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What is a piered slab?

A piered slab uses concrete or steel piers drilled into stable soil below the reactive or unstable surface layer. The slab then sits on these piers rather than on the reactive soil, providing a more stable foundation.

When is a piered slab required?

Piered slabs are typically required for P class (problem) soils, heavily reactive E class sites, sites with significant fill, areas with deep reactive clay, or sites with poor bearing capacity at shallow depth.

How much does a piered slab cost?

Significantly more than a standard raft slab. Piered slabs can cost $25,000–$60,000 or more depending on the number of piers, depth required, and site conditions.

Can a piered slab be avoided?

Sometimes. An alternative is to remove problem fill and replace with engineered fill, or to use a heavier raft slab design. A geotechnical engineer will advise on the most cost-effective solution for your site.

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Home›Build Knowledge Hub›Building Process›Slabs & Siteworks›Piered Slab vs Standard Slab

Slabs & Siteworks

Piered Slab vs Standard Slab

When standard slabs aren't enough — what piered foundations are, when they're required, and what they cost.

5 min·Compare My Build — Independent Resource· Updated June 2025

Introduction

For most residential builds, a raft slab or waffle pod slab is sufficient. But on problem sites — those with deep reactive clay, uncontrolled fill, low bearing capacity, or P class soils — a piered slab may be required. Piers anchor the slab to stable ground below the reactive surface layer, providing a foundation that won't move with soil moisture changes.

Key Differences

Feature	Standard Raft Slab	Piered Slab
Foundation	Sits on prepared ground surface	Piers drilled to stable bearing layer
Soil class	M, H1, H2	P class, E class, fill sites
Construction complexity	Moderate	High
Cost	$12,000 – $25,000	$25,000 – $60,000+
Pier depth	N/A	2m – 12m+ depending on site
Risk of differential movement	Higher on reactive soils	Very low when correctly designed

Pros and Cons

Pros

+ Piered slabs provide very stable foundations on problem sites
+ Reduces risk of slab cracking, door jams and wall cracking over time
+ May be the only viable solution on sites with deep fill or poor bearing capacity
+ Provides peace of mind on challenging sites

Cons

− Significantly more expensive than standard slabs
− Construction takes longer due to drilling requirements
− Not all volume builders can accommodate piered slab designs
− Costs can be difficult to fix — pier depths may vary once drilling begins

Cost Comparison

Standard waffle pod slab (M class)$8,000 – $14,000

Raft slab (H1/H2 class)$13,000 – $25,000

Piered slab (P class / fill sites)$25,000 – $60,000+

Each pier (approximate)$800 – $2,500 per pier

Piered slab costs depend on the number and depth of piers required — determined after drilling commences. Final costs can vary from the initial estimate.

Common Builder Inclusions

›Volume builders do not include piered slabs in base prices
›P class sites trigger a complete engineering redesign — expect significant extras
›Piered slab costs are almost always Provisional Sums — confirm a realistic budget range
›Some builders will refer complex sites to specialist contractors

Watch Out

If your site has fill material or problem soils, get a full geotechnical investigation before purchasing the land. Discovering a piered slab is required after signing can add $30,000–$50,000 to your build cost.