My climate (Alabama) is different from yours, but I have never had a rot problem with PT lumber in ground contact. The new formula is an unknown, but I think it will perform comparably. I have used it for a fence, and my biggest complaint is the corrosiveness -- you must use double galvanized or stainless fasteners. I don't have freeze-thaw worries, or frost heave, but I imagine you have less termite and other organic problems to deal with. The biggest threat to PT lumber in my experience is the effects of UV radiation from the sun, which won't be a factor in your application. I think if you dig your holes deeply enough to get below the frost line by eighteen to twenty-four inches, put a little gravel in the bottom, set the poles or posts, fill with concrete up to the frost line, then fill the rest of the hole with gravel, you will have a trouble free arrangement that will last indefinitely. You must have good compacted or undisturbed soil under the gravel under the post and good drainage away from the building perimeter. My guess (I'm no engineer) about the load is that it would be wise to have the holes, and thus the concrete around the posts, be two to three times the diameter of the posts. Also, I wouldn't rely much on the lateral force resistance of this arrangement unless you use either cross-diagonal bracing or sheathing to prevent racking. This may sound like a lot of work, but I think it is still less trouble than other options. I am assuming that you are not going with beams, floor joists and plywood subflooring. If you are, then I have a great design that uses no PT lumber. Let me know and I will share it.

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