Erecting a shed on a slope is not safe, particularly if it's steep. However, you can dig out a level area large enough to accommodate a shed. If you build the foundations properly, the shed will be perfectly stable. Furthermore, the surrounding terrain may help to shield the shed from wind.
Choose a suitable part of the slope. The flatter it is already, the less digging you will have to do. Mark out the area with ground marking spray. Water based spray is better for the environment.
Dig horizontally into the slope to begin making a level base for the shed. Chop into the ground vertically to cut away and remove sections of earth. Continue this process until you have established a flat area bigger than the shed.
Work out where a corner of the foundations will be. Knock a wooden peg into the ground at this point. This will serve as the reference point for other measurements.
Knock other pegs into the ground every 30 cm (12 inches) in a rectangular configuration slightly smaller than the shed base. Professional Joinery suggests the concrete base should be 25 mm (1 inch) smaller than the shed base all the way around. This is so rain will drip off the shed beyond the foundations rather than directly onto them.
Nail 150 mm (6 inches) wide planks to the insides of the pegs, thereby erecting formwork in preparation for laying the foundations. Use a spirit level to ensure the formwork is at the same height all the way around.
Lay 75 mm (3 inches) of hardcore within the formwork. According to Buy Sheds Direct, a concrete base needs this amount of hardcore. Compact the hardcore with a hand tamper.
Order your ready-mix concrete. You will need to lay 75 mm (3 inches) of concrete onto the hardcore. At least one ready-mix concrete supplier provides a useful quantity calculation tool on its website. You enter the length, width and depth of your foundations and the tool will tell you how many cubic metres of concrete you need to order.
Smooth the concrete out with a concrete float when it is poured by the ready-mix concrete lorry driver. Leave the concrete to “go off” or set before erecting your shed. The Concrete Society recommends waiting at least three days.
Remove the formwork with care. Do not accidently chip the edges. You can now erect your shed.
You may need a mini digger for the excavation, depending on size of plot, accessibility and ground conditions. You can also use a mechanical plate compactor to tamp the hardcore. Not all ready-mix concrete suppliers are willing to supply a small quantity, so make sure you choose a supplier who will. You can mix the concrete yourself, if you wish, using a concrete mixer.