While inspecting newly laid vapor barrier, I noticed one of the support post
is floating -- there is a gap between the bottom of the post and the
concrete base.

Here are some photos of this:
http://www.smugmug.com/gallery/12773777_Laj6w

The other posts all have a piece of black tar(?) sheet (about 2mm thick)
between the bottom of the post and the concrete. Perhaps the workers
accidentally pulled out that black sheet while removing the old vapor
barrier.

What is that black sheet called? I think I need to buy one and quickly
insert it back into the gap before gravity pull the joist and the post down.
I believe this black sheet is to prevent moisture from wicking up into the
wooden post

On Jul 2, 12:43*am, "james" wrote:
While inspecting newly laid vapor barrier, I noticed one of the support post
is floating -- there is a gap between the bottom of the post and the
concrete base.

Here are some photos of this:http://www.smugmug.com/gallery/12773777_Laj6w

The other posts all have a piece of black tar(?) sheet (about 2mm thick)
between the bottom of the post and the concrete. Perhaps the workers
accidentally pulled out that black sheet while removing the old vapor
barrier.

What is that black sheet called? I think I need to buy one and quickly
insert it back into the gap before gravity pull the joist and the post down.
I believe this black sheet is to prevent moisture from wicking up into the
wooden post

You don't specify climate or ground condition. In South Carolina, we
recommend sealing the vapor barrier to the walls.
As for the post, the cedar shingles will shim the post tight, so that
it actually supports the load from above.
Cedar will resist rot if there is condensation or moisture moving up
through the concrete.
As a cautious person, I would add a sheet of poly against the concrete
to stop moisture.
T