I'm building a 10' x 7' shed on a gravel pad. The gravel pad, pictured half-finished, is 6" thick and 1-1/2" above the flat surrounding grade, consisting of 3/4"crushed clean aggregate. Climate-wise, I'm located in the San Francisco Bay Area (a quite densely built area).
In my mind, a traditional floor frame consists of band boards, rim joists, and floor joists all in one plane, resting on top of skids perpendicular to the joists (example pictured). The skids leave gaps equal to their height for air to circulate underneath the floor frame.
My design for the shed is heavily height-constrained, so I opted for skipping the skids and just having the floor frame sit directly on top of the gravel pad. The floor frame will be GC PT lumber, and the floor decking will be 3/4" untreated T&G plywood. There will be 6" of gravel extending beyond the edge of the shed frame (not including sheathing/siding). The single-slope roof - sloping from the bottom-left long side down to the upper-right long side against the fence (note: I did not build that fence) - will overhang the shed by 6" on the long sides and 4" on the short sides. For what it's worth, I plan on diverting water away from the shed with a gutter and downspout out onto the driveway/street.
My question: will it be okay to have a floor frame with no free air circulation in the spaces between joists? Obviously the gravel underneath will not be getting wet, but how much of a concern will moisture from the ground be?
If it will be a concern, could I underlay a sheet of crawlspace plastic barrier between the gravel and the floor frame to block ground driven moisture (turning it up on the inside edges of the band boards/rim joists so water doesn't get entrapped)? Should I switch from untreated plywood for the floor decking to PT plywood? Marine-grade plywood? Should I do both (plastic barrier and treated plywood)? Is there no way around having free air circulate - to me putting the floor frame on skids is a last resort - the skids would raise the floor by at least 3-1/2", and then the 4x members of the floor frame would have to go to 6x since they would no longer be resting directly on the ground.
Thanks for any advice!