The most stable and long-lasting base for a shed is a concrete pad. The pad should be of the same dimensions or slightly larger than the shed to provide a proper anchor for it. Once built, the shed will be a permanent structure, so take your time with the planning phase of the project to ensure that the shed is placed in the best spot relative to your needs.
Building the form
If the shed is quite small, place the assembled shed on the ground in the desired location. Mark the perimeter of the shed with marking paint, spraying the paint approximately 5 to 7.5 cm (2 to 3 inches) from the base of the shed. Remove the shed from the location. If the shed will be built later, mark the dimensions on the ground, being sure that the corners are perfectly square. If you need the visual guidance, make another set of marks 5 to 7.5 cm (2 to 3 inches) out from the first set.
Dig out the area within the marked perimeter to a depth of 15 cm (6 inches). Compact the ground with a hand tamper, which is a long pole with a flat base and two handles on the side, or a mechanical tamper, which is available for rent from most hardware shops.
Spread landscape fabric along the base of the hole, overlapping seams by 7.5 to 10 cm (3 to 4 inches).
Lay 5 cm by 15 cm (2 inch by 6 inch) boards on edge around the perimeter of the hole to build the basic form. At corners, drive 7.5 cm (3 inch) nails through an exterior edge into the ends of the adjacent board. Drive nails at an angle through the narrow edge of one board into another board butted up against it to create a side that is more than one board long.
Measure the inside perimeter of the wood form. The measurement should equal or be larger than the exterior perimeter of the shed. Adjust the form as needed to obtain the proper dimensions.
Drive wood stakes with a mallet along the exterior edge of the forms, spaced every 60 to 90 cm (2 to 3 feet). Place a wood stake along each seam or joint between wood beams. The top of each stake must sit flush with or below the top of the wood form.
Place a level on the top of the wood form to gauge side-to-side and front-to-back level. Raise and lower the form inside the hole until the proper level is obtained. Anchor the position of the form by hammering nails through the stakes into the form.
Laying the pad
Pour a 2.5 cm (1 inch) thick layer of crushed gravel over the base of the hole. Compact the gravel with a tamper and check the depth. The depth of the hole should be 12.5 cm (5 inches) below the top of the wood form. Add and remove gravel as needed to obtain a uniform depth.
Mix concrete and pour it into the hole until the form is half full. Lay out lengths of rebar in a grid pattern across the top of the concrete. Space the rebar every 45 to 60 cm (18 to 24 inches) in both directions, creating squares.
Finish filling the form with concrete. Tap the edges of the form with the rubber mallet while pouring the concrete to help remove air pockets.
Drag a spare board across the top of the form to remove excess material and to leave a level surface. Move a float in wide arcs across the surface to smooth the concrete.
Smooth the surface with a trowel once the surface water has evaporated and the concrete takes on a dull, grey appearance. Drag a push broom over the surface to create small grooves for a skid-resistant surface. Allow the concrete to cure before placing or building a shed on it.
Cover the concrete pad with plastic for seven days to optimise the curing process. The plastic is not required but will assist in keeping the concrete moist while the chemical reaction solidifies the material. Do not wet the surface of the concrete while it is curing. Excess moisture will prolong the process. Allow the concrete to cure and remain stable for two to three weeks before applying any type of sealant or paint.