Dry from the ground up
Your shed comes with a very solid floor.
The floor is made with 18mm weather-proof plywood (WBP). A coat of wood preservative on the top surface gives an extra layer of protection. This plywood is fitted to tanolised timber bearers and then a waterproof membrane is fitted between: This waterproof membrane also laps up around the edges where it is joined with the same membrane that is fitted within the walls.
The key to all outside buildings and the associated damp issues is to ensure that there is a good airflow under the shed. To this end the support beams built into the floor need to sit on a solid surface. There are many suitable surfaces, shingle, slabs, concrete, decking, tarmac to name but a few, these are what we call the sub base.