If mould and mildew begin to sprout below your deck, in other words, on the ground below, this is an indication of too much moisture. An over-abundance of moisture is often caused by substandard drainage systems or incorrect grading below.
Ventilation is one of the best ways to prevent this problem from becoming worse and is especially helpful for structurally impaired decks that are built too close to the ground. Ventilation allows moisture and heat trapped below your deck to escape and effectively makes the ground below your deck an inhabitable place for mould or mildew to reproduce.
Installing an adequate drainage system to take excess water away from the ground below your deck is also an effective method to combat mould and mildew.
If you have mould on the deck surface area itself, the material of the deck will determine what mould removal technique you should use. For wooden decks, ensure that your deck has not begun to rot already, as these sections will need to be replaced for safety reasons. If you are lucky enough to have spotted the mould or mildew before rotting has set in, you can save your deck by spraying it with a bleach solution, which kills mould spores.
If you have composite decking, mould removal will entail using a deck cleaner that has Sodium Hypochlorite in its ingredients. Use a stiff bristled brush to scrub away at the problem area. Getting rid of mould this way may take a few repeated applications, but will eventually be successful if you are persistent.