The structure of a domestic building: the substructure (foundations and ground floor) and the superstructure (walls, upper floors and roof), and what each part does.

What this dot point is asking

CCEA wants you to know how a house is built up from the ground. You should be able to divide a building into its substructure (the parts below ground that carry the load into the soil) and its superstructure (the parts above ground that form the usable building), and to describe the function of the main parts: foundations, ground floor, walls, upper floors and roof.

The answer

Substructure and superstructure

The substructure

Foundations spread the weight of the building over a larger area of firm ground so it does not sink or settle unevenly. The most common type for a house is a strip foundation: a continuous strip of concrete under the load-bearing walls. Other types include trench fill (a deep, narrow trench filled with concrete) and a raft (a single concrete slab under the whole building, used on weaker ground).

The ground floor sits on the substructure and provides the surface of the lowest storey. It can be a solid floor (a concrete slab with insulation and a damp-proof membrane) or a suspended floor (a timber or beam-and-block floor raised above the ground with a ventilated gap below to keep it dry).

A damp-proof course (DPC) is built into the walls just above ground level, and a damp-proof membrane (DPM) is laid under the floor slab. Together they stop moisture rising from the ground into the building (rising damp).

The superstructure

How loads travel down

The whole structure works as a path for loads. The roof and upper floors carry their loads onto the walls; the walls carry the combined load down to the foundations; the foundations spread it into the ground. Every part must be strong enough to pass on the load from the part above it.

Worked example: tracing the load of a roof

Examples in context

Example 1. Digging the foundations first

A house is always built from the ground up. The substructure (foundations and ground floor) is constructed first, because everything above it relies on it to carry the load into the ground.

Example 2. A cavity wall keeping out damp and cold

The brick outer leaf takes the weather, the cavity and insulation cut heat loss, and the inner block leaf supports the floors. Wall ties stop the two leaves moving apart. The DPC near the bottom stops rising damp.

Example 3. A load-bearing wall removed for an extension

If a builder wants to remove an internal load-bearing wall to open up a kitchen, a steel beam must be put in first to carry the load that the wall was carrying. This shows how the load path must always be maintained.

Understanding the structure as a load path, from roof to foundations to ground, explains why each part is built the way it is and why the materials are chosen as they are. It also underpins reading construction drawings, which show all of these parts in plan and section.

Try this

Q1. State whether the foundations are part of the substructure or the superstructure. [1 mark]

• Cue. The substructure.

Q2. Give the function of a foundation. [1 mark]

• Cue. To spread the building's load over firm ground so it does not sink or settle unevenly.

Q3. Name the three main loaded parts of the superstructure. [2 marks]

• Cue. External walls, upper floors and the roof.