleaning Video

Watch the following video to show you why pressure washers hurt wood decks and how Stain Solver oxygen bleach is the better method.

Periodic Cleaning

opens in a new windowOxygen Bleach

Stain Solver is MADE in the USA with USA ingredients that are food-grade quality. CLICK THE IMAGE to order some NOW.

At the very least, maintenance involves periodic cleaning. You can clean some things with regular liquid dish soap and water. Tougher stains from barbecue grills, tree sap, mold, mildew and algae may need Stain Solver, a powerful powdered oxygen bleach you mix with water.

Stain Solver will not hurt any vegetation near the boardwalk nor any fish or other wildlife in the lake. It's certified organic and Made in the USA with USA ingredients.

You mix Stain Solver with warm tap water, stir until dissolved and then apply with a hand-pump sprayer. Work in the early morning allowing the Stain Solver solution to work on it's own for 15-30 minutes before scrubbing and rinsing.

The issue in your case is your composite deck, and several other brands, contain untreated lumber fibers that are encapsulated with recycled plastic. However, not all the wood gets coated with the plastic and as I mentioned earlier, the sun breaks apart the plastic exposing the wood.

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Wood + Water + Spores = Mildew

Mix wood, spores and water and you get black spots. You can clean them off, but the spots come back because the mold and mildew spores are constantly floating down onto the deck and you have a nearly constant supply of water. Overnight dew that covers everything outdoors many days out of the year is perhaps the biggest source of the problem.

I think you can get some peace of mind, but it’s going to take a little periodic work on your part. Fortunately it’s easy to do.

Clipper Ship Clue

Years ago I had a fascination with clipper ships. I have two boxes filled with small parts of two ships I intend to build when I’m retired - the Cutty Sark and the Thermopylae. Both of these tea clippers had copper plating on their hulls.

Copper is a natural biocide. It prevented barnacles and other organisms from growing on the wood hulls of these fast ships. This nasty growth would slow other ships and reduce the owner’s profits.

Apply Copper Sulfate

You can introduce this copper to your boardwalk in a way that should not harm anything that’s next to the boardwalk. I’d clean the decking first and then apply a mist of copper sulfate solution to the decking.

opens in a new windowcopper sulfate

This is exactly what you need to keep algae, mildew and mold from decking and patios. CLICK THE IMAGE NOW TO BUY IT.

Copper sulfate is readily available and farmers have used it for decades. It controls fungus diseases, makes up for copper deficiencies in animals, and other things. If you drink wine, then you have to appreciate copper sulfate! Vintners use copper sulfate.

I’d mix up a solution and put it in a garden hand-pump sprayer. Set the nozzle tip to a fine mist. On a sunny warm day, spray the surface of the decking. Minimize overspray. The solution will soak into the wood fibers and possibly the tiny voids of the decking.

The only unknown is how often you need to respray. It could be once every six weeks, but it’s far easier to do than scrubbing that decking. Good luck!

opens in a new windowCLICK HERE to get FREE & FAST BIDS from local deck cleaning companies.

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