Concrete bases for screeds
These are covered by BS 8204, Screeds, bases and in-situ floorings, Part 1: Concrete bases and cement sand levelling screeds to receive floorings– code of Practice (2004).
A concrete base is the building element that provides the support for a screed and floor finishes. The thickness of a base slab should be determined by the loading conditions, the minimum thickness of any slab on the ground being 100mm.
In situ concrete ground bearing slabs are the most common base used for floors in the UK. Inert, well-graded fully compacted granular material (hardcore) or lean material is used to provide a level base upon which to cast the concrete base, to raise levels and to minimise capillary rise of ground moisture. A Damp Proof Membrane (DPM) will also be required either above or below the concrete base. Thermal insulation may also be used and in some locations it may be necessary to take precautions against radon or methane seeping through the slab.
Where differential ground movement is likely to occur the slab is reinforced to act as a raft foundation.
Further reading: PYE, PW and HARRISON, HW. Floors and Flooring, BRE, 2003.