What is it?
The foundation is the buried concrete base that spreads a wall's load into the ground beneath it. It sits below the footing blockwork, wide enough that the load per square metre of soil stays within what the ground can safely bear.
For a typical low-rise domestic wall, this is a strip foundation: a continuous concrete strip poured in a trench, centred under the wall above.
- 1Strip Foundation
- 2Earth
What does it do?
A foundation transfers the combined weight of the wall, and everything the wall in turn carries (floors, roof), down into the ground without the building settling unevenly or the ground failing beneath it. Its width and depth are sized so that the pressure it puts on the soil stays within that soil's safe bearing capacity.
Depth is also driven by frost heave and, on clay soils, by seasonal ground movement. The foundation needs to sit below the depth the ground itself moves, not just below the point it stops being disturbed by construction.
Where is it used?
Beneath every loadbearing wall, at a depth and width set by the soil conditions on that specific site. This will always include the external walls, but may also be required for any loadbearing internal walls if there are any.
Typical sizes
A typical domestic strip foundation is around 600 mm wide and at least 450–600 mm below finished ground level, though both figures are set project-by-project from a site investigation.
How to specify
Foundation depth and width are set by a Structural Engineer from ground conditions (usually a trial-hole or borehole investigation) rather than by a standard rule of thumb. Concrete strength (mix grade) is specified against the ground conditions too, since sulfates naturally present in some soils can attack ordinary concrete over time and need a sulfate-resisting mix.
Other considerations
Trees near a foundation are a common complicating factor on clay soils: as roots dry the clay out, the ground shrinks, and a foundation that isn't deep enough can settle unevenly as a result. This is one of the main reasons foundation depth is a site-specific engineering decision rather than a fixed number.
Interacting elements
- Footing blockwork: built up from the top of the foundation to ground level
- Hardcore: the compacted sub-base often placed above the foundation before the ground floor slab is poured
- Concrete slab: bears indirectly on the same ground the foundation is designed for