Shed on raised decking frame

I'm going to build a large timber shed on a part raised platform at the back of my garden.

Picture this - I've built an part elevated frame in my garden using 6x6 columns which is roughly 3.6m by 4.8m. Half of this lengthwise is raised 1-1.5m above ground and the other half is just above ground level. I've framed it using 6x2 joists with a span of no more than 1.2m at 600mm centres.

What I would like to know is shall I put the 18mm plywood floor down for the base of the shed straight on the joists or would it be better to build a frame using 4x2 on top of the joists?

The walls then are going to sit on top of the plywood.

Sorry if this isn't too clear

Build a frame using tanalised timber otherwise the plywood will get wet and rot.

You could get away with the floor straight onto the frame as long as the cladding of the shed sides overhangs the edges of the ply.The water running down the sides of the shed would run straight down off the deck.