On Tue, 20 Jul 2010 19:18:27 -0400, Larry wrote:

It's likely Trex if it's that old and that is recycled plastic and wood
fiber. It will certainly expand with the heat. The joists should be on
16" centers for the 5/4" boards. Closer (12") for commercial applications.

Just my 2 cents worth but I'd be concerned about the weight. 3/4 inch
plywood is much lighter, probably less than half. Decent plywood
properly sealed with epoxy or good paint will last a long time.