Cleaning Mold Stains Off a Composite Deck

We had a new deck put on the back of our house in 2012.  Our old deck was wood, and was rotting in spots.  My husband had spent many hours every summer trying to stain the old deck and keep it up, which was an entirely unpleasant job, so when it was time to get a new one, we opted for Trex composite.  Composite decks are marketed as “no maintenance” which sounds nice, but in practice we (and everyone else that publishes rants on the Internet) have found that they do require some maintenance.  Our backyard is very shady and filled with mature trees that produce a lot of tree pollen and other detritus.  It turns out these are optimal conditions for growing mold on your deck.  In particular, what seems to be called leopard spotting mold (see the picture, which is a bit deceiving with the shady and sunny patches, but you can see the spots).

We’ve scrubbed the deck each year trying various basic cleaners, without any impact on the mold spotting.  This year we decided to get serious and look for a product that was more likely to help us. 

I did some Internet research and found 3 candidates for us to consider:

·         Olympic Deck Cleaner

·         Corte-Clean Composite Deck Cleaner

·         Expert Chemical Composite Deck Cleaner

The Olympic Deck Cleaner was not formulated for composite, and didn’t really excite us- middling reviews on various websites for mold.  We eliminated it right away.  The other two were formulated for composite decks, but the Corte-Clean had much fussier application instructions and sounded more likely to kill plants, so we opted for the Expert Chemical.  I stopped by a few local stores, but nobody carried it, so I ended up ordering 3 gallons off the Home Depot website and had it delivered to the store.  Our total bill was $61.08.

It took about a week and a half before it was delivered, and when we picked it up, it had a 4 week shelf life, so we understood that it is more-or-less made to order.  It came with a DVD demonstrating how to apply it.  We cleared all of the furniture off the deck, and my husband put on his grubbiest work pants and got busy spraying and scrubbing with a long brush. 

The whole job took about 3 hours.  He had enough cleaner for the deck, but not enough to do the rails.  We think the next time we do this that we’re still going to order 3 gallons, though, and he won’t apply quite as heavily.  It worked great, restoring the deck to what it looked like when it was new.  No more spots!

Now we’re ready to sit on the deck and sip a gin and tonic.

2020 update:
We're finding that we can do this big application every other year, and scrub the deck with a good soaping on the off years. This spring, there was some buildup of green mildew in a shady spot on the South side. Andy made a basic bleach solution (1 c bleach to 1 gallon water) and scrubbed this section- it helped a good bit.

Before cleaning

After cleaning with a bleach solution