Proper ventilation is needed to prevent moisture buildup:
    Standing water can lead to premature wood damage. Gaps between boards must be free from dirt, debris and twigs; allowing water to drain freely. Air needs to flow in, and around, your deck to prevent moisture buildup. If your deck is over 24" high, make sure air can enter from the sides. Gaps between the boards can be opened, and vents can be installed to solve moisture problems.