Sheds are built in many sizes and styles, depending on the use. Storage sheds, potting sheds, garden sheds and other types share common elements -- they are useful and they must have a good roof and sturdy foundation. You can build sheds on foundations of concrete blocks, concrete stem walls, precast concrete piers, even sleepers or treated wood pilings. The best foundation, however, is a concrete slab, formed and poured specifically for the shed.
Mark the outline for the shed foundation with wood stakes and builder's twine. Measure with a tape measure from corner to corner to make sure the outline is square; if the two diagonal lengths are the same, the shed is square. If there is a difference, adjust the sides until a square outline is achieved. Make the outline a few inches larger than the actual shed will be; it will be a guide for concrete forms.
Excavate inside that perimeter with a shovel, at least 8 inches deep, deeper if the shed will be large and tall or will store heavy equipment like tractors; 8 inches should suffice for most garden and storage sheds. Fill the hole to within 4 inches of the top with medium gravel, 1/2 to 3/4-inch diameter, and compact it solidly with a hand tamper. Extend the gravel base about 4 inches outside the finished perimeter of the shed foundation.
Build forms with 2-by-4-inch lumber to the exact size of the shed foundation. Let the two long sides extend past the front and back lines; cut 2-by-4s to fit inside those and nail them together. Check again to make sure the form is square. Secure the form with wood stakes driven into the ground outside the 2-by-4s about every two feet. Brace those stakes with angled stakes for extra support. Check all sides of the form with a level to make sure it is level; put a board across the corners to double check that level.
Make a slight dip or trench on the outside edges of the gravel, all the way around the forms. Pour concrete into the forms up to the tops of the 2-by-4s. Smooth or screed the concrete by moving a 2-by-4 across the top of the form, end to end and side to side. Jiggle the board as you move it to settle the concrete. Finish the concrete with a trowel when it begins to set, when water starts to show on the surface. Put a board on the concrete for support and start trowelling from the centre, working toward the edges. Let the concrete set at least overnight before starting any shed construction.