A pier foundation is a concrete block that serves as a foundation point for a timber or steel structure. Instead of a continuous strip of concrete (strip foundation), individual piers are placed at strategic points — usually under the corners and intersections of the structure. Pier foundations are the most commonly used foundation for garden buildings, covered structures, decks, and lightweight sheds.
How does a pier foundation work?
A pier distributes the weight of the structure over a larger ground area:
- The post or column stands on the pier (or is anchored into it)
- The pier spreads the point load over its full surface area
- The ground carries the weight
The principle is the same as snowshoes: the larger the contact area, the less the structure sinks.
Types of pier foundations
Precast concrete pier
Ready-made concrete blocks available at building merchants. Available with and without a hole or socket for the post.
| Type | Dimension (indicative) | Application |
|---|---|---|
| Small pier | 30x30x30 cm | Decks, light pergolas |
| Medium pier | 40x40x40 cm | Garden buildings, covered structures |
| Large pier | 50x50x50 cm | Sheds, carports |
Self-poured pier
For heavier structures or when you need to go deeper, you can pour piers yourself:
1. Dig a hole below the frost line (60-80 cm)
2. Compact the base
3. Build formwork
4. Add reinforcement if needed
5. Pour concrete (minimum C20/25)
Screw pier (ground screw)
A steel screw that is turned into the ground. Quick installation, no excavation needed. Suitable for light to medium-weight structures.
Placing pier foundations — step by step
1. Set out — Mark the exact positions with stakes and string line. Check the cross measurement (diagonals must be equal).
2. Dig — Dig a hole per pier. Depth: minimum 30 cm for light structures, 60-80 cm where frost is a concern.
3. Compact — Compact the base with a hand tamper.
4. Gravel bed — Lay 5-10 cm of gravel as a drainage layer.
5. Place the pier — Set the pier level. Check with a spirit level or laser that all piers are at the same height.
6. Mount the post bracket — Fix an adjustable post bracket on the pier (keeps the timber away from the concrete).
Common mistakes
- Piers not level with each other — The entire structure will then be skewed. Use a line laser or straight edge with spirit level.
- Placed too shallow — In frost, the pier can be pushed upward. Minimum 30 cm deep, preferably 60 cm.
- Timber directly on concrete — Moisture wicks from the concrete into the timber. Always use a post bracket or lead strip as a barrier.
- Cross measurement not checked — The structure will then be out of square.
Related terms
- Concrete pier
- Post bracket
- Shallow foundation
- Strip foundation
- Cross measurement
- Spirit level