Thanks everyone. The decks are #2 cypress and probably 40% of the boards have mild to severe rot. The three decks wrap around the south side (rear) of my house in a U-shape. The PT framing is in good shape I am reasonably confident although I have not seen it. Trouble comes from the weather of course. Most of the rot came about because the boards swell with moisture and then close up the gaps. With no place for the water to drain it sits in the cracks until it is absorbed by the boards or evaporates. The original spacing between deck boards was about a 10-penny nail apart, which was probably not enough.
So far I have ruled out Trex and cypress/cedar. The biggest deck is faced with a nice brick step border, and is only 12" off the ground. I am considering taking out the deck boards and filling the space with sand or some such. I could leave the framing in place and use it to help screed the surface level. Then I could put some sort of paving stone over that. I think the cost would be comparable, perhaps even better than a plastic deck. I am open to ideas on the stones - even thought of trying broken granite counter tops except I don't know where to find them.
In the other decks I would have to use planks. Has anyone seen or have any experience with CorrectDeck?
CorrectDeck® Composite Decking & Railing Systems I talked with a disty y'day. He states that Trex is polyethylene and CD is polypropylene, with the latter being superior of course, although exactly why eludes me . Main claim to fame is that the "modulus of elasticity" (can I say that with a straight face?) gives the CD product greater stiffness. And that it doesn't heat up like Trex so you can walk on it barefoot in the sun.
I have seen benches made from composites where the planks split on the ends, along lines from where they were screwed down. An obvious case of thermal expansion problems. I imagine, but don't know for sure, that a hidden system would have limitations as well. Comments?
That's the state of the investigation so far. Anyone care to add to the discussion?