Quick Answer
Pier and beam footings for an Australian residential build typically use 450-600mm diameter piers at 1.8-2.4m centres, with N16 reinforcement. For a 100m² pier-and-beam slab on Class M soil, expect ~25 piers, 8 m³ concrete, and 350kg of steel reinforcement.
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When to Use Pier and Beam vs Slab on Ground
Pier and beam (suspended slab on piers) is the right choice when:
- Reactive soil (Class H or P): Highly reactive clays move enough to crack a standard slab
- Sloping site: Avoids massive cut-and-fill operations
- Flood-prone: Lifts dwelling above flood planning levels
- Subfloor access required: For services, ducting, future works
- Existing tree roots: Spreads load away from root zones
Pier Spacing & Sizing per AS 2870
| Site Class | Pier Diameter | Typical Spacing | Min Depth |
|---|---|---|---|
| A / S (sand/rock) | 300-450mm | 2.4m | 900mm |
| M (moderate clay) | 450-500mm | 1.8-2.4m | 1500mm |
| H1 / H2 (highly reactive) | 500-600mm | 1.5-1.8m | 1800-2400mm |
| E / P (extreme/problem) | 600mm+ | 1.2-1.5m | Engineered (often >3m) |
FAQs
How much does a pier-and-beam footing cost in Australia?
For a 100m² residential dwelling on Class M soil in 2026, expect $14,000-$22,000 supplied and installed including concrete, steel, formwork, and labour. Class H or P sites can reach $35,000+.
How deep should pier holes be on reactive clay?
Minimum 1,500mm on Class M, 1,800-2,400mm on H1/H2, 3m+ on Class E/P. Always per the engineer’s certified design.
What concrete strength for piers?
25 MPa (N25) standard for residential pier and beam per AS 3600. Higher MPa (32-40) for commercial or coastal exposure.
Do I need an engineer for pier and beam footings?
Always — for any structural slab. Engineering fees in 2026 are $1,500-$5,000 for residential pier-and-beam design including AS 2870 compliance.