Thanks. In that case, if the oak becomes a problem, Boral TruExterior it is.You mentioned using miratec for the risers that are in contact with the concrete and I would have to say not to.
I used miratec on the outside of my own home in 2008 with PVC in places as well. It has held up well but it will certainly swell with continued moisture exposure and fall apart. If the miratec is allowed to dry it should hold up well. The bulk of mine is holding up good but its not as bulletproof as PVC is when it come to wet areas. It seems that its most susceptible along cut edges.