Garden cabin with oak subfloor perimeter on concrete pilings

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I'm installing a 5m x 5m garden cabin in my back garden. There are height restrictions. In order to accommodate these and to accommodate the depth of a 6x2 subfloor resting on concrete pilings, I've had to excavate the cabin area about a foot below grade. Since the life of the cabin depends on the life of its foundation, the perimeter of the subfloor will be oak sleepers. This is the only part of the subfloor that the cabin walls rest upon. These sleepers can't be typical green oak which is sold for garden purposes. They have to be air dried. I plan to coat just the top of the pilings and the area of the sleepers in contact with the pilings with bitumen and also have a layer of flexible, lead-like, roof flashing between them as additional moisture barrier that will also outlast the bitumen. I have a question: With no moisture getting into the area under the cabin other than what may come up through and around a thick roll-plastic moisture barrier, is there likely to accumulate any moisture in that minimal space between the flashing and the sleepers that would lessen the life of the oak? An air dried oak beam is said to have a life of over 50 years. There will be some air circulation from the 6 inch gap between the cabin walls and the surrounding retaining wall made of concrete paving slabs stood on end and bolted together. There'll be a board covering that 6 inch gap but there will be space under it for air to get through. Unfortunately, there won't be enough air circulation to air dry green oak, so I have to go with the more expensive air dried oak. What do you think of this plan? Does anyone recommend that I do any of it differently?
Screw in piles are the way forward , my mates business is booming .

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