How to Clean Black Spots on Composite Decking

Updated February 21, 2017

Composite decking is a popular deck material because it is very durable. However, the material is susceptible to mould and mildew. Even if you clean your composite deck regularly, you will likely notice the appearance of small, black spots or streaks over time. These are mould and mildew growth, and they can be removed with a simple addition to your regular deck cleaner.

Sweep your deck. This will make the task easier because you will be able to spot all of the areas that need treatment. Many people prefer to treat the entire deck to help avoid repeating the cleaning just a short while later.

Mix your cleaning solution. Most standard composite deck cleaners require dilution. Start out with a gallon of cleaner and the manufacturer-specified amount of water. Then add three cups of bleach and two cups of white vinegar. These will kill the mould and remove the black spots.

Test the cleaning solution on a small part of the deck that is not very noticeable. This will let you see if the cleaner is going to discolour your decking in an unacceptable way.

Scrub down your deck with the bleach-and-vinegar cleaning solution. Use the long-handled bristle brush to remove the black spots and any other build-up on the deck. If necessary, use the hand-held scrub brush to attack stubborn black spots, but be sure to wear your rubber gloves if you are going to be getting the cleaning solution on your hands.

Rinse off the deck with the garden hose. Use plenty of water. Once you have rinsed off the deck, you can give it a few hours to dry.

Mix the mildew inhibitor solution. Generally, this will just involve adding water, but some brands can be applied straight from the bottle.

Apply the mildew inhibitor. You can use a cleaning rag or the push broom to do so. This will help prevent the mould and mildew from returning.

Let the deck dry completely. This may take a full 24 hours or longer if your area is humid. Once the deck is dry, you may elect to re-stain the deck or leave it as is. Either way, your black spots should be gone, and you will have made sure that they will not be back for some time to come.


If your deck is less than 10 years old, check the warranty before cleaning, since you may void the warranty with the use of certain products.

Things You'll Need

  • Broom
  • Long-handle bristle brush
  • Scrub brush
  • Rubber gloves
  • 5 gallon bucket
  • Bleach
  • Garden hose
  • Mildew inhibitor
  • White vinegar
  • Standard composite deck cleaner
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