What is foundation depth?
Foundation depth is the depth at which a foundation is constructed, measured from ground level to the underside of the foundation. The foundation depth must be sufficient to protect the foundation from frost and to reach a load-bearing soil layer.
Why is foundation depth important?
A foundation that is too shallow can cause problems:
- Frost damage — water in the ground can freeze and expand, pushing a shallow foundation upward (frost heave)
- Insufficient bearing capacity — the upper soil layers are often less load-bearing than deeper layers
- Washout — soil beneath a shallow foundation can be eroded by water flow
Application
Minimum foundation depth in the UK
In the UK, a typical minimum foundation depth is 600-900 mm below ground level, depending on soil type and local conditions. In colder climates, the frost-free depth may be deeper.
| Structure | Typical foundation depth |
|---|---|
| Houses | 600-1000 mm |
| Garden walls | 450-600 mm |
| Sheds, garages | 450-600 mm |
| Lightweight structures | 300-450 mm |
Factors affecting foundation depth
- Frost line — in the UK around 450 mm, in Scandinavia and North America significantly deeper
- Soil type — on clay or peat, deeper foundations may be needed to reach a bearing layer
- Water table — a high water table can limit foundation depth
- Loading — heavier structures may require a deeper foundation
- Tree roots — nearby trees can cause shrinkage in clay soils, requiring deeper foundations
Related terms
- Foundation
- Strip foundation
- Frost line
- Concrete pier
- Bearing capacity
Learn more about building terms in our knowledge base at fredsdiyplans.com.