I need to build a 16" X 6" garden shed on about one-fourth of the area of a former basketball court concrete slab. I want to ensure that the shed's floor is always dry, and know I have to elevate the floor off the slab to ensure this. I propose to make the foundation out of two 2" X 4" pressure-treated and insect repellant wood -- one stacked on the first. I propose to attach the first 2" X 4" to the concrete slab with 3/16" x 3-1/4" Hex Head Tapcon screws -- spaced about every 16". I will bead the outside perimeter of this wood base with a waterproof silicone or polyurethene caulk. I may add a thick layer of felt paper between the concrete slab and the first 2" X 4" to lessen the wood contact with moisture that will undoubtedly penetrate through the caulk as it dries and cracks. I will then build a floor over this floor perimeter with floor joists. Any critique of my idea?