When building the floor frame, which includes the mudsill, floor joists and perimeter band joists, use 2 x 6 or 2 x 8 pressure-treated lumber. Many prefab sheds use untreated construction-grade lumber for the floor frame, which is fine--if you plan to keep your shed indoors. Even in ideal conditions on the perfect site, a shed floor will be exposed to some moisture, and in time, untreated lumber will rot.
Firewood sheds are simple sheds designed to help your firewood dry out and also keep it from getting a lot of water on the wood when it rains or snows. The simple open front design allows the wood to be easily accessible and increases air flow around the stacked wood. The floors are designed using 2x4 boards spaced 1" apart to provide plenty of strength and allow allow air to circulate up through the floor and around the wood to season it properly. The roofs are corrugated metal but they can also be roofed with asphalt shingles.
For a freestanding shed, you can build the shed on top of a concrete slab (see Pouring a Concrete Slab) instead of the pressure-treated skids and floor joists; this will actually result in a sturdier—more permanent— structure. Otherwise, for a freestanding shed, the foundation should be constructed on concrete piers or poured footings (see Pouring Concrete Footings & Piers). Be aware that building on footings will raise the height of the shed up from the ground.
Then, you will need to install a series of floor joists across the entire length of the support beams; these will need to be the same length as the distance between the two rim joints so that they’ll fit between them. In the example design, the floor joists are all separated by 14.5-inch gaps except for the outermost two, which are 13 3⁄4 inches (34.9 cm) from their immediate neighbors; this is to allow a standard piece of plywood to line up with the outermost edge of the outermost joist but only cover half of an interior joist, allowing its neighbor to cover the other half so that both can be supported properly.
Working from ladders is more dangerous than working from scaffolding. Plus, having to constantly move ladders around is time consuming. When you get to the roof construction, consider renting a set of scaffolding with wheels. You’ll be amazed at how much easier it is to have a stable platform to work from and to set your tools and materials on. You can rent a 5-ft.-tall section of scaffold with three planks and wheels for about $110 per week.