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Existing conditions:

  1. 2000 sq.ft of deck, 3/4" x 4" cumaru planks with 3/8" spacing, over 2x8 pt joist and double 2x12 pt beams, over helical piles.
  2. depth of deck to bot of dirt is approximately 18"-20".
  3. there is very little ventilation on all sides.
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I don't understand the issue. Is water standing under your deck? Can you provide a picture? – DMoore Mar 4 '14 at 17:37

May I just add, providing spacing between the decking?

When we moved into our home, the deck which has two sides connected to the house had some standing water beneath it. The decking was pushed together and heavily painted, so there were no gaps for water to just drip off. Instead it all flowed to the lowest corner and created a puddle underneath the deck.

Once the decking was correctly spaced, water would drip evenly beneath the deck, now had a third option when evaporating which was up.

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Lacking much detail in the question, I'd suggest making sure the area under the deck all slopes away from the building (if it's attached to a building) or to the outside of the deck if it's freestanding, or to catch-basins and drains if neither of those can be managed. This will be lots of fun if there's only 18" to crawl around in and it wasn't done in advance.

If that's already been done, move to adding an impervious surface to clear water away more quickly. This will also be fun with that little space to work in, but it's not impossible.

Both would be better to have done in advance.

Depending on the exact situation, it may be easier to jack the whole deck up to do the work needed - or to jack the whole deck up and reset it so there's more space underneath to improve the ventilation. Especially if what you mean is that there's 18" from the deck surface to the dirt, with 16 inches of that full of framing and ~2" below the bottom of the 2x12. The other option in that case is to dig trenches between the piles.

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