[glend (Glen)]

Allan my ROR obs is built on timber bearers sitting on treated pine post footings set (600mm deep) in concrete. There is an air space under the floor to allow circulation, and the ground underneath is covered by weedmat and a layer of gravel on top, this keeps it dry and prevents weed growth near the edges. The floor is heavy tongue and groove structural ply flooring panels. And your right there are no thermal issues to contend with. The question is how heavy is that big dob going to be, and what steps are required to make the floor strong enough to support it? With my setup i use a concrete pier to support my EQ mount, and this pier rises up through the floor cutout - the pier was obviously constructed first and the obs built over it. Interestingly when i recently bought a new mount (Celestron CGX), i set it up in the obs on its tripod for initial testing, because i was waiting for a new pier adaptor to arrive. The tripod and mount head combo are pretty heavy but the obs floor structure had no problems supporting it with no vibration or PA consequences due to weight shifts while slewing. I even did some long exposure image testing and it worked fine. You want to avoid walking around it while imaging but i could supervise it from the house using Teamviewer. For a purely visual setup it would have been fine. Based on the CGX combined weight plus the scope and imaging gear, it easily weighed more than my old 16" dob, so structurally there is no reason a large dob cannot be used in that type structure.

Considerations, do you want to roll it out to a pad as a consideration? This would not require a ROR. A ROR built for a dob may have to have lower walls to allow the low slung dob to get down to lower altitudes, which is not an issue for high mounted EQ setups. A Roll-out building can be less complicated to build than a ROR one, just open the barn doors and roll out the dob to a small concrete pad. As we know in places like Bretti, the land can take some time to cool down, especially in the warmer months, and this is probably a greater impediment to ideal seeing than a small concrete pad immeadiately under your dob. In fact a small concrete pad might be useful for dew prevention in cooler months as it would release its stored heat beneath the optics gradually.
Just a few thoughts. Good luck with the project and we will want photos.