Sheds are popular in yards and gardens because you can store tools and materials out of the elements. Metal sheds are often sold as kits that the homeowner can assemble. These kits typically require the homeowner to provide a floor for the shed, which usually is a flat concrete foundation. The metal shed is then bolted or built on top of the concrete.
Measure and stake the corners of the floor foundation for the metal shed. Add 4 inches to the width and length to allow for the wood form. Tie string between the stakes to create straight lines. Measure on the diagonal to make sure the stakes (lines) are at right angles. Both diagonal measurements should be the same. Dig 4 inches deep with a shovel and remove all of the soil.
Build a 2-by-6-inch form whose inside measurements match the size of your shed floor. Place the form inside your hole and level the form. Hammer pegs along the outside of the frame on all sides and nail through the outside of the peg into the form. Each side should have a peg every 2 to 3 feet. Check your level and then saw off the top of the pegs so they are flush with the top of the form.
Place rebar 2 inches from each side. The rebar should run parallel with the walls. Elevate the rebar 2 inches above the base soil, using a plastic rebar chair. Rebar sections can be wired together with twists of wire and a wire cutter. Cut wire fencing to fit inside the rebar. Elevate the wire fencing 2 inches above the base soil. This will help strengthen the floor.
Mix and fill the foundation with concrete. Mix the concrete as specified by the type of concrete you choose and the manufacturer's recommendations. Fill to the top of the form. Spread the concrete to be as even as possible. Drag a piece of 2-by-4 board that is longer than the width of the form across the concrete to level it out. This is called a screed. You'll need an assistant for this.
Smooth the concrete with a steel trowel. You may need to smooth the concrete several times as it dries to achieve the finish you want. Allow the concrete to set up if it is too wet when you start. Allow the concrete to dry for several days before removing the form and installing the shed.
Shed floors built above grade may require ramps at the doors. Shed floors near grade must be designed so that water does not enter the shed. Grade the yard so that the slab is at least 2 to 3 inches above the finish grade.
Concrete is caustic. Wear gloves, boots, a long-sleeve shirt and trousers and protective goggles when handling. Keep concrete off the skin.