My Harbor Freight greenhouse is build on top of treated-wood decking. It is sturdy and not slippery. However it was cold and leaky in the winter. I covered it with rubber mats, the kind you put together like a puzzle, also from Harbor Freight. We had a mild winter last year but it stayed warm inside with the floor covered, bubble wrap on the walls, and mylar across the back to reflect the sun. I find that firm flooring makes it easier for me to drag large heavy pots across the floor.
