from: http://www.mrdeck.com/products_files...isons.htm#Trex

"Both Timbertech and Trex are made from plastic and wood sawdust. Both turn gray with the weather, not to a "silver" as claimed by their manufacturers, but to an ugly musty gray. Both products are susceptible to speckles of black mildew because the resins leach out at the surface and this exodus forms surface micro-pockets. Now the wood chips are exposed and the chips, being organic, bring on the mildew which then thrives in these pockets!"


there's lots of stuff like this all over the internet. plywood isn't perfect but it's time tested.