Depending whether the foundation for a new detached building has been
cast as deep-fill or shallow-fill, there could be a small or large
amount of wall construction needed below ground level (referred to as
substructure), on which the above ground walls (referred to as
superstructure) will be built.
The principal requirement of the substructure is to ensure adequate
support is provided to the superstructure. The substructure (bricks or
blocks and mortar) should be effective and be resistant to frost and
also to sulphates within the ground.