Building a shed requires a sturdy base. While a wood platform can serve this purpose, a concrete floor will last longer and endure more extreme weather conditions. Laying a concrete floor for a shed uses much the same process as creating any other concrete slab, the primary difference being scale. With the proper procedure, though, your concrete floor will likely outlast the shed itself, providing a base you can rely on for decades to come.
Measure the length and width of the base of your shed. Add 4 inches to each measurement and mark down the new size. Mark out the new dimensions on a piece of ground. Place wooden stakes into the ground at each corner and then attach a string to the tops of the stakes, connecting the stakes into a square.
Excavate the area within the square to a depth of 4 inches, using a spade. Dig a trench around the outside perimeter of the excavation a further 3 inches deep and 5 inches wide to form a footing for the slab. Tamp down the bottom of the excavation with a plate compactor to provide a firm foundation for the concrete floor of the shed.
Cut and place 2-by-4-inch boards around the inside perimeter of the foundation to create a wooden wall 4 inches in height. Use a table saw to cut the boards and then nail them together at the joints to keep them stable.
Pour a 2-inch layer of gravel into the base of the excavation. Level the gravel and then compact it with the plate compactor. The gravel will help drain water from beneath the slab, keeping the moisture from causing damage during cold weather cycles.
Use a water hose to wet down the gravel slightly to prevent the stones from absorbing the moisture from the concrete.
Mix the concrete in a wheelbarrow with the spade. Fold the dry sand, cement and aggregate mixture over the water as you add it, until you have a mixture with the consistency of oatmeal.
Pour the concrete into the excavation from a corner of the hole. Fill the excavation to the level of the wood frame.
Drag a screed bar across the top of the concrete using a sawing motion over the wood frame. The screed will level the concrete's surface by pushing high bits of the pour before it to fill in any voids.
Smooth the surface of the slab with a steel trowel. Remove any lines or gouges left in the surface from the screed bar.
Finish the surface with a slight nonslip texture by brushing the surface of the concrete with the broom.
Cover the shed floor with a sheet of plastic to slow the drying out of the concrete during the curing period. Secure the edges of the plastic on the ground around the slab with bricks. Wait one week for the slab to cure with the sheet on and then remove the sheet and allow the slab to cure a second week before using the surface.