Hi,

I need to build a deck over a 16' x 20' concrete cistern. It is perfectly square and the concrete walls are intact. It fills with water to various levels during the year, and I am unable to put any support posts, etc. down into the pit.

What sort of beams will I need, and how close together sould they be to support the deck. i.e. Would 4 x 8, 16" on center be enough? Do I need more? Can I get away with less?

Because of the water, I expect to use pressure treated lumber. (They will never be sitting in water, but it is a damp area.)

Thanks in advance!