I've had a shed in this space for a few years. It rested on soil with concrete lintels for a base. I dismantled it to have the driveway done and now I have a good solid base for the shed. I was planning on laying Pondliner, shed on top and the liner tucked up about 4cm on the outside back & sides. Then painted shiplap over that. Then lay chipboard on top of the liner for a floor. Liner would keep it all dry. Is there any reason I SHOULDN"T do it like this?