I'm new here so this may have been already covered but I could not find an answer through the site's search. The floor structure is a bit unusual which is the main for my specific question.

I just discovered this site and it is outstanding!

Hopefully other readers will benefit from this question and the discussion!

I’m removing carpet and installing 5/8"x 3 ¾“ solid horizontal bamboo.

The floor structure is:
4x6 posts at 6' spacing
Double 2x6 beams/joists @ 48" spacing
2x8 T&G decking
1/2" particle board (will be removed!)
6 mil visqueen on the ground

The bamboo floor will be nailed with a pneumatic nailer with 2" nails.

Felt will be used under the bamboo.

What thickness plywood or other underlayment/subfloor should be used over the T&G?
Must the underlayment/subfloor be extensively screwed down or lightly screwed down with the 2" flooring nails through the bamboo and underlayment/subfloor providing the principal hold-down to prevent movement and squeaks? (screws @ 6" or 8" spacing takes a lot of time.....)
Could Lauan ply be used for the underlayment/subfloor?

Any other thoughts, concerns, or suggestions would be appreciated.

Thanks for your assistance and interest in my questions.[/list][/list]

3/8 minimum, but 1/2" ply would be much better. Lay it at a 45º to the 2x8 decking. Screwed 6-8" centers.

If you go the opposite direction of the 2x8 plank subfloor, you can go directly on top of it, with 15# asphalt impregnated felt between the two.

I think I may be missing something here. The structure you are describing sounds more like an out door deck or something, that has been converted to indoor space.
If so, have investigated the structure itself to ensure that you have proper insulation, vapour barrier, cross ventilation, etc, under the space?
If you have, then I think Austin is right on with the 1/2 inch ply on top. I dont think however, that I would go directly on top of this decking material though. I dont think it could provide a flat enough surface for 5/8th inch bamboo.

The entire house is built this way.

As I mentioned in my original post, it may be a bit unusual construction.

A photo of my crawl-space access cover is attached,

As you can see, there is a vapor barrier and insulaton. The crawl space has vents through the concrete stem wall that supports the house walls.

What do you mean "I dont think it could provide a flat enough surface for 5/8th inch bamboo." Are you suggesting there needs to be something else over the 2x8's?

[img http://home.teleport.com/~russell2/www/ ... %20900.jpg /img]

Just the 1/2 inch ply, as suggested earlier. But will look at pic later in case im missing something here.

What the OP is describing is a type of subfloor used in the USA in the 50's and early 60's. It was not used every where but was used occasionally and especially in housing tracks. It consists of 4 x 6 girders/beams supported by 4 x 4 ( usually ) verticle posts that are spaced every 6' or so. The posts are supported by concrete piers dug into the earth to a depth of 3'+, depending on soil conditions. Then the decking is, which is usually 5" x 1.5" nominal, is blind nailed through the tongue with #16 common nails. I have never seen one with a 7.5" width. They will often go back and face nail with #16's. The beams/girders are are spanned 4' oc usually but I've seen up to 5'. If the deck is installed green, it will shrink quite a bit, causing some of the T&G's to disengage, thereby getting noises. These could be quite uneven. I've seen many that had been drum sanded first before carpet could be installed. But the correct method is to overlay with 1/2" plywood. One needs to check for valleys between where the beams are and fill in BEFORE installing the plywood. Then sand the plywood as needed to smooth out seams and make flat. When I've done these, I use a construction stapler and staple the snot out of it. Screws hold better but are much more time consuming and expensive. Consider gluing the 1/2" ply as well with some wiggly beads of Liquid Nails.

AHHH, a light comes on in this old head, lol. I now can picture that type of construction (was having a problem before). We used that in cottage country sometimes when building over rock and on slopes.
Very sturdy construction method, but ya i can imagine a lot of shifting and irregularities to the flatness of the floor over time.