On firm non-cohesive subsoils and rocks such as sand, gravel and sound rock beds which are near the surface, under vegetable top soil and are well drained or dry it is satisfactory to lay the concrete oversite directly on a bed of hardcore or broken rock rubble as there is little likelihood of any appreciable amount of moisture rising and being absorbed by the concrete. The concrete is laid within the confines of the external walls and load bearing internal walls and consolidated and levelled to a thickness of 100 mm ready for solid floor finishes or a raised ground floor.
On much of the low lying land that is most suitable for building, the subsoil such as clay retains moisture which will tend to rise through a hardcore bed to concrete oversite. The damp concrete will be cold underfoot and require additional energy from heating systems to maintain an equable indoor temperature. It is practice today to form a continuous layer of some material that is impervious to water under, in or over the concrete oversite as a damp-proof membrane on the site of all inhabited buildings where there is a likelihood of moisture rising to the concrete.