installing some trex patio flooring.

i'm in southern AZ and the heat causes problems.   I wanna install new flooring on the patio; thought about using plastic underneath the flooring for a moisture barrier.   I think the plastic would desinagrate under the trex flooring.  I'm thinking about using tar paper under the trex for a moisture barrier.  I'm looking for opinions.  there will be a solid plywood floor under the trex!

Be careful in choosing the product and the color. plastic lumber can act as a heat sink in sunny areas, and can broil your tootsies, if you walk barefoot on it!

prolly best to go with a slate type flooring? that's about all i've ever seen down there.

Many people use plastic for such purposes, and I have never had anyone problem with that. There should be intensive sun and moist impact for it to erode and have holes.
But if you will use it like this, then it should be fine. Also, OP, did you still use the plastic or the tar paper? I think tar paper is a little excessive, but I might work too.
I have just talked to a guy who works for a patio roofing company in Sydney. He told me that using plastic film is completely fine for the basement. It will prevent the moisture from ruining the basement.