It is essential to have a level and dry foundation. It is impossible to erect a shed on an uneven site as screw holes connecting the wall panels will not line up. For a lightweight shed it may be sufficient to use some paving slabs laid on heavy-duty polythene sheeting to act as a damp-resisting barrier. For larger buildings especially where the shed is to be used as a workshop a concrete slab is best. The slab needs to be just smaller than the floor area of the building so that rainwater running off the wall panels does not drip onto the concrete.
A concrete base needs to have a 75mm (3in) base of compacted hardcore. The base can be level with the ground or raised above it. If it is to be level you will need to excavate 150mm (6in) to allow for the hardcore layer and 75mm (3in) thickness of concrete.
Set up levelled formwork to contain the concrete and check diagonals (1). Then lay thoroughly compacted hardcore and cover with sand.
You can mix your own concrete using one part cement to five parts ballast (which is most economical) or use bags of dry-mixed concrete to which you add water.
The concrete should be kept on the dry side - not too much water or it will be sloppy.
Spread the concrete slightly proud of the formwork (2), then level it off with a long plank resting on the formwork using a sawing motion (3).
If rain is expected cover the concrete with polythene for 24 hours. In warm weather cover it with wet sacks and sprinkle them with water over the 24 hour period. The base is then ready for the shed.
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