A good quality base will support your building against stress caused by ground movement or settlement, deter rot and will increase the lifespan of your building and investment. Your base should be level, as gauged by a spirit level - not just flat faced but sloping for example. In theory, if your base slopes, the building will also lean to the slope - not ideal. In conclusion, any prepared base area should be level, of an even surface, squared (check diagonals are equal, corner to corner) and of a size no less than the plan size of the building.
Garden building base construction can generally be undertaken by two material means, either by paving or concrete.
For a paved base, the area should be stripped of any turf or planting, with excavation extending below ground level, giving consideration to the incorporation of a weed suppressant membrane. The sub-ground area should be filled with compacted hardcore, scalpings or brick rubble to act as a sound foundation. The surface of the hard cored area can then be levelled with compacted sand if appropriate. Paving slabs should be laid to a level plane upon mortar dabs, to an area no less than the building size, with the surface being higher than the surrounding ground to encourage rainwater drainage. As the slabs will be hidden from view by the building, we would recommend the purchase of economically priced standard 2' x 2' (600 x 600mm) pressed concrete paving slabs. An 8' x 6' shed would require only 12no.2' x 2' slabs, laid to a 3 x 4 rectangular format for example.
For a concrete base, once again the area should be stripped of any turf or planting, with excavation extending below ground level, giving consideration to the incorporation of a damp proof membrane (dpc). The sub-ground area should be filled with compacted hardcore, scalpings or brick rubble to act as a sound foundation. The base area should then be accurately set out and shuttered (using timber rails or steel shuttering) to a minimum depth of 4" (100mm) and to a plan size no less than that of the building. Remember to check the shuttering profile is level and the diagonals are equal. The shuttered area can now be flood-filled with concrete, making sure the concrete is well spread and tamped down across the shuttering. Finish the top edge (4"-6" wide) by trowel to provide a smooth surface for the proposed building to sit, especially if a metal, base railed product. Cover the base with supported polythene to drain rainwater away during curing. Ideally, leave for about a week before building installation. An 8' long x 6' wide x 4" deep base will require 0.45m3 of concrete.