[mrcaptainbob]

Why is treated wood preferred for this application? I thought treating was to keep from being infested. Should be no infestation indoors. I would think untreated pine 2x's and sheet wood would be fine. I did similar in my place with a double layer of 3/4 ply on top of sistered joists. Sisters were glued and through bolted every staggered 18". Solid like a rock. If I do another, though, I'll lay the first layer diagonal.