What is a shallow foundation?
A shallow foundation (fundering op staal) is a type of foundation that rests directly on the load-bearing soil layer, without the use of piles. Despite the Dutch name (which includes “staal”, meaning steel), it has nothing to do with steel — “op staal” is an old Dutch construction term meaning “on firm ground”.
How does a shallow foundation work?
With a shallow foundation, you dig a trench until you reach a soil layer that is firm enough to support the weight of the structure. You then pour concrete into this trench, creating a solid base.
The depth depends on two factors:
- The bearing capacity of the soil — Clay and sandy soil are usually suitable. Peat soil often is not.
- The frost line — The bottom of the foundation must be at least 60 to 80 cm below ground level, so that frost cannot lift the foundation.
When do you use a shallow foundation?
A shallow foundation is suitable for:
- Garden buildings and log cabins — Provided the soil has sufficient bearing capacity
- Sheds — For both timber and masonry sheds
- Garages — Often combined with a concrete slab as the floor
- Canopies — For heavier structures that need more stability
In the west of the Netherlands (peat soil), a shallow foundation is often not possible and you must resort to a pile foundation.
Shallow foundation vs. pile foundation
| Shallow foundation | Pile foundation | |
|---|---|---|
| Principle | Rests on load-bearing soil layer | Piles reach a deep load-bearing layer |
| Suitable for | Sandy and clay soil | Weak soil (peat, clay) |
| Depth | 60-80 cm below ground level | 6-20 metres deep |
| Cost | Lower | Higher |
| DIY | Very doable | Usually not a DIY job |
Types of shallow foundations
- Strip foundation — A continuous strip of concrete beneath the load-bearing walls. The most common variant.
- Pier foundation — Individual concrete blocks (piers) at strategic points. Suitable for lighter structures such as a garden building or canopy.
- Raft foundation — A full concrete slab that distributes the weight over a large area.
Building a shallow foundation yourself
For a garden building, shed or garage, you can often lay a shallow foundation yourself. The basic steps are:
- Setting out — Mark the exact position with profile boards and string line
- Digging — Dig the trenches to the correct depth (minimum 60-80 cm)
- Compacting — Tamp the bottom of the trench
- Placing formwork — Build timber formwork for the correct shape
- Installing reinforcement — Lay reinforcement bars in the formwork for extra strength
- Pouring concrete — Use at least C20/25 concrete
- Curing — Let the concrete cure for at least one week
> Tip: Always check with your local authority whether you need a building permit before you start the foundation. Specific rules apply to structures that are exempt from permit requirements.
Related terms
- Strip foundation
- Pier
- Reinforcement
- Concrete slab (floor slab)
- Frost line