Why Do Deck Boards Turn Black? (What You Need To Know)

Having a nice deck in the yard is a good accessory, especially in the summer. But what if the deck boards turn black? In this blog post, we will explore some of the causes of this happening, and what you can do about it.

Answer to the question why do deck boards turn black:

The number one reason deck boards turn black is mildew and mold. However, that is not the only reason, there can be several other issues to check for such as rust and tannic acid. 

If you are struggling with a dark discoloration of your deck boards, check out this article where we will be discussing, what makes deck boards turn black and things you can do to prevent this from happening.

What Makes Deck Boards Turn Black?

Over time, many homeowners may begin to notice that their deck boards are beginning to turn black and before they can treat them, they need to know why. While several issues can lead to your deck dulling, changing colors, etc., there are three main reasons for them to turn black.

  • Rust
  • Mildew
  • Tannic Acid

Once you can determine which one of these culprits are doing the damage, it will be simple to figure out the best solution for the job, giving you back a beautiful deck.

Rusty Spotting

It is very important to clean your deck thoroughly before repainting or staining it. While most people know this and try their best, they can oftentimes use a wired bristled brush or scouring pads to get the job done. 

If the metal bristles or wires get loose and then painted over, there is a good chance they will begin to rust over time. Once the rust starts, it will end up spreading, creating large spots of black stains.

Rusting can also be caused by metal screws or nails that are untreated and left in the weather uncovered. Making sure you use the proper gear when finishing your deck is going to keep this mess from happening.

Mildew and Mold Build Up

Everyone knows how frustrating a mold and mildew infestation can be. Once the fungus starts to grow it can be extremely difficult to stop it. This is probably the most significant issue homeowners run into when dealing with black spots or stains on their deck, and mold growth can be caused by a number of different reasons.

  • Inadequate Materials: Unfortunately, despite what the labels say, not all stains and finishes are alike. There are products out there that are not quality-made. These will not provide the type of seal your wood needs to keep bacteria from developing. 
  • Wrong Sealent Used: If you are sealing your deck on your own, you must do your homework. It’s important to choose a sealant that is made for the job at hand. There is a variation of sealants, stains, and paints that are designed for specific materials. If you don’t use the proper product, you will not maintain a good seal and water can seep into the wood.
  • Not Applied Properly: Even if you use the best products on the market, a sealant is only as good as the person applying it. If a section of the deck isn’t sealed correctly or is missed completely, mold spores will start to develop.
  • Improper Curing: Not only is too little stain a mold and mildew haven but so is over staining your deck. If you apply the product too heavy on the wood, it won’t cure fast enough and can create a thick and sticky substance. This substance will catch and hold onto dirt, grime, and other small particles that will eventually develop bacteria and form mold.

Tannic Acid Reaction

Tannic Acid is a chemical typically found in high amounts in red and white oak, cherry, and walnut wood. When tannic acid comes into contact with metal and water, it will create a black residue, often with red, green, or brown streaks. This residue can and often will spread, creating black splotches all over your deck.

How To Keep Your Deck From Turning Black

Now that you know what the main reasons are for your deck boards turning black, it should be pretty simple to figure out what you need to do to prevent it from happening or stop it once it starts.

Wash Your Deck thoroughly before ever adding any type of paint or stains. This means chipping off all of the previous coating (If this isn’t your first time.), washing it completely with a deck detergent and a power washer if you have one, then allowing it to dry for at least 24-hours before the application.

Next, you want to read the label of the product you are planning on applying to your deck to ensure it is right for the materials you have and it will keep your deck from absorbing water or weathering.

Lastly, make sure you take your time to apply the product perfectly and go over it a second time if necessary, giving it time to cure before adding the next layer.

As long as your porch is protected the right way, you shouldn’t have any problems with future black stains.

How To Fix Black Deck Staining

Fixing the black staining on your porch doesn’t have to be an all-day job. If the issue is due to mold and mildew on the surface a good scrub down may be all it takes. 

If by chance the black stain goes into the wood, it could be remedied with deep sanding, some scrubbing with soap and water, and a re-paint or stain job.

How big is the spot? If it’s too deep or large, you may have to remove the damaged boards and replace them with new ones. After you replace the boards, sand, stain, and seal them right away.

The best way to avoid any discoloration issues with your deck wood is to understand what type of wood you are using, what chemicals it may or may not have in it, and the best ways to keep them safe and sealed.


There are several reasons that can contribute to the discoloration of your deck boards, keeping them clean, dry, and covered is the best way to reduce the chance of black spots popping up, and will help keep your deck in tip-top shape all year long. 

We hope this blog post has been helpful. If you have any questions or comments, please leave them in the section below.

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Why Do Deck Boards Turn Black? (What You Need To Know)
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